Approximately 95% of Lyme disease cases in the United States occur in the Northeast. While Lyme disease is also endemic to the West coast, the large disparity in prevalence is often attributed to differences in climate suitability between these two regions. Inherent differences between the two vector species – Ixodes scapularis in the East and Ixodes pacificus in the West – as well as differences in virulence between the pathogen strains endemic to each region may contribute greatly to disease risk. However, these factors have not previously been investigated. In this study, we compare attachment rates and molting success between I. scapularis and I. pacificus, as well as pathogen acquisition rates between these species on both the East and West coast type strains of Borrelia burgdorferi, the causative agent of Lyme disease. In a fully crossed experimental design, we fed ticks of each species on mice infected with either B31 or CA4, the East and West coast type strains, respectively and quantified B. burgdorferi prevalence and loads in all replete ticks via qPCR. To compare vector behavior and physiology, we fed known numbers of I. pacificus and I. scapularis on uninfected Peromyscus spp. mice and recorded engorgement and molting rates.
Results/Conclusions
We found significant differences in vector behavior and pathogen acquisition between the two vector species. Namely, I. scapularis had significantly higher attachment rates than I. pacificus and higher prevalence of B. burgdorferi for each strain. We found no significant differences in molting rates between the two species. Additionally, we found no significant difference in pathogen acquisition between strains for a given vector species and no significant difference in B. burgdorferi loads between any groups. Given that pathogen colonization did not differ by type strain, these results suggest that regional differences in Lyme disease prevalence are unlikely to be driven by pathogen-specific differences found in these areas. However, we do find support for vector-specific differences in terms of biting behavior and overall vector competence. The relatively greater ability of I. scapularis to acquire B. burgdorferi from an infected host may be a previously overlooked but important factor explaining the higher prevalence of Lyme disease on the East coast, and prompts further investigation into causal factors underlying these observed differences. A deeper understanding of the factors governing vector behavior and competence is needed to inform the development of novel strategies aimed at mitigating the risk of tick-borne diseases.