The pathogenic spirochete, Borrelia burgdorferi, is the causative agent of Lyme Disease (LD), the most prevalent vector-borne infection in the United States. A key virulence factor of the bacteria is attributed to its polymorphic outer surface protein C (ospC), which plays a role in pathogen transmission and host immune evasion. High genetic diversity in the ospC gene is hypothesized to be maintained by a type of balancing selection called multiple-niche polymorphism, where diverse vertebrate communities represent multiple ecological niches allowing for new genotypes to arise. A study looking at the infectivity of B. burgdorferi in rodents found that among three isolates (A, H3, and F) each varied in the transmission and dissemination of the spirochete. To date, 22 ospC genotypes have been described in the United States and of these 12 have been found in California and only three (A, F, and Hb) have been isolated from human patients from this region. As a result, the spatial distribution of ospC genotypes has important and direct impacts on human disease risk. In this study, we characterized the frequency and distribution of ospC genotypes in two diverse regions in Northern California and across several years. Questing Ixodes pacificus nymphal ticks were collected between 2006 and 2015, screened for B. burgdorferi, and genotyped by sequencing the ospC locus.
Contrary to previous studies in California that found the highest prevalence of ospC genotype H3 in Mendocino County, our preliminary data from Marin County revealed genotype A (48.8%) to be the most prevalent among I. pacificus ticks. In contrast, genotype F (39.4%) was found to be the most prevalent in Sonoma County. Notably, both Marin and Sonoma displayed human invasive strains in highest frequency whereas Mendocino showed low prevalence of human invasive strains in the tick population. Both Marin and Mendocino County also revealed greater ospC diversity compared to Sonoma County. Gathering a better understanding of the diversity in the ospC gene of B. burgdorferi circulating between I. pacificus, vertebrate hosts, and humans is important in understanding how LD is maintained and transmitted to greatly inform disease ecology.