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County-Scale Distribution of Ixodes scapularis and Ixodes pacificus (Acari: Ixodidae) in the Continental United States

Rebecca J. Eisen, Lars Eisen, Charles B. Beard
DOI: http://dx.doi.org/10.1093/jme/tjv237 tjv237 First published online: 18 January 2016

Abstract

The blacklegged tick, Ixodes scapularis Say, is the primary vector to humans in the eastern United States of the Lyme disease spirochete Borrelia burgdorferi, as well as causative agents of anaplasmosis and babesiosis. Its close relative in the far western United States, the western blacklegged tick Ixodes pacificus Cooley and Kohls, is the primary vector to humans in that region of the Lyme disease and anaplasmosis agents. Since 1991, when standardized surveillance and reporting began, Lyme disease case counts have increased steadily in number and in geographical distribution in the eastern United States. Similar trends have been observed for anaplasmosis and babesiosis. To better understand the changing landscape of risk of human exposure to disease agents transmitted by I. scapularis and I. pacificus, and to document changes in their recorded distribution over the past two decades, we updated the distribution of these species from a map published in 1998. The presence of I. scapularis has now been documented from 1,420 (45.7%) of the 3,110 continental United States counties, as compared with 111 (3.6%) counties for I. pacificus. Combined, these vectors of B. burgdorferi and other disease agents now have been identified in a total of 1,531 (49.2%) counties spread across 43 states. This marks a 44.7% increase in the number of counties that have recorded the presence of these ticks since the previous map was presented in 1998, when 1,058 counties in 41 states reported the ticks to be present. Notably, the number of counties in which I. scapularis is considered established (six or more individuals or one or more life stages identified in a single year) has more than doubled since the previous national distribution map was published nearly two decades ago. The majority of county status changes occurred in the North-Central and Northeastern states, whereas the distribution in the South remained fairly stable. Two previously distinct foci for I. scapularis in the Northeast and North-Central states appear to be merging in the Ohio River Valley to form a single contiguous focus. Here we document a shifting landscape of risk for human exposure to medically important ticks and point to areas of re-emergence where enhanced vector surveillance and control may be warranted.

  • Ixodes scapularis
  • Ixodes pacificus
  • distribution
  • Lyme disease

The blacklegged tick, Ixodes scapularis Say, is the primary vector to humans in the eastern United States of the Lyme disease spirochete Borrelia burgdorferi, as well as the relapsing fever spirochete, Borrelia miyamotoi; causative agents of anaplasmosis (Anaplasma phagocytophilum) and babesiosis (Babesia microti); and Powassan encephalitis virus (Piesman and Eisen 2008, Ebel 2010, Krause et al. 2015). Its close relative in the far western United States, the western blacklegged tick Ixodes pacificus Cooley and Kohls, is the primary vector to humans in that region of Lyme disease and anaplasmosis agents, as well as relapsing fever spirochetes (B. miyamotoi; Lane et al. 1994, Teglas and Foley 2006, Krause et al. 2015). Lyme disease is the most commonly reported vector-borne disease in the United States. It is a geographically focal illness, with the majority of cases reported from the Northeastern and North-Central states and discrete areas of risk in the Pacific Coast states (Mead 2015). Since 1991, when standardized surveillance and reporting began, Lyme disease case counts have increased steadily from roughly 10,000 cases in 1991 to more than 30,000 cases in 2008 and subsequent years (Bacon et al. 2008, Mead 2015); the true burden of disease is estimated to be roughly 10-fold greater (Hinckley et al. 2014, Nelson et al. 2015). In addition to the increase in case counts over time, the geographical foci of high-incidence counties have expanded both in the North-Central and in the Northeastern United States (Kugeler et al. 2015).

Rising case counts and geographical expansion of Lyme disease endemic areas have been attributed to range expansion of I. scapularis in the eastern United States (Bacon et al. 2008, Rydzewski et al. 2012, Lee et al. 2013, Brinkerhoff et al. 2014, Robinson et al. 2014, Wang et al. 2014, Khatchikian et al. 2015, Kugeler et al. 2015, Stone et al. 2015). However, because of a lack of systematic surveillance of I. scapularis and I. pacificus, national trends in the geographic distribution of these medically important ticks are difficult to document. To better understand the changing landscape of risk of human exposure to I. scapularis and I. pacificus in the United States, and to document changes in their distribution over the past two decades, we updated the reported distribution of these species from the map previously published by Dennis et al. (1998).

Materials and Methods

County Status Definitions

The definitions used to classify I. scapularis or I. pacificus as “established” or “reported” in a county follow Dennis et al. (1998). Counties were classified as established if at least six individual ticks or at least two of the three host-seeking life stages had been identified in a single collection period. Here, a single collection period is defined as a single year. Counties were classified as reported if they failed to meet the criteria for established but if at least one tick of any life stage had been identified at any time in that county, or if county records did not specify the number of ticks or life stages collected. Lack of tick records from a county—“no records”—does not imply that ticks are absent from that county, only that records of ticks having been collected in the county are lacking.

The county status (i.e., established, reported, or no records) given by Dennis et al. (1998) was used as the basis for our updated county status. If a county was classified as established by Dennis et al. (1998), it remained established in the updated classification regardless of whether more recent tick records were available. A county classified as reported by Dennis et al. (1998) retained this status in the updated classification, unless more recent collection records changed the county’s classification from reported to established. Herein, the term county refers to counties and county equivalents corresponding with five-digit Federal Information Processing Standard (FIPS) coding.

Data Sources

Six independent literature searches were conducted using Scopus and PubMed databases with the following key words “Ixodes scapularis,” “Ixodes pacificus,” and “tick” to identify relevant articles and abstracts published from 1996 through 25 August 2015. We also conducted a search to ensure that papers using the junior synonym “Ixodes dammini” rather than Ixodes scapularis were included. All reports that explicitly presented county-specific tick data were included in our database. In addition, we visited individual state health department Web sites to identify county-level tick surveillance data, and contacted public health officials, acarologists, and Lyme disease investigators throughout the United States to assess county-level tick collection data.

GIS Mapping

Our final database containing state, county, county FIPS code, county status as per Dennis et al. (1998), and the updated county status was joined based on FIPS codes to a continental United States county map using ArcMap 10.3 (ESRI, Redlands, CA).

Results

Counties With Recorded Presence of I. scapularis or I. pacificus

Our updated county status records show that I. scapularis now has been collected from 37 states, from the eastern seaboard to the eastern edge of the Great Plains, and I. pacificus from six western states (Tables 1–3; Fig. 1). No single state has records of both tick species, and five states in the Rocky Mountain region lack records for either I. scapularis or I. pacificus: Colorado, Idaho, Montana, New Mexico, and Wyoming. The presence of I. scapularis has now been documented from 1,420 (45.7%) of the 3,110 continental United States counties, as compared with 111 (3.6%) counties for I. pacificus. Combined, these primary vectors of B. burgdorferi and other tick-borne disease agents now have been identified in a total of 1,531 (49.2%) counties spread across 43 states. This marks a 44.7% increase in the number of counties that have recorded the presence of these ticks since the survey conducted by Dennis et al. (1998), when 1,058 counties in 41 states reported the ticks to be present. Nebraska and North Dakota are the two states where I. scapularis was recorded only after the Dennis et al. (1998) survey.

Fig. 1.

Distribution by county of recorded presence of I. scapularis and I. pacificus in the continental United States (a) 1907–1996 (from Dennis et al. 1998), (b) 1907–2015. Counties classified as established (red or green) for a given tick species had at least six ticks or two life stages recorded within a single calendar year. Counties with fewer ticks of a single life stage were classified as reported (blue or yellow) for the tick species. Counties shown in white indicate“no records.”

Counties Where I. scapularis Is Classified as Established or Reported

Ixodes scapularis now is classified as established in 842 counties (27.1% of counties in the continental United States) distributed across 35 states (Tables 1–3; Fig. 1). This more than doubles the number of counties in which the tick is classified as established since the previous survey by Dennis et al. (1998), when it was considered established in 396 counties (12.7% of counties in the continental United States) spanning 32 states (Tables 1–2; Fig. 1–2). In total, 446 counties were updated from either no records (n = 262) or reported (n = 184) to established, and 208 counties were updated from no records to reported (Table 2; Fig. 2). Counties with I. scapularis classified as established were added for three states: Kentucky, North Dakota, and Ohio.

Fig. 2.

Changes in county status for I. scapularis and I. pacificus from December 1996 (Dennis et al. 1998) to August 2015 (our data). Black or gray color indicates that county status already was established (black) or reported (gray) for I. scapularis or I. pacificus by Dennis et al. (1998) and considered to be the same in this study. Red or orange color indicates that the status of a county changed from no records to established (red) or from reported to established (orange). Green color indicates that the status of a county changed from no records to reported.

The data presented here suggest that I. scapularis over the past two decades has expanded from its northeastern focus northward into upstate New York, Vermont, New Hampshire, and northern Maine; westward across Pennsylvania, eastern Ohio, and New York; and south- and southwestward into West Virginia, Virginia, and North Carolina (Fig. 2). A similar geographic expansion for I. scapularis appears to have occurred from the long-established focus in the North-Central states, with notable spread of counties where the tick is now classified as established in all four cardinal directions (Fig. 2). The two previously distinct foci in the Northeast and North-Central states appear to be merging in the Ohio River Valley to form a single contiguous focus. In striking contrast to the Northeast and North-Central states, in the far South and South-Central states, counties where I. scapularis is classified as established have remained relatively stable since the survey by Dennis et al. (1998) (Figs. 1–2).

Ixodes scapularis is now classified as reported in 578 counties (18.6% of counties in the continental United States) distributed across 30 states (Table 1; Fig. 1). Counties classified as reported for I. scapularis generally clustered around counties classified as established for this tick. The overall ratio of counties in which I. scapularis is classified as established versus reported (established:reported) was 1.41:1 in this study, as compared with 0.71:1 in the previous Dennis et al. (1998) survey.

View this table:
Table 1.

Number (%) of continental United States counties in which I. scapularis or I. pacificus were classified as reported or established, by December 1996 (from Dennis et al. 1998) and August 2015

Species/StateNo. (%) counties with reported statusNo. (%) counties with established status
By 1996By 2015By 1996By 2015
I. scapularis556  (17.8)578  (18.6)396  (12.7)842  (27.1)
Alabama20  (29.9)21  (31.3)25  (37.3)25  (37.3)
Arkansas37  (49.3)25  (33.3)9  (12.0)27  (36.0)
Connecticut0  (0)0  (0)8  (100)8  (100)
Delaware0  (0)0  (0)3  (100)3  (100)
Florida22  (32.8)15  (22.4)35  (52.2)52  (77.6)
Georgia31  (19.5)35  (22.0)23  (14.5)35  (22.0)
Illinois47  (46.1)29  (28.4)4  (3.9)35  (34.3)
Indiana25  (27.2)37  (40.2)8  (8.7)29  (31.5)
Iowa17  (17.2)25  (25.3)7  (7.1)14  (14.1)
Kansas14  (13.3)14  (13.3)1  (1.0)1  (1.0)
Kentucky2  (1.7)4  (3.3)0  (0)14  (11.7)
Louisiana12  (18.8)23  (36.0)12  (18.8)12  (18.8)
Maine3  (18.8)0  (0)13  (81.3)16  (100)
Maryland2  (8.3)2  (8.7)21  (87.5)21  (91.3)
Massachusetts3  (21.4)0  (0)9  (64.3)14  (100)
Michigan22  (26.5)16  (19.3)5  (6.0)24  (28.9)
Minnesota12  (13.8)3  (3.5)9  (10.3)45  (51.7)
Mississippi72  (87.8)71  (86.6)10  (12.2)11  (13.4)
Missouri8  (7.0)8  (7.0)21  (18.3)23  (20.0)
Nebraska0  (0)3  (3.2)0  (0)0  (0)
New Hampshire5  (50.0)1  (10.0)5  (50.0)9  (90.0)
New Jersey0  (0)0  (0)21  (100)21  (100)
New York20  (32.3)1  (1.6)31  (50.0)61  (98.4)
North Carolina23  (23.0)16  (16.0)7  (7)43  (43.0)
North Dakota0  (0)3  (5.7)0  (0)5  (9.4)
Ohio5  (5.7)31  (35.6)0  (0)33  (37.5)
Oklahoma36  (46.8)36  (46.8)3  (4.0)3  (4.0)
Pennsylvania26  (38.8)0  (0)23  (34.3)67  (100)
Rhode Island0  (0)0  (0)5  (100)5  (100)
South Carolina14  (30.4)19  (41.3)13  (28.3)14  (30.4)
South Dakota2  (3.0)2  (3.0)2  (3.0)0  (0)
Tennessee6  (6.3)27  (28.4)1  (1.1)16  (16.8)
Texas39  (15.4)45  (17.7)24  (9.5)26  (10.2)
Vermont6  (43.0)2  (14.3)1  (7.1)11  (78.6)
Virginia4  (3.0)29  (21.6)8  (6.0)43  (32.1)
West Virginia2  (3.6)20  (36.4)2  (3.6)23  (41.8)
Wisconsin16  (22.2)15  (20.8)29  (40.3)51  (70.8)
I. pacificus16  (0.5)16  (0.5)90  (2.9)95  (3.1)
Arizona1  (6.7)1  (6.7)0  (0)0  (0)
California1  (1.72)1  (1.72)55  (94.8)55  (94.8)
Nevada2  (11.8)2  (11.8)0  (0)0  (0)
Oregon4  (11.1)4  (11.1)18  (50.0)18  (50.0)
Utah4  (13.8)3  (10.3)4  (13.8)4  (13.8)
Washington5  (12.8)6  (15.3)12  (30.8)16  (41.3)
View this table:
Table 2.

