Genotypic associations of Borrelia burgdorferi in mammalian and avian hosts
Borrelia burgdorferi s. s., the bacterium that causes Lyme disease in North America, occurs as multiple co-circulating genotypes of the ospC gene, an important antigenic outer surface protein C. The diversity of ospC genotypes is thought to arise from fairly specific associations between genotypes and vertebrate hosts, such that different host species act as different “ecological niches” for the pathogen. To evaluate the degree of specificity of B. burgdorferi-host associations, we sampled genotypes of bacteria transmitted to ticks by several mammalian and avian host species. We also examined how the subset of genotypes known to infect humans, considered as human-invasive strains (HIS), is distributed among host species and higher taxonomic levels (birds, shrews, rodents). We adapted a patch occupancy model used for species detection to test for the occurrence probabilities (ψ) and transmission efficiencies (ε) associated with each ospC type. We found that the frequency of specific ospC genotypes varied among host species, demonstrating some support for the niche concept. Indeed, examination of all ospC types concurrently indicated that the ospC frequencies associated with birds and rodents were more similar within the taxonomic group than between groups based on principal components analysis. Conversely, HIS frequencies were more similar across all host species than within a host taxonomic level, implying weaker support for the niche concept. The occupancy model suggests that HIS types occurred more frequently than non-HIS types, but that HIS types generally had lower transmission efficiencies (from hosts to ticks). In several cases, rare genotypes had higher transmission efficiencies, suggesting an evolutionary trade-off between transmission efficiency and maintenance within the host. Our study highlights the importance of American robins (Turdus migratorius) and short-tailed shrews (Blarina brevicauda) in contributing large proportions of HIS types, and offers a novel way of examining occurrence and transmission efficiencies of ospC types within tick vectors, using a likelihood approach.