Many pathogens employ a mechanism known as antigenic variation in order to avoid detection by the host’s immune response and infect the same host. Such antigenic variation is observed in an important virulence factor, the outer surface protein C (ospC), of Borrelia burgdorferi, the etiological agent of Lyme disease. Genotypic variation in B. burgdorferi ospC has important implications for human health as previous studies have found varying levels of human infectivity and dissemination to be associated with different genotypes. Specifically, ospC genotypes A, B, I, and K are associated with systemic disease in humans while other genotypes rarely cause human disease or cause only local infections. The fundamental processes maintaining the observed high levels of genetic diversity at the ospC locus remain unresolved, despite important public health implications. This study investigated the potential role of multiple niche polymorphism, a form of balancing selection in which ospC genotypes are partitioned by host type, in maintaining genetic diversity in this pathogen. To investigate these host-genotype associations, we characterized the frequency and distribution of ospC genotypes for three years in two Lyme endemic areas in Northern California. Questing Ixodes pacificus nymphal ticks and mammal tissue samples were collected between 2006 and 2009, screened for B. burgdorferi, and genotyped by sequencing the B. burgdorferi ospC locus.
Results/Conclusions
Our characterization of B. burgdorferi ospC genotype distribution revealed that ospC genotype A, a human invasive strain, had the highest prevalence among questing I. pacificus ticks in Marin County (37.4%) and genotype F, another human invasive strain, was the most prevalent in Sonoma County (36.1%). We also identified two genotypes in questing ticks (Hb and Db) not previously detected in California. These site-specific differences demonstrate differential spatial structure in the distribution of B. burgdorferi genotypes in different regions, however, the precise mechanisms driving these differences is unknown. Investigation into host-genotype associations identified a dominance of genotypes A and H in Neotoma fuscipes, the dusky-footed woodrat, collected in Marin County. Interestingly, genotypes A and H were also found in high prevalence in questing ticks collected from the same area suggesting a maintenance of these genotypes is may be driven by host-specialization to N. fuscipes. Further investigation in these areas is needed to determine the extent that host-genotype association is driving the geographic distribution and antigenic variation of ospC in B. burgdorferi.