Status for I. scapularis by continental United States county. Fields left blank indicate that status was inherited from Dennis et al. (1998)

State and countyStatus by August 2015aStatus change from Dennis et al. (1998)bSource for change of status from Dennis et al. (1998) survey
Alabama
 AutaugaEstablished
 BaldwinReported
 BarbourEstablished
 BibbEstablished
 BullockEstablished
 ButlerEstablished
 ChambersEstablished
 ChiltonReported
 ChoctawEstablished
 ClarkeEstablished
 ClayReported
 CleburneReported
 CoffeeReportedN-Rhttp://vectormap.nhm.ku.edu/vectormap/
 ColbertEstablished
 ConecuhEstablished
 CoosaEstablished
 CovingtonEstablished
 CrenshawReported
 DaleReported
 ElmoreEstablished
 EscambiaEstablished
 FranklinEstablished
 GenevaReported
 HaleEstablished
 HenryEstablished
 HoustonReported
 JacksonEstablished
 JeffersonReported
 LawrenceReported
 LeeEstablished
 LowndesReported
 MadisonReported
 MarengoReported
 MobileEstablished
 MonroeEstablished
 MontgomeryReported
 PickensEstablished
 RandolphReported
 RussellEstablished
 SumterReported
 TalladegaReported
 TallapoosaEstablished
 TuscaloosaReported
 WashingtonReported
 WilcoxReported
 WinstonEstablished
Arkansas
 AshleyEstablished
 BaxterEstablished
 BentonEstablishedR-E(Trout and Steelman 2010)
 BooneEstablishedR-E(Trout and Steelman 2010)
 CalhounReported
 CarrollReported
 ClarkEstablishedN-E(Trout and Steelman 2010)
 CleburneEstablishedN-E(Trout and Steelman 2010)
 ColumbiaReported
 ConwayReported
 CrawfordEstablishedR-E(Trout and Steelman 2010)
 DallasReported
 DrewReported
 FaulknerEstablishedN-E(Trout and Steelman 2010)
 FranklinEstablished
 FultonEstablishedR-E(Trout and Steelman 2010)
 GarlandEstablishedR-E(Trout and Steelman 2010)
 GrantEstablishedR-E(Trout and Steelman 2010)
 HempsteadEstablishedN-E(Trout and Steelman 2010)
 HowardReported
 IzardReported
 JeffersonReported
 JohnsonReported
 LafayetteReported
 LawrenceEstablishedR-E(Trout and Steelman 2010)
 LincolnReported
 LoganReported
 MadisonEstablished
 MarionEstablishedR-E(McAllister et al. 2013)
 MontgomeryReported
 NevadaReported
 NewtonReported
 OuachitaReported
 PerryReported
 PikeReported
 PoinsettEstablishedN-E(Trout and Steelman 2010)
 PolkEstablished
 PopeEstablishedR-E(Trout and Steelman 2010)
 PrairieEstablishedR-E(Trout and Steelman 2010)
 PulaskiReported
 RandolphReported
 SalineEstablishedN-E(Trout and Steelman 2010)
 ScottReported
 SearcyEstablished
 SebastianEstablished
 SevierEstablishedR-E(Trout and Steelman 2010)
 SharpReported
 StoneEstablished
 UnionEstablishedR-E(Trout and Steelman 2010, McAllister et al. 2013)
 Van BurenReported
 WashingtonEstablished
 YellReported
Connecticut
 FairfieldEstablished
 HartfordEstablished
 LitchfieldEstablished
 MiddlesexEstablished
 New HavenEstablished
 New LondonEstablished
 TollandEstablished
 WindhamEstablished
Delaware
 KentEstablished
 New CastleEstablished
 SussexEstablished
District of ColumbiaEstablishedN-ET. L. Johnson, unpublished
Florida
 AlachuaEstablished
 BakerEstablished
 BayEstablished
 BradfordReported
 BrevardEstablishedN-EJ. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
 BrevardReported
 BrowardReported
 CalhounEstablished
 CharlotteReported
 CitrusReported
 ClayEstablished
 CollierEstablished
 ColumbiaEstablished
 DeSotoEstablishedN-EJ. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
 DixieEstablished
 DuvalReported
 EscambiaEstablished
 FlaglerEstablished
 FranklinEstablished
 GadsdenEstablished
 GilchristReportedN-RK. Sayler, unpublished
 GladesEstablished
 GulfEstablished
 HamiltonEstablishedR-EJ. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
 HardeeReported
 HendryEstablishedR-EJ. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
 HernandoEstablished
 HighlandsEstablishedR-EJ. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
 HillsboroughEstablishedR-EJ. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
 HolmesReportedN-RJ. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
 Indian RiverEstablishedN-EC. Lord, unpublished
 JacksonEstablished
 JeffersonEstablished
 LafayetteEstablished
 LakeEstablishedR-EJ. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
 LeeEstablishedR-EJ. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
 LeonEstablished
 LevyEstablishedR-EK. Sayler, unpublished
 LibertyEstablished
 MadisonEstablishedN-EK. Sayler, unpublished
 ManateeReported
 MarionEstablished
 MartinEstablished
 Miami-DadeEstablished
 MonroeEstablishedR-EJ. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
 NassauEstablished
 OkaloosaEstablished
 OkeechobeeEstablishedN-EWisely, Cleveland, Satterlee, and Lord, unpublished
 OrangeEstablished
 OsceolaEstablished
 Palm BeachEstablished
 PascoReported
 PinellasEstablishedN-EJ. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
 PolkEstablished
 PutnamEstablishedR-EJ. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
 Santa RosaEstablished
 SarasotaEstablishedR-EJ. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
 SeminoleEstablishedR-EJ. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
 St. JohnsReported
 St. LucieReportedN-RL. Durden, unpublished
 SumterEstablished
 SuwanneeReportedN-RL. Durden, unpublished
 TaylorEstablished
 UnionReported
 VolusiaEstablished
 WakullaEstablished
 WaltonEstablished
 WashingtonReportedN-RJ. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
Georgia
 ApplingReportedN-RL. Durden, unpublished
 AtkinsonReported
 BaconReportedN-RL. Durden, unpublished
 BakerReported
 BaldwinReported
 BibbReported
 BleckleyEstablished
 BrantleyEstablished
 BrooksReported
 BryanReported
 BullochEstablished
 BurkeEstablishedR-EL. Durden, unpublished
 CalhounReported
 CamdenReported
 CandlerEstablishedN-EL. Durden, unpublished
 CharltonEstablished
 ChathamEstablished
 ChattahoocheeReported
 ClarkeReportedN-RL. Durden, unpublished
 ClinchEstablished
 CoffeeReported
 ColumbiaReported
 CookReported
 CrispReported
 DecaturReported
 DoughertyEstablished
 EcholsReported
 EffinghamEstablished
 EmanuelEstablishedN-EL. Durden, unpublished
 EvansEstablishedR-EL. Durden, unpublished
 ForsythReportedN-RL. Durden, unpublished
 GlynnEstablished
 GradyEstablished
 HancockReported
 HoustonEstablished
 IrwinEstablished
 JasperEstablished
 Jeff DavisReported
 JeffersonEstablishedN-EL. Durden, unpublished
 JenkinsEstablishedN-EL. Durden, unpublished
 JohnsonEstablishedN-EL. Durden, unpublished
 JonesReported
 LaurensEstablishedN-EL. Durden, unpublished
 LibertyEstablished
 LongReported
 LowndesEstablished
 McDuffieEstablished
 McIntoshEstablished
 MonroeEstablished
 MontgomeryEstablishedN-EL. Durden, unpublished
 MorganReported
 MuscogeeReported
 PierceReported
 PulaskiReported
 PutnamEstablished
 TalbotEstablished
 TaliaferroReported
 TattnallReportedN-RL. Durden, unpublished
 TelfairEstablished
 TerrellReported
 ThomasEstablished
 TiftReportedN-RL. Durden, unpublished
 ToombsEstablishedN-EL. Durden, unpublished
 TreutlenEstablishedN-EL. Durden, unpublished
 TwiggsReported
 WareEstablished
 WashingtonEstablishedN-EL. Durden, unpublished
 WayneReported
 WilkesReported
 WilkinsonReported
Illinois
 BooneReported
 BrownReported
 BureauEstablishedR-E(Cortinas and Kitron 2006)
 CarrollEstablished
 CassReportedN-R(Cortinas and Kitron 2006)
 ChampaignEstablishedR-Ehttp://www.idph.state.il.us/envhealth/pdf/ Deer_Tick_Range.pdf
 ClarkEstablishedR-Ehttp://www.idph.state.il.us/envhealth/pdf/ Deer_Tick_Range.pdf
 ColesEstablishedR-Ehttp://www.idph.state.il.us/envhealth/pdf /Deer_Tick_Range.pdf
 CookEstablishedR-E(Rydzewski et al. 2012); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
 CrawfordReported
 CumberlandReported
 De WittEstablishedN-Ehttp://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
 DuPageEstablishedR-E(Rydzewski et al. 2012); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
 EdgarReported
 FayetteReported
 FranklinReported
 FultonEstablishedN-E(Cortinas et al. 2002)); (Cortinas and Kitron 2006); http:// www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
 GallatinReported
 GrundyEstablishedR-E(Cortinas and Kitron 2006); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
 HenryEstablishedR-Ehttp://www.idph.state.il.us/envhealth/pdf/ Deer_Tick_Range.pdf
 IroquoisReported
 JacksonReported
 Jo DaviessEstablishedR-Ehttp://www.idph.state.il.us/envhealth/pdf/ Deer_Tick_Range.pdf
 KankakeeEstablishedR-E(Cortinas and Kitron 2006); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
 KnoxReported
 LakeEstablishedN-E(Rydzewski et al. 2012); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
 LaSalleEstablishedR-E(Cortinas and Kitron 2006); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
 LawrenceReported
 LeeEstablishedR-Ehttp://www.idph.state.il.us/envhealth/pdf/ Deer_Tick_Range.pdf
 MacoupinEstablishedN-Ehttp://www.idph.state.il.us/envhealth/pdf/ Deer_Tick_Range.pdf
 MadisonReported
 MarshallEstablishedR-E(Cortinas and Kitron 2006); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
 MasonEstablishedN-Ehttp://www.idph.state.il.us/envhealth/pdf/ Deer_Tick_Range.pdf
 McDonoughEstablishedN-Ehttp://www.idph.state.il.us/envhealth/pdf/ Deer_Tick_Range.pdf
 McHenryReported
 McLeanEstablishedR-Ehttp://www.idph.state.il.us/envhealth/pdf/ Deer_Tick_Range.pdf
 MenardReported
 MercerReported
 MonroeEstablished
 MontgomeryReportedN-Rhttp://www.idph.state.il.us/envhealth/pdf/ Deer_Tick_Range.pdf
 MorganReportedN-R(Cortinas and Kitron 2006)
 OgleEstablished
 PeoriaEstablishedR-E(Cortinas and Kitron 2006); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
 PerryReported
 PiattEstablishedR-E(Rydzewski et al. 2011); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
 PopeReported
 PutnamEstablishedR-E(Cortinas and Kitron 2006); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
 RandolphReported
 Rock IslandEstablished
 SalineReported
 SangamonReported
 SchuylerEstablishedR-E(Cortinas and Kitron 2006)
 ScottReported
 ShelbyEstablishedN-Ehttp://www.idph.state.il.us/envhealth/pdf/ Deer_Tick_Range.pdf
 St. ClairReported
 StephensonReported
 TazewellEstablishedR-E(Cortinas et al. 2002, Cortinas and Kitron 2006); http:// www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
 UnionReported
 VermilionEstablishedN-Ehttp://www.idph.state.il.us/envhealth/pdf/ Deer_Tick_Range.pdf
 WabashEstablishedN-Ehttp://www.idph.state.il.us/envhealth/pdf/ Deer_Tick_Range.pdf
 WillEstablishedR-E(Cortinas and Kitron 2006); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
 WilliamsonReported
 WinnebagoEstablishedR-Ehttp://www.idph.state.il.us/envhealth/pdf/ Deer_Tick_Range.pdf
 WoodfordEstablishedN-E(Cortinas and Kitron 2006); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Indiana
 AdamsReportedN-RR. Pinger, unpublished
 BartholomewEstablishedR-ER. Pinger, unpublished
 BentonReportedN-RR. Pinger, unpublished
