Lyme disease is caused by the spirochete Borrelia burgdorferi, which is maintained in an enzootic transmission cycle consisting of vertebrate reservoir species and Ixodes spp. ticks. In eastern North America, white-footed mice (Peromyscus leucopus) are among the most competent reservoir hosts, although other mammalian and avian species are also able to support and transmit B. burgdorferi infection. There is substantial genotypic variation among B. burgdorferi lineages and this variation is linked to differential dissemination in humans and differential persistence in some reservoir hosts. Songbirds are capable of infecting and transporting larval ticks over hundreds of kilometers, but it is unclear whether birds commonly transmit the B. burgdorferi genotypes that cause severe disease in humans. We investigated genotypic variation in B. burgdorferi detected in larval Ixodes scapularis ticks collected from songbirds to determine whether 1) birds are capable of transmitting all known B. burgdorferi genotypes and, 2) there is evidence for differential transmission of potentially ‘bird-adapted’ B. burgdorferi genotypes. We sampled birds at 19 sites throughout eastern North America and screened all bird-derived I. scapularis larvae for B. burgdorferi DNA by PCR amplification of the 16S-23S intergenic spacer region. We sequenced resulting amplicons and characterized the sequences by phylogenetic comparison to known variants.
I. scapularis ticks were detected on 517 individual birds sampled at 17 of 19 study sites and 609 larvae were collected from 276 individual birds collected from 11 sites. We detected Borrelia burgdorferi DNA in 98 out of 609 (16.1%) bird-derived I. scapularis larvae collected from 52 of 276 (18.7%) individual birds. We obtained unambiguous sequence data from 63 samples and found that eight of the nine major B. burgdorferi IGS groups are transmissible by birds, although not at equal frequencies. Comparison with genotype frequencies detected in 781 host-seeking I. scapularis nymphs collected throughout the eastern United States suggests that some IGS types are over-represented in birds, possibly indicating host-specialization to avian rather than mammalian hosts. However, genotype frequencies are known to vary in space and time, so site specific variation may also partly explain the observed genotype-host association patterns.