Impacts of vector range expansion on pathogen transmission dynamics

Background/Question/Methods

For vector-borne pathogens, shifts in vector or reservoir host species distributions can affect interactions between reservoirs and ultimately impact pathogen transmission dynamics.  Likewise, introduction of alternative competent vectors can result in expanded opportunities for the pathogen to infect new host types if the introduced vector has different host preferences from the original vector species.  Ixodes scapularis, the principal vector for the Lyme disease bacterium in the eastern United States has existed in eastern Virginia for decades, but the congener I. affinis has only recently been reported in Virginia after apparently expanding from southern source populations.  I. affinis may be responsible for maintenance of B. burgdorferi in the absence of I. scapularis, but interactions between these species and variation in preference for different rodent hosts is largely unstudied in areas where both species occur.  Our goal was to sample rodents in an area of southeastern Virginia where both Ixodes species are now sympatric to explore differences in: 1) patterns of host parasitism, and 2) infection prevalence with and genotypic variation of B. burgdorferi between vector species.

Results/Conclusions

We have sampled several hundred small mammals in this system and preliminary results suggest that there are significant differences in infection prevalence with B. burgdorferi among mammalian species, with shrews among the most commonly infected species.  We have also detected differences in B. burgdorferi genotypic structure compared with other parts of Virginia where I. affinis does not occur.  In general, I. affinis is more abundant than I. scapularis as a small mammal parasite in southeastern Virginia, however, certain host species seem to be preferentially parasitized by one or the other species: cotton rats (Sigmodon hispidus) and harvest mice (Reithrodontomys humulis) were parasitized exclusively by I. affinis whereas house mice (Mus musculus) were parasitized primarily by I. scapularis. We did not see significant a difference in infection prevalence in host-derived ticks of each species (I. affinis = 17.1%, I. scapularis = 21.4%; G = 0.0002, P > 0.5).  We suggest that both tick species in this system are competent vectors of B. burgdorferi with substantial overlap in host species utilization.  We also suggest that the northward expansion of I. affinis may have introduced novel genotypic B. burgdorferi variants that are unobserved in areas where I. affinis is absent.  Future efforts will be aimed at using this dataset to parameterize matrix-based models to quantify impacts of each tick species on B. burgdorferi transmission dynamics.