 BooneReportedN-RR. Pinger, unpublished
 BrownReported
 CarrollReportedN-RR. Pinger, unpublished
 CassEstablishedR-E(Raizman et al. 2012); E. Raizman, unpublished
 ClarkReportedN-RR. Pinger, unpublished
 ClayEstablishedR-E(Raizman et al. 2012); E. Raizman, unpublished
 DaviessReportedN-RR. Pinger, unpublished
 DearbornReportedN-RR. Pinger, unpublished
 DuboisEstablishedR-E(Raizman et al. 2012); E. Raizman, unpublished
 ElkhartReportedN-RR. Pinger, unpublished
 FountainReported
 FranklinEstablishedR-E(Raizman et al. 2012); E. Raizman, unpublished
 FultonEstablishedR-E(Raizman et al. 2012); E. Raizman, unpublished
 GrantReported
 GreeneReportedN-RR. Pinger, unpublished
 HamiltonReportedN-R(Raizman et al. 2012); E. Raizman, unpublished
 HendricksReportedN-RR. Pinger, unpublished
 HowardReportedN-RR. Pinger, unpublished
 HuntingtonReported
 JasperEstablished
 JeffersonEstablishedN-E(Raizman et al. 2012); E. Raizman, unpublished
 JenningsEstablishedN-E(Raizman et al. 2012); E. Raizman, unpublished
 JohnsonReportedN-RR. Pinger, unpublished
 KnoxReportedN-RR. Pinger, unpublished
 KosciuskoEstablishedN-E(Raizman et al. 2012); E. Raizman, unpublished
 LaGrangeReported
 LakeEstablishedR-E(Raizman et al. 2012); E. Raizman, unpublished
 LaPorteEstablished
 LawrenceReportedN-R
 MadisonReported
 MarshallReported
 MartinEstablishedN-E(Raizman et al. 2012); E. Raizman, unpublished
 MonroeEstablishedN-E(Raizman et al. 2012); E. Raizman, unpublished
 MontgomeryEstablishedN-E(Raizman et al. 2012); E. Raizman, unpublished d
 MorganEstablishedR-E(Raizman et al. 2012); E. Raizman, unpublished
 NewtonEstablished
 OrangeReported
 OwenEstablishedN-E(Raizman et al. 2012); E. Raizman, unpublished
 ParkeEstablishedR-E(Raizman et al. 2012); E. Raizman, unpublished
 PikeReported
 PorterEstablished
 PoseyReported
 PulaskiEstablished
 PutnamEstablishedN-E(Raizman et al. 2012); E. Raizman, unpublished
 RandolphReportedN-RR. Pinger, unpublished
 RipleyReportedN-RR. Pinger, unpublished
 ShelbyReportedN-R(Raizman et al. 2012); E. Raizman, unpublished
 St. JosephEstablishedR-E(Raizman et al. 2012); E. Raizman, unpublished
 StarkeEstablished
 SteubenReportedN-RR. Pinger, unpublished
 SullivanEstablishedN-E(Raizman et al. 2012); E. Raizman, unpublished
 SwitzerlandReportedN-RR. Pinger, unpublished
 TippecanoeEstablishedR-E(Raizman et al. 2012); E. Raizman, unpublished
 UnionReportedN-R(Raizman et al. 2012); E. Raizman, unpublished
 VanderburghReportedN-RR. Pinger, unpublished
 VermillionReported
 VigoEstablished
 WabashReported
 WarrenEstablishedN-E(Raizman et al. 2012); E. Raizman, unpublished
 WarrickReported
 WashingtonReported
 WhiteEstablished
 WhitleyReportedN-R(Raizman et al. 2012); E. Raizman, unpublished
Iowa
 BentonReportedN-R(Lingren et al. 2005)
 BremerEstablished
 Buena VistaReported
 CalhounReportedN-R(Lingren et al. 2005)
 CedarReportedN-R(Lingren et al. 2005)
 Cerro GordoEstablished
 ClaytonReported
 ClintonReportedN-R(Lingren et al. 2005)
 CrawfordReportedN-R(Lingren et al. 2005)
 DallasReported
 DelawareReported
 Des MoinesEstablished
 DubuqueReported
 FayetteReported
 FloydReported
 GreeneReportedN-R(Lingren et al. 2005)
 GuthrieReportedN-R(Lingren et al. 2005)
 HancockReportedN-R(Lingren et al. 2005)
 IowaReported
 JacksonEstablished
 JasperReportedN-R(Lingren et al. 2005)
 JohnsonEstablishedR-E(Lingren et al. 2005)
 KeokukEstablishedN-E(Lingren et al. 2005)
 KossuthReportedN-R(Lingren et al. 2005)
 LinnEstablished
 LouisaReported
 MarshallReported
 MuscatineEstablished
 Palo AltoReportedN-R(Lingren et al. 2005)
 PolkEstablishedR-E(Lingren et al. 2005)
 ScottEstablishedR-E(Lingren et al. 2005)
 StoryEstablishedR-E(Lingren et al. 2005)
 TamaReportedN-R(Lingren et al. 2005)
 WashingtonReportedN-R(Lingren et al. 2005)
 WebsterEstablishedR-E(Lingren et al. 2005)
 WinnebagoReported
 WinneshiekEstablishedR-E(Lingren et al. 2005)
 WoodburyReportedN-R(Lingren et al. 2005)
Kansas
 BourbonReported
 ChautauquaReported
 CherokeeReported
 CoffeyReported
 CowleyReported
 CrawfordReported
 DouglasEstablished
 JeffersonReported
 JohnsonReported
 LabetteReported
 LinnReported
 MiamiReported
 MontgomeryReported
 RileyReported
 ShawneeReported
Kentucky
 BallardReported
 BooneEstablishedN-EL. Townsend, unpublished
 CarrollEstablishedN-EL. Townsend, unpublished
 ChristianReported
 ClayEstablishedN-EL. Townsend, unpublished
 EstillEstablishedN-EL. Townsend, unpublished
 FayetteEstablishedN-EL. Townsend, unpublished
 HancockReportedN-RL. Townsend, unpublished
 HardinEstablishedN-EL. Townsend, unpublished
 JacksonEstablishedN-EL. Townsend, unpublished
 KnoxEstablishedN-EL. Townsend, unpublished
 LeeEstablishedN-EL. Townsend, unpublished
 McCrearyEstablishedN-EL. Townsend, unpublished
 MeadeEstablishedN-EB. Pagac and M. Miller, unpublished
 OwsleyEstablishedN-EL. Townsend, unpublished
 PulaskiEstablishedN-EL. Townsend, unpublished
 TrimbleEstablishedN-EL. Townsend, unpublished
 WarrenReportedN-R(Onwu 2012)
Louisiana
 AllenEstablished
 AvoyellesEstablished
 BienvilleEstablished
 BossierReported
 CaldwellEstablished
 CatahoulaReportedN-R(Mackay and Foil 2005)
 ClaiborneReported
 ConcordiaEstablished
 De SotoReportedN-R(Mackay and Foil 2005)
 East Baton RougeReported
 East CarrollReportedN-R(Mackay and Foil 2005)
 GrantEstablished
 JacksonEstablished
 Jefferson DavisReportedN-R(Mackay and Foil 2005)
 LafayetteReportedN-R(Mackay and Foil 2005)
 LincolnEstablished
 MadisonReported
 MorehouseEstablished
 NatchitochesEstablished
 OuachitaReported
 Pointe CoupeeReportedN-R(Mackay and Foil 2005)
 RapidesReported
 SabineReported
 St. John the BaptistReportedN-R(Mackay and Foil 2005)
 St. LandryReported
 St. MartinReportedN-R(Mackay and Foil 2005)
 St. MaryReportedN-R(Mackay and Foil 2005)
 St. TammanyReported
 TangipahoaReported
 TensasReported
 UnionEstablished
 VermilionReportedN-R(Mackay and Foil 2005)
 VernonEstablished
 WashingtonReportedN-R(Mackay and Foil 2005)
 WinnReported
Maine
 AndroscogginEstablished
 AroostookEstablishedR-Ehttp://www.maine.gov/dhhs/mecdc/infectious-disease/epi/vector-borne/lyme/documents/2014-lyme-legislature.pdf
 CumberlandEstablished
 FranklinEstablishedR-Ehttp://www.maine.gov/dhhs/mecdc/infectious-disease/epi/vector-borne/lyme/documents/2014-lyme-legislature.pdf
 HancockEstablished
 KennebecEstablished
 KnoxEstablished
 LincolnEstablished
 OxfordEstablished
 PenobscotEstablished
 PiscataquisEstablishedR-Ehttp://www.maine.gov/dhhs/mecdc/infectious-disease/epi/vector-borne/lyme/documents/2014-lyme-legislature.pdf
 SagadahocEstablished
 SomersetEstablished
 WaldoEstablished
 WashingtonEstablished
 YorkEstablished
Maryland
 AlleganyReported
 Anne ArundelEstablished
 Baltimore City
 BaltimoreEstablished
 CalvertEstablished
 CarolineEstablished
 CarrollEstablished
 CecilEstablished
 CharlesEstablished
 DorchesterEstablished
 FrederickEstablished
 GarrettReported
 HarfordEstablished
 HowardEstablished
 KentEstablished
 MontgomeryEstablished
 Prince George'sEstablished
 Queen Anne'sEstablished
 SomersetEstablished
 St. Mary'sEstablished
 TalbotEstablished
 WashingtonEstablished
 WicomicoEstablished
 WorcesterEstablished
Massachusetts
 BarnstableEstablished
 BerkshireEstablishedR-Ehttp://stats.tickdiseases.org/
 BristolEstablished
 DukesEstablished
 EssexEstablished
 FranklinEstablished
 HampdenEstablished
 HampshireEstablished
 MiddlesexEstablishedR-Ehttp://stats.tickdiseases.org/
 NantucketEstablished
 NorfolkEstablishedN-Ehttp://stats.tickdiseases.org/
 PlymouthEstablished
 SuffolkEstablishedN-Ehttp://stats.tickdiseases.org/
 WorcesterEstablishedR-Ehttp://stats.tickdiseases.org/
Michigan
 AlgerReportedN-R(Schaar 2012)
 AlleganEstablishedR-E(Foster 2004, Hamer et al. 2010)
 BaragaReported
 BarryEstablishedN-EJ. Tsao, S. Hamer, I. Arsnoe, and G. Hickling, unpublished
 BenzieEstablishedN-E(Hamer et al. 2010); J. Sidge unpublished
 BerrienEstablished
 CassEstablishedN-E(Hamer et al. 2009); E. Foster, unpublished
 CharlevoixEstablishedN-EE. Foster and J. Sidge, unpublished
 ChippewaEstablished
 ClintonReported
 DeltaEstablished
 DickinsonEstablishedR-EI. Arsnoe, unpublished
 EmmetReportedN-RJ. Sidge, unpublished
 GeneseeReported
 GogebicEstablished
 HillsdaleReportedN-RE. Foster, and J. Tsao, unpublished
 HoughtonReported
 InghamEstablishedR-EE. Foster, and J. Tsao, unpublished
 IoniaEstablishedN-E(Hamer et al. 2010)
 IronReportedN-R(Walker et al. 1998)
 JacksonReported
 KalamazooEstablishedN-EJ. Tsao and G. Hickling, unpublished
 KentReported
 LapeerReported
 LeelanauEstablishedR-EE. Foster, J. Tsao and J. Sidge, unpublished
 LivingstonReported
 MackinacReported
 ManisteeEstablishedR-E(Hamer et al. 2010)
 MarquetteReported
 MasonEstablishedN-EJ. Tsao and J. Sidge, unpublished
 MenomineeEstablished
 MidlandReported
 MuskegonEstablishedR-E(Hamer et al. 2009, Hamer et al. 2010)
 OaklandReported
 OceanaEstablishedR-EJ. Tsao, and J. Sidge, unpublished
 OntonagonEstablishedR-EE. Foster, unpublished
 OttawaEstablishedR-E(Hamer et al. 2009); E. Foster, unpublished
 SchoolcraftEstablishedR-EI. Arsnoe, unpublished
 St. JosephEstablishedN-EJ. Tsao and E. Foster, unpublished
 Van BurenEstablishedN-E(Foster 2004, Hamer et al. 2009, Hamer et al. 2014)
Minnesota
 AitkinEstablishedR-ED. Neitzel, unpublished
 AnokaEstablished
 BeckerEstablishedN-E(Sanders and Guilfoile 2000), D. Neitzel, unpublished
 BeltramiEstablishedR-E(Sanders and Guilfoile 2000), D. Neitzel, unpublished
 BentonEstablishedN-ET. Johnson and D. Neitzel, unpublished
 BrownReportedN-RD. Neitzel, unpublished
 CarltonEstablishedR-E(Sanders and Guilfoile 2000), D. Neitzel, unpublished
 CarverEstablished
 CassEstablishedN-E(Sanders and Guilfoile 2000)
 ChisagoEstablished
 ClearwaterEstablishedN-E(Sanders and Guilfoile 2000), D. Neitzel, unpublished
 CookEstablishedN-ED. Neitzel, unpublished
 Crow WingEstablishedR-E
 DakotaEstablished
 DouglasEstablishedR-ED. Neitzel, unpublished
 FillmoreEstablishedN-ED. Neitzel, unpublished
 GoodhueEstablishedN-ED. Neitzel, unpublished
 HennepinEstablishedN-ED. Neitzel, unpublished
 HoustonEstablishedR-ED. Neitzel, unpublished
 HubbardEstablishedN-E(Sanders and Guilfoile 2000), D. Neitzel, unpublished
 IsantiEstablishedR-ED. Neitzel, unpublished
 ItascaEstablishedN-E(Sanders and Guilfoile 2000)
 KanabecEstablishedR-ED. Neitzel, unpublished
 KandiyohiEstablishedN-E(Diuk-Wasser et al. 2006); D. Neitzel, unpublished
 KoochichingEstablishedN-E(Sanders and Guilfoile 2000)
 LakeEstablishedN-ED. Neitzel, unpublished
 Lake of the WoodsEstablishedN-E(Sanders and Guilfoile 2000), D. Neitzel, unpublished
 MahnomenEstablishedN-ED. Neitzel, unpublished
 Mille LacsEstablishedR-E(Sanders and Guilfoile 2000)
 MorrisonEstablished
 NicolletReportedN-RT. Johnson and D. Neitzel, unpublished
 OlmstedEstablishedR-ED. Neitzel, unpublished
 Otter TailEstablishedN-ED. Neitzel, unpublished
