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Host Contributions to the Force of Borrelia burgdorferi and Babesia microti Transmission Differ at Edges of and within a Small Habitat Patch

Habitat fragmentation is thought to be a main factor in the emergence of Lyme disease and other deer tick-transmitted infections. The patchwork of forest and edges promotes altered biodiversity, favoring the abundance of generalist rodents, such as white footed mice, heretofore considered a key tick and reservoir host in the northeastern United States. ABSTRACT In the northeastern United States, the emergence of Lyme disease has been associated, in part, with the increase of small forest patches. Such disturbed habitat is exploited by generalist species, such as white-footed mice, which are considered the host with the greatest reservoir capacity for the agents of Lyme disease (Borrelia burgdorferi sensu stricto) and human babesiosis (Babesia microti). Spatial risk analyses have identified edge habitat as particularly risky. Using a retrotransposon-based quantitative PCR assay for host bloodmeal remnant identification, we directly measured whether the hosts upon which vector ticks fed differed at the edge or within the contiguous small habitat patch. Questing nymphal deer ticks, Ixodes dammini, the northern clade of Ixodes scapularis, were collected from either the edge or within a thicket on Nantucket Island over 3 transmission seasons and tested for evidence of infection as well as bloodmeal hosts. Tick bloodmeal hosts significantly differed by site as well as by year. Mice and deer were identified most often (49.9%), but shrews, rabbits, and birds were also common. Ticks from the edge fed on a greater diversity of hosts than those from the thicket. Surprisingly, mice were not strongly associated with either infection at either sampling site (odds ratio [OR] < 2 for all). Although shrews were not the most common host utilized by ticks, they were highly associated with both infections at both sites (OR = 4.5 and 11.0 for B. burgdorferi and 7.9 and 19.0 for B. microti at the edge and thicket, respectively). We conclude that reservoir hosts may differ in their contributions to infecting ticks between edge and contiguous vegetated patches. IMPORTANCE Habitat fragmentation is thought to be a main factor in the emergence of Lyme disease and other deer tick-transmitted infections. The patchwork of forest and edges promotes altered biodiversity, favoring the abundance of generalist rodents, such as white footed mice, heretofore considered a key tick and reservoir host in the northeastern United States. We used tick bloodmeal analyses to directly identify the hosts from which nymphal deer ticks became infected. We demonstrate that there is considerable microfocality in host contributions to the cohort of infected ticks and that shrews, although they fed fewer ticks than mice, disproportionately influenced the force of pathogen transmission in our site. The venue of transmission of certain deer tick-transmitted agents may comprise a habitat scale of 10 m or fewer and depend on alternative small mammal hosts such as shrews.