Apparent range expansion of Ixodes scapularis in Virginia – inferences from field sampling and vector phylogenetic analysis

Background/Question/Methods

Black-legged ticks (Ixodes scapularis) serve as vectors for a number of pathogenic agents including viruses, bacteria, and eukaryotes.  Among the more notorious pathogens transmitted by this vector is Borrelia burgdorferi, the agent of Lyme disease, a zoonotic bacterium that is maintained in a wide variety of vertebrate host species in the eastern United States.  Recent modeling and empirical evidence suggests that Lyme disease is undergoing geographic expansion to the north, south, and west from its principal focus in the northeastern US.  Virginia is at the southern edge of the current expansion and has seen dramatic rise in human Lyme disease cases since 2007, potentially owing to a recent increase in vector abundance.  Ixodes scapularis is known throughout the eastern US but behavioral or physiological variation between northern and southern lineages might lead northern-variant ticks to more frequently parasitize (and thus infect) humans.  We hypothesized that the recent spatial and numerical increase in Lyme disease cases is associated with a southward expansion of northern-variant I. scapularis and that this spatial expansion will be detectable in population genetic signals.  We drag-sampled ticks at over 20 field sites distributed throughout central Virginia in summer 2012 and supplemented our collections with I. scapularis removed from deer at hunter check stations in fall 2011 and 2012 for the purposes of addressing spatial variation in occurrence and population genetic structure of this vector.

Results/Conclusions

We collected 16S sequence data from at least 25 I. scapularis individuals collected at field sites distributed across central Virginia from the coast through the front range of the Appalachian Mountains and north to the Maryland border.  We found significantly lower haplotypic diversity at high-elevation western sites than at eastern sites, potentially resulting from founder effects and/or demographic expansion in west-central Virginia.  Additionally, we detected mtDNA haplotypes exclusively associated with northern-lineage ticks at high elevation sites whereas eastern sites showed a mixture of northern- and southern-lineage I. scapularis.  Analysis of spatial distribution of ticks revealed that elevation and latitude are positively associated with I. scapularis presence.  These results are consistent with recent increases in human Lyme disease cases in the Blue Ridge Mountains of Virginia as well as recent data showing significantly higher density and B. burgdorferi infection prevalence at high-elevation sites in central Virginia.  These results lead us to conclude that northern-variant I. scapularis may be expanding into Virginia and that this expansion may account for recent increases in human disease.