 PineEstablished
 PopeEstablishedN-ED. Neitzel, unpublished
 RamseyEstablished
 RiceReportedN-RT. Johnson and D. Neitzel, unpublished
 ScottEstablished
 SherburneEstablishedN-E(Diuk-Wasser et al. 2006), D. Neitzel, unpublished
 SibleyEstablishedN-ET. Johnson and D. Neitzel, unpublished
 St. LouisEstablishedN-E(Sanders and Guilfoile 2000), D. Neitzel, unpublished
 StearnsEstablishedN-ED. Neitzel, unpublished
 ToddEstablishedR-ED. Neitzel, unpublished
 WabashaEstablishedN-ED. Neitzel, unpublished
 WadenaEstablishedN-E(Sanders and Guilfoile 2000), D. Neitzel, unpublished
 WashingtonEstablished
 WinonaEstablishedR-ED. Neitzel, unpublished
 WrightEstablishedN-ED. Neitzel, unpublished
Mississippi
 AdamsReported
 AlcornReported
 AmiteReported
 AttalaReported
 BentonReported
 BolivarEstablished
 CalhounReported
 CarrollReported
 ChickasawReported
 ChoctawReported
 ClaiborneReported
 ClarkeReported
 ClayReported
 CoahomaReported
 CopiahEstablished
 CovingtonReported
 DeSotoReported
 ForrestReported
 FranklinReported
 GeorgeReported
 GreeneReported
 GrenadaReported
 HancockReported
 HarrisonReported
 HindsEstablished
 HolmesReported
 HumphreysReported
 IssaquenaReported
 ItawambaReported
 JacksonEstablished
 JasperReported
 JeffersonReported
 Jefferson DavisReported
 JonesReported
 KemperReported
 LafayetteReported
 LamarReported
 LauderdaleReported
 LawrenceReported
 LeakeReported
 LeeReported
 LefloreReported
 LincolnReported
 LowndesReported
 MadisonReported
 MarionReported
 MarshallEstablishedR-E(Goltz and Goddard 2013, Goltz et al. 2013)
 MonroeReported
 MontgomeryReported
 NeshobaReported
 NewtonReported
 NoxubeeEstablished
 OktibbehaEstablished
 PanolaReported
 Pearl RiverReported
 PerryEstablished
 PikeReported
 PontotocReported
 PrentissReported
 QuitmanReported
 RankinEstablished
 ScottEstablished
 SharkeyReported
 SimpsonReported
 SmithReported
 StoneReported
 SunflowerReported
 TallahatchieReported
 TateReported
 TippahReported
 TishomingoReported
 TunicaReported
 UnionReported
 WalthallReported
 WarrenReported
 WashingtonReported
 WayneReported
 WebsterReported
 WilkinsonReported
 WinstonEstablished
 YalobushaReported
 YazooReported
Missouri
 AdairEstablishedN-ES. Fore and H.-J. Kim, unpublished
 BentonEstablished
 BollingerEstablished
 BooneReported
 CallawayEstablished
 Cape GirardeauEstablishedR-E(Kollars et al. 1997, Kollars et al. 1999)
 ClarkReported
 DallasEstablished
 DentEstablished
 DouglasEstablished
 GasconadeEstablished
 GreeneEstablished
 HowellEstablished
 JasperEstablished
 JohnsonEstablished
 LacledeEstablished
 MontgomeryReported
 New MadridReported
 NewtonReportedN-Rhttp://vectormap.nhm.ku.edu/vectormap/
 OsageEstablished
 PulaskiEstablished
 ScottReported
 St. CharlesEstablished
 St. ClairReported
 St. LouisReported
 StoddardEstablished
 StoneEstablished
 TaneyEstablished
 TexasEstablished
 WayneEstablished
 WebsterEstablished
Nebraska
 CassReportedN-R(Cortinas and Spomer 2014)
 LancasterReportedN-R(Cortinas and Spomer 2014)
 PawneeReportedN-R(Cortinas and Spomer 2014)
New Hampshire
 BelknapEstablishedR-Ehttp://www.dhhs.nh.gov/dphs/cdcs/lyme/documents/tick-borne-bulletin-2014.pdf
 CarrollEstablishedR-Ehttp://www.dhhs.nh.gov/dphs/cdcs/lyme/documents/tick-borne-bulletin-2014.pdf
 CheshireEstablishedR-Ehttp://www.dhhs.nh.gov/dphs/cdcs/lyme/documents/tick-borne-bulletin-2014.pdf
 CoosReported
 GraftonEstablished
 HillsboroughEstablished
 MerrimackEstablished
 RockinghamEstablished
 StraffordEstablished
 SullivanEstablishedR-Ehttp://www.dhhs.nh.gov/dphs/cdcs/lyme/documents/tick-borne-bulletin-2014.pdf
New Jersey
 AtlanticEstablished
 BergenEstablished
 BurlingtonEstablished
 CamdenEstablished
 Cape MayEstablished
 CumberlandEstablished
 EssexEstablished
 GloucesterEstablished
 HudsonEstablished
 HunterdonEstablished
 MercerEstablished
 MiddlesexEstablished
 MonmouthEstablished
 MorrisEstablished
 OceanEstablished
 PassaicEstablished
 SalemEstablished
 SomersetEstablished
 SussexEstablished
 UnionEstablished
 WarrenEstablished
New York
 AlbanyEstablished
 AlleganyEstablishedR-EM. Prusinski/New York Department of Health, unpublished
 BronxEstablished
 BroomeEstablished
 CattaraugusEstablished
 CayugaEstablishedN-EM. Prusinski/New York Department of Health, unpublished
 ChautauquaEstablishedR-EM. Prusinski/New York Department of Health, unpublished
 ChemungEstablishedN-EM. Prusinski/New York Department of Health, unpublished
 ChenangoEstablishedR-EM. Prusinski/New York Department of Health, unpublished
 ClintonEstablished
 ColumbiaEstablished
 CortlandEstablishedN-EM. Prusinski/New York Department of Health, unpublished
 DelawareEstablished
 DutchessEstablished
 ErieEstablished
 EssexEstablished
 FranklinEstablishedR-EM. Prusinski/New York Department of Health, unpublished
 FultonEstablished
 GeneseeEstablishedR-EM. Prusinski/New York Department of Health, unpublished
 GreeneEstablished
 HamiltonEstablishedN-EM. Prusinski/New York Department of Health, unpublished
 HerkimerEstablishedR-EM. Prusinski/New York Department of Health, unpublished
 JeffersonEstablished
 KingsEstablished
 LewisEstablishedR-EM. Prusinski/New York Department of Health, unpublished
 LivingstonEstablishedR-EM. Prusinski/New York Department of Health, unpublished
 MadisonEstablishedR-EM. Prusinski/New York Department of Health, unpublished
 MonroeEstablishedR-EM. Prusinski/New York Department of Health, unpublished
 MontgomeryEstablished
 NassauEstablished
 New YorkEstablishedN-EM. Prusinski/New York Department of Health, unpublished
 NiagaraEstablishedN-EM. Prusinski/New York Department of Health, unpublished
 OneidaEstablished
 OnondagaEstablishedR-EM. Prusinski/New York Department of Health, unpublished
 OntarioEstablishedR-EM. Prusinski/New York Department of Health, unpublished
 OrangeEstablished
 OrleansReportedN-RM. Prusinski/New York Department of Health, unpublished
 OswegoEstablishedR-EM. Prusinski/New York Department of Health, unpublished
 OtsegoEstablished
 PutnamEstablished
 QueensEstablishedR-EM. Prusinski/New York Department of Health, unpublished
 RensselaerEstablished
 RichmondEstablishedR-EM. Prusinski/New York Department of Health, unpublished
 RocklandEstablished
 SaratogaEstablished
 SchenectadyEstablished
 SchoharieEstablished
 SchuylerEstablishedN-EM. Prusinski/New York Department of Health, unpublished
 SenecaEstablishedN-EM. Prusinski/New York Department of Health, unpublished
 St. LawrenceEstablishedR-EM. Prusinski/New York Department of Health, unpublished
 SteubenEstablishedN-EM. Prusinski/New York Department of Health, unpublished
 SuffolkEstablished
 SullivanEstablished
 TiogaEstablishedR-EM. Prusinski/New York Department of Health, unpublished
 TompkinsEstablishedR-EM. Prusinski/New York Department of Health, unpublished
 UlsterEstablished
 WarrenEstablished
 WashingtonEstablished
 WayneEstablishedN-EM. Prusinski/New York Department of Health, unpublished
 WestchesterEstablished
 WyomingEstablishedR-EM. Prusinski/New York Department of Health, unpublished
 YatesEstablishedR-EM. Prusinski/New York Department of Health, unpublished
North Carolina
 AlamanceEstablishedN-EB. Harrison/NCPHPMc, unpublished
 AnsonReported
 BeaufortEstablished
 BertieEstablishedN-EB. Harrison/NCPHPMc, unpublished
 BladenEstablished
 BrunswickEstablished
 CamdenEstablishedR-EB. Harrison/NCPHPMc, unpublished
 CarteretEstablishedR-EB. Harrison/NCPHPMc, unpublished
 CatawbaReported
 ChathamEstablishedR-E(Smith et al. 2010)
 ChowanEstablishedR-EB. Harrison/NCPHPMc, unpublished
 ColumbusEstablishedR-EB. Harrison/NCPHPMc, unpublished
 CravenEstablishedR-EB. Harrison/NCPHPMc, unpublished
 CumberlandEstablished
 CurrituckReported
 DareEstablished
 DuplinEstablishedN-EB. Harrison/NCPHPMc, unpublished
 DurhamEstablishedN-EB. Harrison/NCPHPMc, unpublished
 EdgecombeEstablishedN-EB. Harrison/NCPHPMc, unpublished
 ForsythReportedN-RB. Harrison/NCPHPMc, unpublished
 GatesEstablishedR-EB. Harrison/NCPHPMc, unpublished
 GranvilleReportedN-RB. Harrison/NCPHPMc, unpublished; http://vectormap.nhm. ku.edu/vectormap/
 GreeneEstablishedN-EB. Harrison/NCPHPMc, unpublished
 GuilfordReportedN-RB. Harrison/NCPHPMc, unpublished
 HalifaxEstablishedN-EB. Harrison/NCPHPMc, unpublished
 HarnettEstablishedN-EB. Harrison/NCPHPMc, unpublished
 HaywoodReportedN-RB. Harrison/NCPHPMc, unpublished
 HertfordEstablishedR-EB. Harrison/NCPHPMc, unpublished
 HokeEstablished
 HydeEstablishedR-EB. Harrison/NCPHPMc, unpublished
 JohnstonEstablishedN-EB. Harrison/NCPHPMc, unpublished
 JonesEstablishedR-EB. Harrison/NCPHPMc, unpublished
 LenoirReported
 MartinEstablishedR-EB. Harrison/NCPHPMc, unpublished
 MecklenburgReportedN-RB. Harrison/NCPHPMc, unpublished
 MooreReported
 NashEstablishedN-EB. Harrison/NCPHPMc, unpublished
 New HanoverEstablishedR-EB. Harrison/NCPHPMc, unpublished
 OnslowEstablished
 OrangeEstablishedN-EB. Harrison/NCPHPMc, unpublished
 PamlicoEstablishedR-EB. Harrison/NCPHPMc, unpublished
 PasquotankReportedN-RB. Harrison/NCPHPMc, unpublished
 PenderEstablishedR-EB. Harrison/NCPHPMc, unpublished
 PerquimansEstablishedN-EB. Harrison/NCPHPMc, unpublished
 PittReportedN-RB. Harrison/NCPHPMc, unpublished
 RandolphReportedN-Rhttp://vectormap.nhm.ku.edu/vectormap/
 RobesonEstablishedN-EB. Harrison/NCPHPMc, unpublished
 RowanEstablishedN-EB. Harrison/NCPHPMc, unpublished
 RutherfordReported
 SampsonEstablishedR-EB. Harrison/NCPHPMc, unpublished
 ScotlandEstablishedN-EB. Harrison/NCPHPMc, unpublished
 StokesEstablishedN-E(Sakamoto et al. 2014); B. Harrison/NCPHPMc, unpublished
 SurryReportedN-RB. Harrison/NCPHPMc, unpublished
 TyrrellEstablishedN-EB. Harrison/NCPHPMc, unpublished
 VanceReportedN-RB. Harrison/NCPHPMc, unpublished
 WakeEstablishedR-EB. Harrison/NCPHPMc, unpublished
 WarrenEstablishedN-EB. Harrison/NCPHPMc, unpublished
 WashingtonEstablishedR-EB. Harrison/NCPHPMc, unpublished
 WayneEstablishedN-EH. Gaff, unpublished; B. Harrison/NCPHPMc, unpublished
North Dakota
 BottineauEstablishedN-EM. Feist and J. Vaughan, unpublished
 EddyEstablishedN-E(Russart et al. 2014)
 Grand ForksEstablishedN-E(Russart et al. 2014, Stone et al. 2015)
 PembinaEstablishedN-E(Russart et al. 2014)
 RamseyEstablishedN-E(Russart et al. 2014)
 RansomReportedN-RM. Feist and J. Vaughan, unpublished
 RoletteReportedN-R(Russart et al. 2014)
 SteeleReportedN-R(Russart et al. 2014)
Ohio
 AdamsEstablishedN-ER. Gary/Ohio Department of Health, unpublished
 AshlandReportedN-Rhttp://www.odh.ohio.gov/lyme
 AshtabulaEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 AthensReportedN-R(Wang et al. 2014), http://www.odh.ohio.gov/lyme
 AuglaizeReportedN-Rhttp://www.odh.ohio.gov/lyme
 BelmontEstablishedN-ER. Gary/Ohio Department of Health, unpublished
 BrownReportedN-Rhttp://www.odh.ohio.gov/lyme
 ButlerReported
 CarrollEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 ClermontReported
 ColumbianaEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 CoshoctonEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 CuyahogaEstablishedN-ER. Gary/Ohio Department of Health, unpublished
 DelawareReportedN-Rhttp://www.odh.ohio.gov/lyme
 ErieReportedN-Rhttp://www.odh.ohio.gov/lyme
 FairfieldReportedN-Rhttp://www.odh.ohio.gov/lyme
 FayetteReportedN-Rhttp://www.odh.ohio.gov/lyme
 FranklinReportedN-RR. Gary, unpublished; http://vectormap.nhm.ku.edu/vectormap/
 GalliaEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 GeaugaReportedN-Rhttp://www.odh.ohio.gov/lyme
 GreeneReportedN-Rhttp://www.odh.ohio.gov/lyme
 GuernseyEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 HamiltonEstablishedR-ER. Gary/Ohio Department of Health, unpublished
 HarrisonEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 HighlandReportedN-R(Wang et al. 2014), http://www.odh.ohio.gov/lyme
 HockingEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 HolmesEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 JacksonEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 JeffersonEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 KnoxEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 LakeEstablishedN-ER. Gary/Ohio Department of Health, unpublished
 LawrenceEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 LickingEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 LucasReportedN-Rhttp://www.odh.ohio.gov/lyme
 MadisonReportedN-Rhttp://www.odh.ohio.gov/lyme
 MahoningReportedN-Rhttp://www.odh.ohio.gov/lyme
 MedinaEstablishedN-ER. Gary/Ohio Department of Health, unpublished
 MeigsReportedN-Rhttp://www.odh.ohio.gov/lyme
 MonroeEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 MontgomeryReportedN-Rhttp://www.odh.ohio.gov/lyme
 MorganEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 MorrowReportedN-Rhttp://www.odh.ohio.gov/lyme
 MuskingumEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 NobleEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 OttawaReportedN-Rhttp://www.odh.ohio.gov/lyme
 PauldingReportedN-R
 PerryEstablishedN-ER. Gary/Ohio Department of Health, unpublished
 PickawayReportedN-Rhttp://www.odh.ohio.gov/lyme
 PikeEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 PortageEstablishedN-ER. Gary/Ohio Department of Health, unpublished
 RichlandEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 RossEstablishedN-E(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
 SciotoReportedN-Rhttp://www.odh.ohio.gov/lyme
 StarkEstablishedN-ER. Gary/Ohio Department of Health, unpublished
 SummitEstablishedN-ER. Gary/Ohio Department of Health, unpublished
 TrumbullEstablishedN-ER. Gary/Ohio Department of Health, unpublished
 TuscarawasReportedN-R(Wang et al. 2014), http://www.odh.ohio.gov/lyme
 UnionReportedN-Rhttp://www.odh.ohio.gov/lyme
 VintonReportedN-R(Wang et al. 2014), http://www.odh.ohio.gov/lyme
 WarrenReportedN-Rhttp://www.odh.ohio.gov/lyme
 WashingtonReportedN-Rhttp://www.odh.ohio.gov/lyme
 WayneEstablishedR-ER. Gary/Ohio Department of Health, unpublished
 WilliamsReported
 WoodReportedN-Rhttp://www.odh.ohio.gov/lyme
Oklahoma
 AdairReported
 AtokaReported
 CaddoReported
 CarterReported
 CherokeeEstablished
 CoalReported
 ComancheReported
 CreekReported
 DelawareReported
 GarfieldReported
 GarvinReported
 HaskellReported
 HughesReported
 JohnstonReported
 LatimerReported
 Le FloreEstablished
 LincolnReported
 LoveReported
 MayesReported
 McClainEstablished
 McCurtainReported
 McIntoshReported
 MurrayReported
 MuskogeeReported
 OklahomaReported
 OkmulgeeReported
 OttawaReported
 PawneeReported
 PayneReported
 PittsburgReported
 PontotocReported
 PottawatomieReported
 PushmatahaReported
 RogersReported
 SeminoleReported
 SequoyahReported
 TulsaReported
 WagonerReported
 WashingtonReported
Pennsylvania
 AdamsEstablishedR-E(Han et al. 2014, Hutchinson et al. 2015)
 AlleghenyEstablishedR-E(Hutchinson et al. 2015)
 ArmstrongEstablishedR-E(Hutchinson et al. 2015)
 BeaverEstablishedN-E(Hutchinson et al. 2015)
 BedfordEstablishedN-E(Hutchinson et al. 2015)
 BerksEstablished
 BlairEstablished
 BradfordEstablishedN-E(Hutchinson et al. 2015)
 BucksEstablished
 ButlerEstablished
 CambriaEstablishedR-E(Hutchinson et al. 2015)
 CameronEstablished
 CarbonEstablishedN-E(Hutchinson et al. 2015)
 CentreEstablished
 ChesterEstablished
 ClarionEstablished
 ClearfieldEstablished
 ClintonEstablishedR-E(Hutchinson et al. 2015)
 ColumbiaEstablishedR-E(Hutchinson et al. 2015)
 CrawfordEstablishedR-E(Hutchinson et al. 2015)
 CumberlandEstablishedR-E(Hutchinson et al. 2015)
 DauphinEstablishedR-E(Hutchinson et al. 2015)
 DelawareEstablished
 ElkEstablished
 ErieEstablished
 FayetteEstablishedN-E(Hutchinson et al. 2015)
 ForestEstablishedN-E(Hutchinson et al. 2015)
 FranklinEstablishedN-E(Hutchinson et al. 2015)
 FultonEstablishedR-E(Hutchinson et al. 2015)
 GreeneEstablishedN-E(Hutchinson et al. 2015)
 HuntingdonEstablishedR-E(Hutchinson et al. 2015)
 IndianaEstablishedN-E(Hutchinson et al. 2015)
 JeffersonEstablishedR-E(Hutchinson et al. 2015)
 JuniataEstablishedN-E(Hutchinson et al. 2015)
 LackawannaEstablishedR-E(Hutchinson et al. 2015)
 LancasterEstablished
 LawrenceEstablishedR-E(Hutchinson et al. 2015)
 LebanonEstablishedR-E(Hutchinson et al. 2015)
 LehighEstablished
 LuzerneEstablishedR-E(Hutchinson et al. 2015)
 LycomingEstablishedR-E(Hutchinson et al. 2015)
 McKeanEstablished
 MercerEstablished
 MifflinEstablishedR-E(Hutchinson et al. 2015)
 MonroeEstablished
 MontgomeryEstablished
 MontourEstablishedN-E(Hutchinson et al. 2015)
 NorthamptonEstablished
 NorthumberlandEstablishedR-E(Hutchinson et al. 2015)
 PerryEstablishedR-E(Hutchinson et al. 2015)
 PhiladelphiaEstablished
 PikeEstablished
 PotterEstablishedR-E(Hutchinson et al. 2015)
 SchuylkillEstablishedR-E(Hutchinson et al. 2015)
 SnyderEstablishedN-E(Hutchinson et al. 2015)
 SomersetEstablishedN-E(Hutchinson et al. 2015)
 SullivanEstablishedN-E(Hutchinson et al. 2015)
 SusquehannaEstablishedN-E(Hutchinson et al. 2015)
 TiogaEstablishedN-E(Hutchinson et al. 2015)
 UnionEstablishedR-E(Hutchinson et al. 2015)
 VenangoEstablishedN-E(Hutchinson et al. 2015)
 WarrenEstablishedR-E(Hutchinson et al. 2015)
 WashingtonEstablishedR-E(Hutchinson et al. 2015)
 WayneEstablished
 WestmorelandEstablishedR-E(Hutchinson et al. 2015)
 WyomingEstablishedN-E(Hutchinson et al. 2015)
 YorkEstablished
Rhode Island
 BristolEstablished
 KentEstablished
 NewportEstablished
 ProvidenceEstablished
 WashingtonEstablished
South Carolina
 AbbevilleReported
 AikenEstablished
 AllendaleReported
 AndersonReported
 BarnwellEstablished
 BeaufortEstablished
 BerkeleyEstablished
 CalhounReportedN-R(Williams et al. 1999)
 CharlestonEstablished
 ChesterEstablished
 ChesterfieldReportedN-R(Williams et al. 1999)
 ColletonReported
 DarlingtonReported
 DorchesterReported
 EdgefieldEstablished
 FairfieldReported
 FlorenceEstablished
 GeorgetownEstablished
 GreenvilleReportedN-R(Williams et al. 1999)
 GreenwoodReported
 HamptonEstablished
 HorryReported
 JasperEstablishedR-EL. Durden, unpublished
 LaurensReported
 McCormickEstablished
 NewberryEstablished
 OrangeburgReported
 PickensReported
 RichlandReportedN-R(Williams et al. 1999)
 SaludaReportedN-R(Williams et al. 1999)
 SpartanburgReported
 SumterReportedN-Rhttp://vectormap.nhm.ku.edu/vectormap/
 UnionEstablished
 South Dakota
 BrookingsReported
 CodingtonReported
Tennessee
 AndersonEstablishedN-E(Rosen et al. 2012)
 BedfordEstablishedR-E(Rosen et al. 2012)
 BledsoeReportedN-R(Rosen et al. 2012)
 CampbellEstablishedN-E(Rosen et al. 2012)
 ClaiborneReportedN-R(Rosen et al. 2012)
 ClayReportedN-R(Rosen et al. 2012)
 CoffeeEstablishedN-EG. Hickling, unpublished
 CumberlandReportedN-R(Harmon et al. 2011, Rosen et al. 2012)
 DavidsonReportedN-R(Rosen et al. 2012)
 DeKalbEstablishedN-E(Rosen et al. 2012)
 FayetteEstablishedN-E(Rosen et al. 2012, Mays et al. 2014)
 FentressReported
 FranklinEstablishedN-EG. Hickling, unpublished
 GilesReportedN-R(Rosen et al. 2012)
 GraingerReportedN-R(Rosen et al. 2012)
 HamiltonEstablishedN-E(Rosen et al. 2012)
 HenryReportedN-R(Rosen et al. 2012)
 HumphreysReportedN-R(Rosen et al. 2012)
 JacksonEstablishedN-E(Rosen et al. 2012)
 KnoxEstablishedN-EG. Hickling, unpublished
 LakeReported
 LauderdaleReportedN-R(Rosen et al. 2012)
 LawrenceEstablishedN-E(Rosen et al. 2012)
 LoudonEstablishedN-E(Rosen et al. 2012)
 MarionEstablishedR-E(Rosen et al. 2012)
 MarshallEstablishedN-E(Rosen et al. 2012)
 MauryReportedN-R(Rosen et al. 2012)
 MontgomeryReportedN-Rhttp://vectormap.nhm.ku.edu/vectormap/
 ObionReportedN-R(Rosen et al. 2012)
 PickettReportedN-R(Rosen et al. 2012)
 RheaEstablishedN-E(Rosen et al. 2012)
 RoaneReportedN-R(Rosen et al. 2012)
 RutherfordReported
 ScottReported
 SequatchieReportedN-R(Rosen et al. 2012)
 ShelbyEstablished
 StewartReportedN-R(Rosen et al. 2012)
 SumnerReportedN-R(Rosen et al. 2012)
 TiptonReportedN-R(Rosen et al. 2012)
 WarrenReportedN-R(Rosen et al. 2012)
 WayneReportedN-R(Rosen et al. 2012)
 WhiteReportedN-R(Rosen et al. 2012)
 WilliamsonReportedN-R(Rosen et al. 2012)
Texas
 AndersonEstablished
 AngelinaEstablished
 AransasEstablished
 AustinReported
 BanderaReported
 BastropReported
 BellEstablished
 BexarEstablished
 BlancoReportedN-RS. Hamer, unpublished
 BowieReported
 BrazoriaEstablished
 BrazosEstablishedR-E(Sanders et al. 2013, Rodriguez et al. 2015)
 CameronReportedN-R(Feria-Arroyo et al. 2014)
 CassEstablished
 CherokeeEstablished
 ColoradoReported
 CoryellReportedN-R(Sanders et al. 2013)
 DallasReported
 EdwardsReported
 Fort BendReported
 FranklinReported
 FreestoneEstablishedN-ES. Hamer, unpublished
 GraysonReported
 GreggReported
 HamiltonReported
 HarrisEstablished
 HarrisonReported
 HaysReported
 HendersonEstablished
 HidalgoReportedN-R(Feria-Arroyo et al. 2014)
 HoustonReported
 HuntReported
 JasperEstablished
 JeffersonReported
 KerrReported
 KlebergReported
 LamarReported
 LampasasReported
 LavacaEstablished
 LeonEstablished
 LibertyReported
 LlanoReported
 MarionEstablished
 MasonReportedN-R(Feria-Arroyo et al. 2014)
 MatagordaReported
 MontgomeryEstablished
 NacogdochesEstablished
 NewtonReported
 Palo PintoReported
 ParkerEstablished
 PolkEstablished
 RealEstablished
 RobertsonReported
 RuskReported
 SabineReported
 San AugustineReported
 ShelbyReported
 SmithEstablished
 SuttonReported
 TarrantReportedN-R(Feria-Arroyo et al. 2014)
 TaylorReported
 TrinityEstablished
 TylerEstablished
 UpshurReported
 UvaldeReported
 VictoriaReported
 WalkerReported
 WallerEstablished
 WashingtonReportedN-RS. Hamer, unpublished
 WoodEstablished
 ZavalaReported
Vermont
 AddisonEstablishedN-Ehttps://apps.health.vermont.gov/gis/vttracking/ticktracker/2014Summary/
 BenningtonEstablishedR-Ehttps://apps.health.vermont.gov/gis/vttracking/ticktracker/2014Summary/
