Differential tick burdens may explain differential Borrelia afzelii and Borrelia garinii infection rates among four, wild, rodent species in Hokkaido, Japan.

The ecologies of Lyme disease Borrelia spp. are very specific to location, as they are dependent upon the spirochete species and genotypes, the vectors and the host vertebrates present. In Hokkaido, Japan, where two human pathogenic, Lyme disease Borrelia spp. are present, and human cases are reported annually, the ecologies have been poorly studied. Our goal was to determine whether variation in borrelial infection rates among rodent species sharing an environment, is due to immunological or ecological differences. To this end, we examined the relationships between tick burden and borrelial infection, by including examination of agreement between nested PCR, as a test for infection, and serology, as a test for exposure. We collected 868 rodents, comprised of four species commonly found in Hokkaido, and tested for infection rates with Borrelia spp. using PCR for the borrelial flaB gene, seroprevalence of Borrelia afzelii and Borrelia garinii using ELISA, and attachment of ticks by direct counts. We noted a correlation between differential nymph and larval burdens and the borrelial infection rates found among the four rodent species. Furthermore, there was significant correlation between infection and seroprevalence of B. afzelii and B. garinii (P<0.01), between infection and Ixodes persulcatus nymph burden (P<0.01), and between seroprevalence and I. persulcatus nymph burden (P<0.01). The close agreement among rodent species seroprevalences with infection rates and tick burdens suggest the differences in infection rates of Borrelia spp. may largely be a direct consequence of differential exposure to vectors.