Variation in black-legged tick (Ixodes scapularis) density and pathogen infection prevalence at a spatial expansion front

Background/Question/Methods

Lyme disease, caused by the bacterium Borrelia burgdorferi and transmitted in the eastern United States by the black-legged tick (Ixodes scapularis) is increasing in incidence and expanding geographically.  Recent environmental modeling efforts based on extensive field collections of host-seeking I. scapularis predicted a coastal distribution of ticks in mid-Atlantic states as well as an elevational limit of 510 m.  However, human Lyme disease cases are increasing most dramatically at higher elevation locations in Virginia, a state that is experiencing rapid emergence of this disease.  Our goal was to explore the incongruity between human case data and predicted and observed I. scapularis distribution.  We hypothesized that density of B. burgdorferi-infected ticks should be highest in locations associated with high disease incidence if epidemiological data represent endemic cases.  However, if ticks are limited by factors associated with higher elevations, we would expect higher densities of infected ticks in locations with fewer Lyme disease cases, suggesting that cases at high elevations are either mis-diagnosed or represent allochthonous exposure.  To test this hypothesis, we sampled I. scapularis nymphs at four field sites along an elevational gradient (~30 – 520 m) in central Virginia by dragging 1 x 1 m corduroy cloths along 10 fixed 100 m transects.  Each site was visited four times to capture temporal variation in nymphal host-seeking activity.  We used PCR amplification of the ospC gene to determine B. burgdorferi infection status in individual ticks.

Results/Conclusions

We found significantly higher densities of infected ticks at our highest elevation site (9.55 nymphs per 200 m²) and overall low densities (average 0.98 nymphs per 200 m² ) at lower elevation sites.  In light of recent studies that detected no ticks at high elevation sites in Virginia, our results suggest that 1) the range of I. scapularis is increasing to the south, mirroring that of observed Lyme disease cases in humans, 2) I. scapularis may not be limited by elevation, at least along the southern Lyme disease expansion front and 3) Lyme disease cases at higher elevation sites likely represent local exposure and enzootic maintenance of B. burgdorferi at these sites.  Specific mechanisms to account for I. scapuaris range expansion have not been identified but probably include dispersal and movement of the mammalian and avian host species on which these ticks feed.  Ultimately, identifying the environmental drivers of tick occurrence and expansion will be critical to understanding changes in human risk to tick-borne diseases.