 CaledoniaEstablishedR-Ehttps://apps.health.vermont.gov/gis/vttracking/ticktracker/2014Summary/
 ChittendenEstablishedR-Ehttps://apps.health.vermont.gov/gis/vttracking/ticktracker/2014Summary/
 FranklinEstablishedN-Ehttps://apps.health.vermont.gov/gis/vttracking/ticktracker/2014Summary/
 Grand IsleEstablishedN-Ehttps://apps.health.vermont.gov/gis/vttracking/ticktracker/2014Summary/
 LamoilleReported
 OrangeEstablishedN-E(Serra et al. 2013)
 OrleansReportedN-Rhttps://apps.health.vermont.gov/gis/vttracking/ticktracker/2014Summary/
 RutlandEstablishedR-Ehttps://apps.health.vermont.gov/gis/vttracking/ticktracker/2014Summary/
 WashingtonEstablishedN-Ehttps://apps.health.vermont.gov/gis/vttracking/ticktracker/2014Summary/
 WindhamEstablishedR-Ehttps://apps.health.vermont.gov/gis/vttracking/ticktracker/2014Summary/
 WindsorEstablished
Virginia
 AccomackEstablished
 AlbemarleEstablishedN-ED. Gaines, unpublished
 AlleghanyReportedN-RD. Gaines, unpublished
 AppomattoxEstablishedN-ER.J. Brinkerhoff, unpublished
 ArlingtonReportedN-Rhttp://vectormap.nhm.ku.edu/vectormap/
 AugustaEstablishedN-ED. Gaines, unpublished
 BathReportedN-RD. Gaines, unpublished
 Bedford CityReportedN-Rhttp://vectormap.nhm.ku.edu/vectormap/
 BedfordEstablishedN-EM. Shanks, D. Gaines, unpublished
 BlandEstablishedN-EM. Shanks, D. Gaines, unpublished
 BrunswickReportedN-RD. Gaines, unpublished
 BuckinghamEstablishedN-ED. Gaines, unpublished; R.J. Brinkerhoff, unpublished
 CarolineEstablished
 Charles CityReportedN-RD. Gaines, unpublished; R.J. Brinkerhoff, unpublished
 ChesapeakeEstablished
 ChesterfieldEstablishedN-E(Kelly et al. 2014)
 CraigEstablishedN-EM. Shanks, D. Gaines, unpublished
 CulpeperReportedN-RH. Gaff, unpublished; http://vectormap.nhm.ku.edu/ vectormap/
 CumberlandReportedN-RR.J. Brinkerhoff, unpublished
 DinwiddieReportedN-Rhttp://vectormap.nhm.ku.edu/vectormap/
 EssexReportedN-RH. Gaff, unpublished
 FairfaxEstablished
 FauquierEstablishedR-ER.J. Brinkerhoff, unpublished
 FloydEstablishedN-ED. Gaines, unpublished
 FluvannaEstablishedN-ER.J. Brinkerhoff, unpublished
 FranklinEstablishedN-EM. Shanks, D. Gaines, unpublished
 FrederickEstablishedN-ED. Gaines, unpublished
 GilesEstablishedN-E(Herrin et al. 2014)
 GloucesterReportedN-RH. Gaff, unpublished; http://vectormap.nhm.ku.edu/vectormap/
 GoochlandEstablishedN-E(Kelly et al. 2014)
 GreensvilleReportedN-RH. Gaff, unpublished
 HamptonEstablishedN-EH. Gaff, unpublished
 HanoverReportedN-RR.J. Brinkerhoff, unpublished
 HenricoReportedN-RD. Gaines, unpublished
 Isle of WightEstablishedN-EH. Gaff, unpublished
 James CityEstablishedN-E(Kelly et al. 2014); H. Gaff, unpublished
 King and QueenReportedN-RH. Gaff, unpublished
 King GeorgeEstablishedN-ED. Gaines, unpublished; H. Gaff, unpublished
 LancasterReportedN-RH. Gaff, unpublished
 LoudounEstablishedN-EH. Gaff, unpublished
 LynchburgEstablishedN-ED. Gaines, unpublished
 MecklenburgReportedN-RD. Gaines, unpublished
 MiddlesexEstablishedN-EH. Gaff, unpublished
 MontgomeryEstablishedN-ED. Gaines, unpublished
 NelsonEstablishedN-EKelly et al. 2014
 New KentEstablishedN-ER.J. Brinkerhoff, unpublished
 Newport NewsEstablished
 NorfolkReportedN-RH. Gaff, unpublished
 NorthamptonEstablishedN-E(Kelly et al. 2014); H. Gaff, unpublished
 NorthumberlandReportedN-RH. Gaff, unpublished; http://vectormap.nhm.ku.edu/vectormap/
 PortsmouthEstablishedN-EH. Gaff, unpublished
 PowhatanEstablishedN-ER.J. Brinkerhoff, unpublished
 Prince EdwardReportedN-RR.J. Brinkerhoff, unpublished
 Prince GeorgeReported
 Prince WilliamEstablishedN-EH. Gaff, unpublished; http://vectormap.nhm.ku.edu/vectormap/
 PulaskiEstablishedN-E(Herrin et al. 2014)
 RappahannockReportedN-Rhttp://vectormap.nhm.ku.edu/vectormap/
 RichmondReportedN-RD. Gaines, unpublished; H. Gaff, unpublished
 RoanokeEstablishedN-ED. Gaines, unpublished
 RockbridgeEstablishedN-ED. Gaines, unpublished
 RockinghamReportedN-RD. Gaines, unpublished
 ShenandoahReportedN-RD. Gaines, unpublished
 SouthamptonEstablishedR-EH. Gaff, unpublished
 SpotsylvaniaReportedN-Rhttp://vectormap.nhm.ku.edu/vectormap/
 StaffordEstablished
 SuffolkEstablishedN-ED. Gaines, unpublished; H. Gaff, unpublished
 SussexReportedN-RH. Gaff, unpublished
 Virginia BeachEstablished
 WarrenEstablishedN-ED. Gaines, unpublished
 WashingtonReportedN-Rhttp://vectormap.nhm.ku.edu/vectormap/
 WestmorelandEstablishedN-EH. Gaff, unpublished
 YorkEstablished
West Virginia
 BarbourEstablishedN-Ehttp://www.dhhr.wv.gov/oeps/disease/Zoonosis/Mosquito/Documents/arbovirus/vectorborne-disease-report.pdf#page=3; E. Dotseth, unpublished
 BerkeleyEstablished
 BooneReported
 BraxtonEstablishedN-EE. Dotseth, unpublished
 BrookeEstablishedN-Ehttp://www.mamca.org/2014Meeting/0306_0840_SR_WV.pdf; E. Dotseth, unpublished
 CabellReportedN-Rhttp://www.dhhr.wv.gov/oeps/disease/zoonosis/documents/wv-zd-summary-2014.pdf
 DoddridgeReportedN-Rhttp://www.dhhr.wv.gov/oeps/disease/Zoonosis/Mosquito/Documents/arbovirus/vectorborne-disease-report.pdf#page=3
 FayetteReportedN-RE. Dotseth, unpublished
 GilmerEstablishedN-EE. Dotseth, unpublished
 GrantReportedN-Rhttp://www.dhhr.wv.gov/oeps/disease/Zoonosis/Mosquito/Documents/arbovirus/vectorborne-disease-report.pdf#page=3
 GreenbrierEstablishedN-Ehttp://www.mamca.org/2014Meeting/0306_0840_SR_WV.pdf; E. Dotseth, unpublished
 HampshireReportedN-Rhttp://www.mamca.org/2014Meeting/0306_0840_SR_WV.pdf
 HancockEstablishedN-Ehttp://www.dhhr.wv.gov/oeps/disease/Zoonosis/Tick/documents/tickborne-disease-summary-2013.pdf
 HardyEstablishedN-Ehttp://www.dhhr.wv.gov/oeps/disease/zoonosis/tick/documents/tickborne-disease-summary-2013.pdf; E. Dotseth, unpublished
 HarrisonEstablishedN-Ehttp://www.dhhr.wv.gov/oeps/disease/zoonosis/tick/documents/tickborne-disease-summary-2013.pdf; E. Dotseth, unpublished
 JacksonReportedN-Rhttp://www.dhhr.wv.gov/oeps/disease/Zoonosis/Mosquito/Documents/arbovirus/vectorborne-disease-report.pdf#page=3
 JeffersonEstablished
 KanawhaEstablishedN-Ehttp://www.dhhr.wv.gov/oeps/disease/Zoonosis/Tick/documents/tickborne-summary-2012.pdf
 LewisEstablishedN-EE. Dotseth & M. Mark-Carew, unpublished
 LincolnReportedN-Rhttp://www.dhhr.wv.gov/oeps/disease/Zoonosis/Mosquito/Documents/arbovirus/vectorborne-disease-report.pdf#page=3
 LoganReportedN-Rhttp://www.dhhr.wv.gov/oeps/disease/Zoonosis/Mosquito/Documents/arbovirus/vectorborne-disease-report.pdf#page=3
 MarionEstablishedN-Ehttp://www.dhhr.wv.gov/oeps/disease/zoonosis/tick/documents/tickborne-disease-summary-2013.pdf; E. Dotseth, unpublished
 MarshallReportedN-Rhttp://www.dhhr.wv.gov/oeps/disease/zoonosis/tick/documents/tickborne-disease-summary-2013.pdf
 MercerEstablishedN-Ehttp://www.dhhr.wv.gov/oeps/disease/Zoonosis/Mosquito/Documents/arbovirus/vectorborne-disease-report.pdf#page=3; E. Dotseth, unpublished
 MineralReportedN-Rhttp://www.dhhr.wv.gov/oeps/disease/Zoonosis/Mosquito/Documents/arbovirus/vectorborne-disease-report.pdf#page=3
 MonongaliaEstablishedN-Ehttp://www.dhhr.wv.gov/oeps/disease/zoonosis/tick/documents/tickborne-disease-summary-2013.pdf; E. Dotseth, unpublished
 MonroeEstablishedN-EE. Dotseth, unpublished
 MorganReported
 OhioReportedN-RE. Dotseth, unpublished
 PocahontasReportedN-RE. Dotseth, unpublished
 PrestonEstablishedN-Ehttp://www.mamca.org/2014Meeting/0306_0840_SR_WV.pdf; E. Dotseth, unpublished
 PutnamReportedN-Rhttp://www.dhhr.wv.gov/oeps/disease/zoonosis/documents/wv-zd-summary-2014.pdf
 RaleighEstablishedN-EE. Dotseth, unpublished
 RandolphReportedN-Rhttp://www.mamca.org/2014Meeting/0306_0840_SR_WV.pdf
 RitchieReportedN-RE. Dotseth, unpublished
 SummersEstablishedN-Ehttp://www.dhhr.wv.gov/oeps/disease/zoonosis/tick/documents/tickborne-disease-summary-2013.pdf; E. Dotseth, unpublished
 TaylorEstablishedN-Ehttp://www.dhhr.wv.gov/oeps/disease/zoonosis/tick/documents/tickborne-disease-summary-2013.pdf; E. Dotseth, unpublished
 TuckerReportedN-Rhttp://www.dhhr.wv.gov/oeps/disease/Zoonosis/Tick/documents/tickborne-disease-summary-2013.pdf
 TylerReportedN-Rhttp://www.dhhr.wv.gov/oeps/disease/zoonosis/documents/wv-zd-summary-2014.pdf
 UpshurEstablishedN-EE. Dotseth, unpublished
 WetzelEstablishedN-EE. Dotseth, unpublished
 WirtEstablishedN-EE. Dotseth, unpublished
 WoodReportedN-RE. Dotseth, unpublished
Wisconsin
 AdamsEstablished
 AshlandEstablishedR-ES. Paskewitz, unpublished
 BarronEstablished
 BayfieldEstablishedN-ES. Paskewitz, unpublished
 BrownEstablishedN-ES. Paskewitz, unpublished
 BuffaloEstablishedR-E(Michalski et al. 2006)
 BurnettEstablished
 ChippewaEstablished
 ClarkEstablished
 ColumbiaEstablished
 CrawfordEstablished
 DaneEstablished
 DodgeReportedN-R(Lee et al. 2013)
 DoorEstablishedR-ES. Paskewitz, unpublished
 DouglasEstablishedN-ES. Paskewitz, unpublished.
 DunnEstablishedN-ES. Paskewitz, unpublished
 Eau ClaireEstablished
 FlorenceReportedN-Rhttp://labs.russell.wisc.edu/wisconsin-ticks/presence-of-ixodes-scapularis-on-hunter-killed-deer-in-wisconsin-2008-09/
 Fond du LacReportedN-RLee et al. 2013; http://labs.russell.wisc.edu/wisconsin-ticks/presence-of-ixodes-scapularis-on-hunter-killed-deer-in-wisconsin-2008-09/
 ForestEstablishedN-ES. Paskewitz, unpublished
 GrantEstablished
 GreenEstablished
 Green LakeEstablishedN-E(Michalski et al. 2006)
 IowaEstablished
 JacksonEstablished
 JeffersonEstablishedR-ES. Paskewitz, unpublished
 JuneauEstablishedN-ES. Paskewitz, unpublished
 KenoshaReportedN-R(Lee et al. 2013)
 La CrosseEstablished
 LafayetteReported
 LangladeEstablishedN-ES. Paskewitz, unpublished
 LincolnEstablished
 ManitowocReported
 MarathonEstablished
 MarinetteEstablished
 MenomineeEstablishedN-ES. Paskewitz, unpublished
 MilwaukeeEstablishedN-ES. Paskewitz, unpublished
 MonroeEstablished
 OcontoEstablishedN-ES. Paskewitz, unpublished
 OneidaEstablishedR-ES. Paskewitz, unpublished
 OutagamieReported
 OzaukeeReportedN-Rhttp://labs.russell.wisc.edu/wisconsin-ticks/presence-of-ixodes-scapularis-on-hunter-killed-deer-in-wisconsin-2008-09/
 PierceReportedN-RS. Paskewitz, unpublished
 PolkReported
 PortageEstablished
 PriceEstablished
 RacineEstablished
 RichlandEstablished
 RockEstablished
 RuskEstablished
 SaukEstablished
 SawyerEstablished
 ShawanoReportedN-R(Lee et al. 2013)
 SheboyganEstablishedR-ES. Paskewitz, unpublished
 St. CroixEstablishedR-ES. Paskewitz, unpublished
 TaylorEstablished
 TrempealeauEstablished
 VernonReported
 VilasEstablishedN-ES. Paskewitz, unpublished
 WalworthEstablishedR-E(Caporale et al. 2005, Lee et al. 2013)
 WashburnEstablished
 WaukeshaEstablishedR-ES. Paskewitz, unpublished
 WaupacaEstablishedN-ES. Paskewitz, unpublished
 WausharaReported
 WinnebagoReported
 WoodReportedN-RS. Paskewitz, unpublished
  • aEstablished: Six or more ticks or two or more tick life stages; Reported: Fewer than six ticks and one tick life stage only.

