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In the United States, tick-borne diseases are increasing in incidence and cases are reported over an expanding geographical area. Avoiding tick bites is a key strategy in tick-borne disease prevention, and this requires current and accurate information on where humans are at risk for exposure to ticks. Based on a review of published literature and records in the U.S. National Tick Collection and National Ecological Observatory Network databases, we compiled an updated county-level map showing the reported distribution of the American dog tick, Dermacentor variabilis (Say). We show that this vector of the bacterial agents causing Rocky Mountain spotted fever and tularemia is widely distributed, with records derived from 45 states across the contiguous United States. However, within these states, county-level records of established tick populations are limited. Relative to the range of suitable habitat for this tick, our data imply that D. variabilis is currently underreported in the peer-reviewed literature, highlighting a need for improved surveillance and documentation of existing tick records.

We identified an established population of the Gulf Coast tick (Amblyomma maculatum Koch) infected with Rickettsia parkeri in Connecticut, representing the northernmost range limit of this medically relevant tick species. Our finding highlights the importance of tick surveillance and public health challenges posed by geographic expansion of tick vectors and their pathogens.

During 2013, we collected and tested ticks for Heartland virus (HRTV), a recently described human pathogen in the genus Phlebovirus (Bunyaviridae), from six sites in northwestern Missouri. Five sites were properties owned by HRTV patients, and the sixth was a conservation area that yielded virus in ticks during 2012. We collected 39,096 ticks representing five species; however, two species, Amblyomma americanum (L.) (97.6%) and Dermacentor variabilis (Say) (2.3%), accounted for nearly all ticks collected. We detected 60 HRTV-positive tick pools and all were composed of A . americanum : 53 pools of nymphs, six pools of male adults, and one pool of female adults. This is the first record of HRTV in adult ticks. Virus was detected at five properties that yielded A. americanum ticks, including properties owned by four of five patients. Virus was detected at two sites that yielded virus in 2012. Detection of virus in multiple years indicates that the virus persists in ticks within a relatively small geographic area, although infection rates (IR) may vary greatly among sites and between years at a site. IR per 1,000 A. americanum in northwestern Missouri during the April–July 2013 study period were as follows: all adults, IR = 1.13; adult females, IR = 0.33; adult males, IR = 1.90; and nymphs, IR = 1.79. The IR in nymphs, the stage with the largest data set, corresponds to 1/559 infected ticks. Having robust estimates of IR in various stages for A. americanum should lead to more accurate public health messaging and a better understanding of virus transmission.

The American dog tick, Dermacentor variabilis (Say), is a vector of spotted fever group (SFG) rickettsiae, including Rickettsia rickettsii the causative organism of Rocky Mountain spotted fever (RMSF). In North Carolina, SFG rickettsioses (including RMSF) are a leading cause of tick-borne illness. Knowledge of the infection rate and geographic distribution of D. variabilis ticks infected with Rickettsia spp. provides information on the spatial distribution of public health risk. Accordingly, we extracted genomic DNA from adult D. variabilis collected from field habitats in 32 North Carolina counties from 2009 to 2013. A nested PCR assay of the 23S-5S intergenic spacer (IGS) region of Rickettsia coupled with reverse line blot hybridization (RLBH) with species-specific probes was used to detect and identify rickettsiae to species. Approximately half of the 532 tick DNA samples exhibited a band of the expected size on agarose gels, indicating infection with Rickettsia spp. RLBH analyses showed R. amblyommatis (formerly ‘Candidatus R. amblyommii’), R. parkeri, and R. montanensis were predominant, while other Rickettsia species detected included R. conorii-like, R. massiliae, R. rhipicephali, R. canadensis, R. bellii, and some unknown Rickettsia spp. Some ticks were infected with more than one Rickettsia species. Notably, several Rickettsia-positive ticks harbored R. rickettsii. DNA sequencing was performed on a portion of the 23S-5S IGS amplicons and the results were concordant with RLB assay results. We conclude that Rickettsia spp. are common in D. variabilis in North Carolina. Geographic patterns in the occurrence of Rickettsia-infected D. variabilis ticks across the counties sampled are discussed.

Ixodid ticks removed from hosts and from vegetation during March-November 1987 at sites in coastal Virginia and North Carolina were examined for Borrelia burgdorferi. B. burgdorferi was evident in nine (22%) Ixodes cookei Packard removed from rice rats (Oryzomys palustris), a white-footed mouse (Peromyscus leucopus), and raccoons (Procyon lotor); four (6%) Amblyomma americanum (L.) removed from raccoons; and two (3%) Dermacentor variabilis (Say) removed from a raccoon and a rice rat. B. burgdorferi was also detected in Ixodes dentatus Marx removed from a brown thrasher (Toxostoma rufum), a Carolina wren (Thryothoros ludovicianus), and a towhee (Piplio erythrophthalamus); and in Haemaphysalis leporispalustris (Packard) removed from a brown thrasher and a white-throated sparrow (Zonotrichia albicollis) netted at Kiptopeke Beach, Va. Two Ixodes dammini Spielman, Clifford, Piesman & Corwin were collected on Parramore Island; one specimen was examined for spirochetes, and it was infected with B. burgdorferi. No spirochetes were detected in host-seeking A. americanum and Amblyomma maculatum Koch removed from vegetation. The plasma of one P. leucopus and sera obtained from two P. lotor contained antibodies to B. burgdorferi. All infected ticks and the seroreactive hosts were collected from the Eastern Shore of Virginia.

The ability of tabanid mouthparts to retain and to transfer mechanically Ehrlichia risticii Holland, Weiss, Burgdorfer, Cole & Kakoma was evaluated by feeding flies on infected and noninfected mice and on capillary tubes containing infected cells and cell-free medium. Flies representing two genera and 29 species were collected at equine boarding stables, farms, and along riding trails in Wake, Johnston, and Duplin counties in North Carolina for the feeding trials. Two species, Tabanus fulvulus Wiedemann and T. pallidescens Philip, fed on mice but failed to transfer the pathogen from infected to susceptible mice. Chrysops vittatus Wiedemann, Tabanus americanus Forster, and T. sulcifrons Macquart transferred E. risticii -infected cells from capillary tubes containing infected cells in medium to tubes containing medium. These studies document that E. risticii -infected cells can be retained on mouthparts and potentially transferred by tabanids.

Larval Dermacentor variabilis (Say) (n = 327) were fed on Balb/C mice inoculated with Ehrlichia risticii, the etiologic agent of equine monocytic ehrlichiosis (Potomac horse fever). All mice displayed clinical signs of E. risticii infection at the time of feeding. After molting, resulting nymphs (n = 74) were fed on susceptible mice. No clinical signs were observed, and the mice remained seronegative for 6 wk after feeding.