Increasing Prevalence of Borrelia Burgdorferi Sensu Stricto–Infected Blacklegged Ticks in Tennessee Valley, Tennessee, USA

Graham J. Hickling; Janetta R. Kelly; Lisa D. Auckland; Sarah A. Hamer

Disclosures

Emerging Infectious Diseases. 2018;24(9):1713-1716. 

In This Article

Conclusions

In eastern Tennessee, public awareness and concern about ticks focuses primarily on the abundant lone star ticks (Amblyomma americanum) and American dog ticks (Dermacentor variabilis) encountered during the spring and summer. Both species can spread pathogens,[9] but neither are vectors of B. burgdorferi spirochetes. Immature I. scapularis ticks are similarly active in the summer, but in southern states, these ticks typically avoid host-seeking above leaf litter and are rarely seen on humans or drag-cloths.[10] For this reason, assessment of I. scapularis distribution in southern states is best achieved by acquiring adult life-stage ticks during cool season drag-cloth surveys (as reported here) or by collecting ticks from deer harvested in the fall. Inspection of hunter-harvested deer is efficient for the detection of low-density I. scapularis ticks.[11] Thus, our drag-cloth sampling for I. scapularis ticks in 14 counties where none were found on deer a decade ago (Figure 1, panels A, B) suggests that tick abundance in these counties has increased. This suggestion is supported by a >3-fold increase in I. scapularis tick counts at the Anderson County site where we have 6 consecutive years of drag-cloth counts.

This study documents emergence of B. burgdorferi senso stricto in tick populations in eastern Tennessee. Infected ticks were predominantly found in high-prevalence hot spots in Union County (36.39°N). Relative to Lyme disease–endemic areas in the north, B. burgdorferi prevalence in the study area was low (10%) and had a patchy distribution (7/49 sites had positive ticks). This distribution could reflect host barriers of B. burgdorferi transmission in the South,[12] or more concerning, the hot spots in Union County might reflect the beginning of an infection surge, similar to that seen in southwestern Virginia during the past decade.[4]

In the United States, Lyme disease is primarily a summertime disease associated with bites from nymphal I. scapularis ticks. In southern states, detection of B. burgdorferi bacteria in adult ticks does not necessarily imply risk to humans; for example, B. burgdorferi cycles in I. scapularis populations on the Outer Banks of North Carolina, yet nymphs in that area cannot be collected on drag-cloths and no locally acquired cases of Lyme disease have been reported.[13] In contrast, infected nymphs have been found on drag-cloths from surveys in Virginia, where Lyme disease incidence has spiked.[14] We speculate that Borrelia-infected I. scapularis populations emerging in southwestern Virginia include immigrant ticks from the North, with some nymphs in these populations exhibiting host-seeking behaviors that lead to contact with humans. A similar invasion process might be under way in eastern Tennessee; the surveillance data reported here provide a baseline for investigating this possibility. Health officials and practitioners need to be vigilant for increasing Lyme disease incidence in Tennessee.

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