  • bN-R, change from No Records to Reported; N-E, change from No Records to Established; R-E, change from Reported to Established.

  • cNCPHPM: North Carolina Public Health Pest Management, terminated in 2011.

  • dSCWDS: Southeastern Cooperative Wildlife Disease Study.

  • eNational Veterinary Services Laboratories.

Counties Where I. pacificus Is Classified as Established or Reported

Ixodes pacificus is now classified as established in 95 counties, and as reported in 16 additional counties, spanning 6 states (Tables 1, 3; Fig. 1). The majority of these counties are in the Pacific Coast states of California, Oregon, and Washington. Our new data mark a very modest increase in the number of western counties in which I. pacificus is classified as established since the survey by Dennis et al. (1998), when the tick was listed as established in 90 counties; the number of counties with reported status remained stable. In total, five counties were updated from either no records (n = 1) or reported (n = 4) to established and four counties were updated from no records to reported (Table 3; Fig. 2). All county status changes occurred in Washington, Oregon, or Utah (Tables 1, 3; Figs. 1–2). The overall ratio of counties in which I. pacificus is classified as established versus reported was 5.94:1 in this study, compared with 5.6:1 (Dennis et al. 1998).

View this table:
Table 3.

Records of I. pacificus by state and county. Fields left blank indicate status was inherited from Dennis et al. (1998)

State and countyStatus by Aug. 2015aStatus change from Dennis et al. (1998)bSource for change of status from Dennis et al. (1998) survey
Arizona
 MohaveEstablished
California
 AlamedaEstablished
 AmadorEstablished
 ButteEstablished
 CalaverasEstablished
 ColusaEstablished
 Contra CostaEstablished
 Del NorteEstablished
 El DoradoEstablished
 FresnoEstablished
 GlennEstablished
 HumboldtEstablished
 ImperialEstablished
 InyoEstablished
 KernEstablished
 KingsEstablished
 LakeEstablished
 LassenEstablished
 Los AngelesEstablished
 MaderaEstablished
 MarinEstablished
 MariposaEstablished
 MendocinoEstablished
 MercedEstablished
 MonoReported
 MontereyEstablished
 NapaEstablished
 NevadaEstablished
 OrangeEstablished
 PlacerEstablished
 PlumasEstablished
 RiversideEstablished
 SacramentoEstablished
 San BenitoEstablished
 San BernardinoEstablished
 San DiegoEstablished
 San FranciscoEstablished
 San JoaquinEstablished
 San Luis ObispoEstablished
 San MateoEstablished
 Santa BarbaraEstablished
 Santa ClaraEstablished
 Santa CruzEstablished
 ShastaEstablished
 SierraEstablished
 SiskiyouEstablished
 SolanoEstablished
 SonomaEstablished
 StanislausEstablished
 SutterEstablished
 TehamaEstablished
 TrinityEstablished
 TulareEstablished
 TuolumneEstablished
 VenturaEstablished
 YoloEstablished
 YubaEstablished
Nevada
 ClarkReported
 LincolnReported
Oregon
 BentonEstablished
 ClackamasEstablished
 ClatsopEstablished
 ColumbiaReported
 CoosEstablished
 CurryEstablished
 DouglasEstablished
 Hood RiverEstablished
 JacksonEstablished
 JeffersonReported
 JosephineEstablished
 LaneEstablished
 LincolnEstablished
 LinnEstablished
 MarionEstablished
 MultnomahEstablished
 PolkReported
 ShermanEstablished
 TillamookEstablished
 UmatillaReported
 WascoEstablished
 WashingtonEstablished
Utah
 BeaverReported
 JuabEstablished
 MillardReported
 PiuteReported
 Salt LakeEstablished
 TooeleEstablishedR-E(Davis et al. 2015)
 UtahEstablished
WashingtonEstablished
 Washington
 ChelanEstablished
 ClallamEstablishedN-EE. Dykstra, unpublished
 ClarkEstablished
 CowlitzEstablishedR-EE. Dykstra, unpublished
 IslandReported
 JeffersonEstablished
 KingEstablishedR-EE. Dykstra, unpublished
 KitsapEstablishedR-EE. Dykstra, unpublished
 KittitasReportedN-RE. Dykstra, unpublished
 KlickitatEstablished
 LewisEstablished
 MasonEstablished
 OkanoganReportedN-RE. Dykstra, unpublished
 PacificReportedN-RE. Dykstra, unpublished
 PierceEstablished
 San JuanEstablished
 SkagitEstablished
 SkamaniaEstablished
 SnohomishReported
 ThurstonEstablished
 WhatcomEstablished
 YakimaReportedN-RE. Dykstra, unpublished
  • Fields left blank indicate status was inherited from Dennis et al. (1998)

  • aEstablished: Six or more ticks or two or more tick life stages; Reported: Fewer than six ticks and one tick life stage only.

  • bN-R, change from No Records to Reported; N-E, change from No Records to Established; R-E, change from Reported to Established.

Discussion

Data on the current geographic distributions of medically important tick vectors, such as I. scapularis and I. pacificus, provide information complementary to epidemiological data on geographic disease case occurrence to inform the medical community and the public of where risk for exposure to tick-borne disease agents may occur. The lack of routine systematic surveillance across the continental United States of ticks of public health importance hampers our ability to define their current geographic distributions and to monitor changes in their ranges and densities over time. Although we are able to report in this paper where I. scapularis and I. pacificus are now known to be present at the county level, our certainty in where the tick is absent is low, especially at the edges of their ranges and in regions where they can be assumed to occur only at low densities. Range contractions, if they occurred, were not quantified in this study because counties that were previously considered established maintained that status here. Nonetheless, using survey methods similar to those of Dennis et al. (1998), specifically, literature review and inclusion of unpublished data from individual researchers and state public health departments, we showed a substantial increase over the past nearly two decades in counties classified as having I. scapularis present. Moreover, the number of counties in which I. scapularis is considered established has more than doubled since the previous national distribution map was published (Dennis et al. 1998). The majority of county status changes occurred in the North, while the distribution in the South remained fairly stable. The North-Central focus for I. scapularis in Minnesota and Wisconsin appears to have expanded in all cardinal directions, and the Northeastern focus has spread inland from the Atlantic seaboard and expanded in both northerly and southerly directions. As a result, the two previously distinct foci in the North-Central and Northeastern United States have now converged in the Ohio River Valley to form a single larger focus. In striking contrast to I. scapularis, increases in counties reporting the presence of I. pacificus in the Far West were very modest.

Population genetic analyses provide support for the theory that I. scapularis was once established across the Northeastern and North-Central United States for thousands of years and likely colonized the region following the recession of the Pleistocene ice sheet (Humphrey et al. 2010). Thus, current trends may represent recolonization of the tick’s historical range. Rapid deforestation and suppression of white-tailed deer during the late 1800s and early 1990s may have restricted I. scapularis to focal refugia (Spielman et al. 1985, Lee et al. 2013). Reforestation and increasing abundance of white-tailed deer, the primary hosts of adult I. scapularis (Spielman et al. 1985), are considered to have contributed to the dramatic expansion of the tick’s range over the past half century (Spielman 1994).

Our updated species distribution map shows a continued range expansion for I. scapularis, particularly in northern states. Given the lack of systematic surveillance for I. scapularis, one might ask if the range expansion suggested by our data is real or merely an artifact of enhanced tick surveillance and research activities in some areas. A true range expansion of I. scapularis in northern states, as described in this report, is supported by the largely concordant changes in the distribution of human Lyme disease cases captured through mandatory reporting of the disease since 1991 (Rand et al. 2007, Raizman et al. 2012, Lee et al. 2013, Serra et al. 2013, Brinkerhoff et al. 2014, Robinson et al. 2014, Wang et al. 2014, Kugeler et al. 2015, Mead 2015). Moreover, as detailed later in the text, true range expansions of I. scapularis have been documented in some areas where tick surveillance was conducted routinely during the period of emergence, or where extensive surveys were conducted at discrete time-points spanning periods from when the tick was absent through invasion and establishment.

The North-Central States

Within the North-Central United States, I. scapularis was first described in a focal area of northwestern Wisconsin in the late 1960s (Jackson and DeFoliart 1970). State-wide surveys of adult I. scapularis collected from hunter-killed deer in Wisconsin from 1981–1989 (French et al. 1992) revealed that the tick had become established in western, southern, and focal parts of the north, but no evidence of the tick was found in other areas in the north or southeastern reaches of Wisconsin. Subsequent surveys of hunter-killed deer documented continued expansion into the north during the early 1990s (Riehle and Paskewitz 1996) and eventual invasion of eastern Wisconsin by 2008–2009 (Lee et al. 2013). Ixodes scapularis now appears to be present throughout most of the habitat that is predicted to be suitable for the tick in the state of Wisconsin (Guerra et al. 2002, Diuk-Wasser et al. 2010).

In neighboring Minnesota to the west, I. scapularis was classified as reported or established primarily in counties bordering Wisconsin in the east-central portion of Minnesota in the mid-1990s (Dennis et al. 1998). Opportunistic sampling during 1998–1999 revealed the presence of I. scapularis in additional northern and central Minnesota counties (Sanders and Guilfoile 2000), and new county records from the south-central portion of the state are presented in this report. Paralleling this expansion of the tick’s known range in Minnesota, Robinson et al. (2014) noted increases in both the numbers and geographical distributions of I. scapularis-borne diseases in Minnesota from 1996 through 2011. The north-westerly expansion appears to have continued into eastern North Dakota (Russart et al. 2014, Stone et al. 2015), beyond or near the limit of habitat previously predicted to be suitable for I. scapularis (Estrada-Pena 2002, Brownstein et al. 2003, Diuk-Wasser et al. 2010).

Wisconsin likely served as a primary source for a southerly invasion of I. scapularis into Illinois, specifically along the Rock River corridor (Cortinas and Kitron 2006). Surveys of hunter-killed deer from Illinois conducted from 1988–1996 showed that infested deer were restricted largely to northern counties (Cortinas et al. 2002). However, similar surveys conducted from 1998 to 2003 showed expansion of the tick’s range to more southern counties and noted that I. scapularis densities decreased along a northern to southern gradient, suggesting that Illinois was first colonized in the northwestern and north-central counties, where I. scapularis indeed was first discovered in the state in the late 1980s (Bouseman et al. 1990, Cortinas and Kitron 2006). Populations of I. scapularis in the extreme northeast along the Illinois River speculatively may have originated from established populations in northwestern Indiana (Cortinas and Kitron 2006). In Indiana, where I. scapularis was first collected from deer in northwestern counties in 1987 (Pinger and Glancy 1989), densities of I. scapularis are greatest along the western border and decrease eastward; expansion to eastern counties was observed between 2005 and 2007 (Pinger et al. 1996, Keefe 2008, Raizman et al. 2012).

Hamer et al. (2010) proposed that established I. scapularis populations in Indiana seeded colonization of lower Michigan, where the tick was first discovered in southwestern lower Michigan in 2002 (Foster 2004). Invasion of I. scapularis northward along the coast of Lake Michigan was documented from 2004–2008; in 2004, ticks were collected only from the southernmost of the sampled sites, whereas they were found in all sites by 2008. Tick densities decreased from south to north, supporting a view that densities would be higher in areas where the tick has been longer established. Notably, although inland transects were also surveyed, no evidence of I. scapularis invasion was observed in these transects (Hamer et al. 2010). Subsequently, I. scapularis has been reported also from inland counties in southern Michigan (Table 2). Colonization of the Upper Peninsula of Michigan most likely occurred via northern Wisconsin and preceded colonization of the Lower Peninsula by more than a decade, as I. scapularis was discovered already in the 1980s in Menominee County in the Upper Peninsula of Michigan (Strand et al. 1992, Walker et al. 1994).

The Northeast

Similar to I. scapularis expansion in the North-Central focus, the tick’s range in the northeastern focus appears to have expanded in all directions, except for eastward, where the Atlantic Ocean prevents further spread. Since the survey by Dennis et al. (1998), I. scapularis appears to have expanded northward in Maine, New Hampshire, and Vermont. In Maine, analysis of public submission of ticks from 1989 to 2006 showed a northerly expansion along the Atlantic coastline, followed by invasion inland along river corridors (Rand et al. 2007). Ixodes scapularis is now considered established in all Maine counties. In Vermont, drag sampling was conducted from June 2011 to June 2012 along a north-south transect following the Connecticut River: densities of I. scapularis generally decreased from south to north, with no ticks collected from the northernmost sites (Serra et al. 2013). Expansion of the tick’s range in these New England states likely contributed, together with increasing tick densities in already established areas, to a 5–10-fold increase in incidence of reported Lyme disease cases in those states during the past decade (Mead 2015).

Since the survey by Dennis et al. (1998), the number of New York counties where I. scapularis is considered established has nearly doubled from 50.0 to 98.4%. At the time of the previous report, the tick was established primarily in the southeastern and eastern portions of the state and appears to have expanded in northerly and westerly directions. In parallel with this observation, from 1990 to 2000, Lyme disease surveillance data revealed a northward and westward expansion in the disease focus from a central cluster in the southeastern portion of the state (i.e., Westchester County). Moreover, during the same timeframe, the primary epidemiological focus shifted northward along the Hudson River (Chen et al. 2005). Population genetic analysis of I. scapularis collected from a transect along the Hudson River from 2004 to 2009 indicated recent rapid expansion of the tick’s range, primarily in a northerly direction along the Hudson River (Khatchikian et al. 2015); expansion appears to be the result of local migration of the ticks, via movements of mammal hosts, but some long-distance migration, perhaps via infestation of birds, was detected. Importantly, the DNA sequence analyses provide evidence for recent range expansion, as opposed to recent detection of in situ populations.

Neighboring New York to the south, Pennsylvania also experienced a recent westward expansion of I. scapularis. In 2003, Lyme disease cases were reported primarily from eastern counties in Pennsylvania. By 2013, human Lyme disease case counts increased markedly in western counties, with cases reported throughout the state (Mead 2015). This mirrors data for range expansion of I. scapularis in Pennsylvania. No I. scapularis were collected during a statewide survey from 1963 to 1967 (Snetsinger 1968), whereas the tick was recorded from 49 of 67 counties by the late 1990s (Dennis et al. 1998). A statewide survey conducted during 2012–2014 (Hutchinson et al. 2015) revealed that the tick now is established in all 67 counties. It is likely that the east-to-west tick expansion across Pennsylvania continued into neighboring Ohio to the west, where active tick surveillance was conducted from 1983 to 2012. Surveillance data showed a dramatic increase in I. scapularis abundance beginning in 2009. Arguing against increasing tick surveillance as a primary source for the observed range expansion in Ohio, the spread of the tick was observed when Ohio’s tick surveillance programs were being considered for termination and their budgets were dwindling (Wang et al. 2014). Currently, the I. scapularis range in Ohio is largely consistent with the range of deciduous forest in the state. As a result of the westward expansion of the previous northeastern focus and the eastward expansion of the previous North-Central focus, the distribution of I. scapularis now appears to be continuous across northern states with convergence of the two previously distinct foci in the Ohio River Valley.

The West Virginia, Virginia, and North Carolina Area

Ixodes scapularis has also expanded its range in West Virginia, where only 4 counties reported the tick previously (Dennis et al. 1998) and now 43 counties are classified as either reported (n = 20) or established (n = 23). Review of Lyme disease and I. scapularis surveillance reports (see references in Table 2) suggests that the tick is expanding westward across the state, with highly Lyme disease endemic counties still focused in the eastern panhandle. Similarly, in Virginia, I. scapularis was considered established primarily in eastern coastal counties previously (Dennis et al. 1998), but the current survey shows the tick to now be established throughout most of Virginia, with the highest densities of openly host-seeking ticks occurring in higher elevation sites (Brinkerhoff et al. 2014, Kelly et al. 2014). The spread of openly host-seeking I. scapularis appears to have proceeded in a southwesterly direction in Virginia, concordant with the expanding geographic distribution of Lyme disease cases and increasing incidence in Virginia (Brinkerhoff et al. 2014, Lantos et al. 2015). Comparison of the previous and current distributions of I. scapularis in North Carolina also suggests an inland incursion of the tick (Fig. 1). One important caveat to these findings for West Virginia, Virginia, and North Carolina is that the observed spread of I. scapularis may have resulted from southerly spread of I. scapularis from states to the north where this tick is more prone, as compared with southern populations, to seek hosts openly from vegetation (Arsnoe et al. 2015) and therefore is more readily contacted by tick dragging or flagging (Diuk-Wasser et al. 2006), or by humans and their pets (Stromdahl and Hickling 2012). Stated differently, this may be an invasion of more easily surveyed northern populations of I. scapularis rather than invasion at the species level in areas where more cryptic southern populations already may have been present but had not been recognized. Regardless, the end result is range expansion of I. scapularis populations that commonly contact and bite humans in West Virginia, Virginia, and North Carolina.

The Allegheny Mountains to Mississippi Valley Area

The authors (Dennis et al. 1998) of the previous survey were intrigued with the lack of I. scapularis records, despite collection efforts, from the Allegheny Mountains to the Mississippi Valley (an area spanning western Pennsylvania southeastward across Kentucky and Tennessee), because of the tick’s large geographical coverage in the eastern United States spanning variable climates and forested habitat types. Habitat suitability modeling suggested that the area ranged from low (Brownstein et al. 2003, Diuk-Wasser et al. 2010) to moderately suitable (Estrada-Pena 2002). Our revised distribution map shows the tick to now be established in this region, attesting to its climate and habitat suitability for I. scapularis. Several studies have documented a trend in which the tick expands along riparian corridors (Cortinas et al. 2002, Cortinas and Kitron 2006, Rand et al. 2007, Hamer et al. 2010, Serra et al. 2013, Kelly et al. 2014, Khatchikian et al. 2015). This provides a hypothesis for how the area from the Allegheny Mountains to the Mississippi Valley could have been or is currently being invaded from areas to the north and east with already established tick populations, first along distinct dispersal corridors followed by more diffuse short-range tick dispersal to suitable habitats across the landscape.

The Southeast

In contrast to the observed concordance between the reported distributions of I. scapularis and human Lyme disease in the North-Central, Northeastern, and Mid-Atlantic States, a wide distribution of this tick vector in the Southeast is not similarly associated with widespread Lyme disease case occurrence (Mead 2015). Lack of concordance in the Southeast could arise for several reasons. First, our map displays a coarse, county-scale representation of where the tick is classified as reported or established. The advantage of the county-scale representation is that it matches the spatial scale at which epidemiological surveillance data are presented. However, in some instances, the vector tick may be established only in limited areas of a particular county, putting very few humans at risk for exposure to tick bites (Eisen and Eisen 2007, Eisen and Eisen 2008). Second, while the presence of at least one tick vector species is a prerequisite for sustaining enzootic transmission of B. burgdorferi, density of B. burgdorferi-infected host-seeking nymphs of a species that commonly bite humans is a better surrogate for human risk of exposure to Lyme disease spirochetes compared with tick presence data alone. At coarse spatial scales such as states or regions of the United States, density of infected I. scapularis nymphs is significantly and positively associated with Lyme disease incidence (Mather et al. 1996, Stafford et al. 1998, Pepin et al. 2012).

A recent systematic field survey of I. scapularis revealed that both the density of host-seeking I. scapularis nymphs and their rate of infection with B. burgdorferi generally were greater in the northern compared with southern states (Diuk-Wasser et al. 2012). This is in agreement with field studies indicating that I. scapularis larvae feed primarily on white-footed mice, Peromyscus leucopus Rafinesque, and other small, highly reservoir-competent mammals in the northeast (Spielman et al. 1985, Giardina et al. 2000), whereas they feed frequently on lizards of, at best, low reservoir-competence for B. burgdorferi in the southeast (Apperson et al. 1993). Moreover, the tick’s host-seeking behavior differs between northern and southern states, such that collection of I. scapularis nymphs by drag sampling (Diuk-Wasser et al. 2006) or from humans (Stromdahl and Hickling 2012) is rare in the south but commonplace in the north. Variable contact rates between humans and nymphal ticks resulting from regional differences in host-seeking behavior could, in large part, explain regional differences in Lyme disease incidence between the northern and southern parts of the eastern United States (Diuk-Wasser et al. 2012, Stromdahl and Hickling 2012, Kelly et al. 2014, Arsnoe et al. 2015). A recent experimental field study showed differences in host-seeking behavior between I. scapularis of northern versus southern origin, such that ticks of northern origin were more likely to ascend vegetation while questing for a host, regardless of whether field release arenas were located in the north or south, suggesting that host-seeking behavior is strongly determined by genetics and to a lesser extent by environmental conditions (Arsnoe et al. 2015). Indeed, population genetic studies show two distinct clades, with the southern clade restricted to the south and the so-called American clade predominant in the north (Norris et al. 1996, Qiu et al. 2002, Humphrey et al. 2010, Van Zee et al. 2013, Sakamoto et al. 2014).

The Far-Western States

The recorded county-level distribution of I. pacificus has changed very little since the previous survey (Dennis et al. 1998). The tick is established primarily in coastal states along the Pacific Ocean (Washington, Oregon, and California), but also can occur locally in especially cool or moist settings in more arid inland states (Arizona, Nevada, and Utah). In contrast to I. scapularis, few studies have sought to define the environmental variables that define the distribution of I. pacificus (Eisen et al. 2006b). Owing in part to sizeable western counties commonly encompassing vast ecological diversity, often with only a portion of a given county presenting risk for human exposure to I. pacificus, there is a lack of concordance between the vector’s range as defined at the county level and the incidence of Lyme disease. For example, in California, although the tick is established in all but three counties, Lyme disease incidence is highest in north-coastal counties (Eisen et al. 2006b). Although B. burgdorferi-infected host-seeking nymphs may be established in limited regions of counties, few humans may be exposed (Eisen et al. 2006b). In addition, densities of host-seeking I. pacificus appear to be much lower in southern (Lane et al. 2013) compared with northern California (Eisen et al. 2006a). Likewise, infection rates with B. burgdorferi also appear to be lower in host-seeking nymphs from southern compared with northern California (Eisen et al. 2010, Lane et al. 2013).

Future Research Needs

The data presented and discussed here provide strong support for systematic sampling to assess the density of host-seeking I. scapularis, and the density of nymphs infected with B. burgdorferi and other I. scapularis-borne human pathogens, in strategic areas where the tick can be expected to invade or increase dramatically in numbers in the near future. We also recognize needs for: 1) improved regional habitat suitability models to better define the likely extent for continued expansion of I. scapularis; 2) population genetic studies aimed at identifying changes in the geographic range of the American clade of I. scapularis, especially in areas previously dominated by the southern clade such as Virginia, and North Carolina where American clade invasion likely results in increased human tick bites; and 3) longitudinal studies aimed at identifying how the convergence of the North-Central and Northeastern tick foci may result in changes in B. burgdorferi genotypes (Pepin et al. 2012), particularly those most likely to cause disease in humans, in the convergence area itself as well as across the North-Central and Northeast states.

Acknowledgments

We thank the following individuals for contributing their unpublished data on collection records for I. scapularis or I. pacificus: I. Arsnoe (Michigan State University), J. Corn (Southeastern Cooperative Wildlife Disease Study), E. Dotseth (West Virginia Department of Health and Human Resources), L. Durden (Georgia Southern University), E. Dykstra (Washington State Department of Health), M. Feist (North Dakota Department of Health), S. Fore (Truman State University), E. Foster (Michigan Department of Health and Human Services), H. Gaff (Old Dominion University), D. Gaines (Virginia Department of Health), R. Gary (Ohio Department of Health), S. Hamer (Texas A&M), B. Harrison (Western Carolina University), G. Hickling (University of Tennessee), T.L. Johnson (Centers for Disease Control and Prevention), H.-J. Kim (Truman State University), C. Lord (University of Florida), J. Mertins (National Veterinary Services Laboratory), M. Miller (United States Army Public Health Command), D. Neitzel (Minnesota Department of Health), B. Pagac (United States Army Public Health Command), S. Paskewitz (University of Wisconsin), R. Pinger (Ball State University), M. Prusinski (New York State Department of Health), E. Raizman (Food and Agriculture Organization of the United Nations), K. Sayler (University of Florida), M. Shanks, J. Sidge (Michigan State University), Southeastern Cooperative Wildlife Disease Study, Unites States Department of Agriculture, L. Townsend (University of Kentucky), J. Tsao (Michigan State University), and J. Vaughn (University of North Dakota).

References Cited

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