Identifying counties with suitable habitat for host-seeking Borrelia burgdorferi sensu stricto-infected Ixodes pacificus nymphs across seven states in the western United States

Background Tick surveillance is used to monitor acarological risk for Lyme disease. Most human infections arise from the bite of a Borrelia burgdorferi sensu stricto (Bbss)-infected nymphal tick. Tick surveillance records documenting the densities of host-seeking Bbss-infected nymphs are limited in the western US, where Ixodes pacificus (Cooley and Kohls, 1943) is the primary vector. Using available tick surveillance records, we sought to identify counties with environmental conditions to support enzootic transmission of Bbss and nymphal host-seeking activity above vegetation, a tick behavior that puts humans at elevated risk for encountering nymphs. Methods To supplement tick surveillance data available in the ArboNET Tick Module national database maintained by the US Centers for Disease Control and Prevention, we reviewed the literature and public health databases to determine the county-level presence of host-seeking nymphs and separately collated records of Bbss-infected I. pacificus in nymphs and adults across seven western states. Host-seeking I. pacificus nymphs were recorded in 54 counties while Bbss-infected I. pacificus were recorded in 36 counties across California, Oregon, and Washington. We developed separate habitat suitability models to identify counties where host-seeking nymphs and Bbss- infected I. pacificus are likely found. Suitability thresholds of all modeling algorithms were set to yield 90% sensitivity, and the models were overlaid to identify counties where the habitat was considered suitable for Bbss-infected I. pacificus host-seeking nymphs. Results The range of suitable habitat for Bbss-infected host-seeking nymphs was found mostly in central and northern California as well as coastal Washington and Oregon and was more limited than the overall range of I. pacificus . Specifically, 113 counties were classified as suitable for I. pacificus in a previous modeling effort, but only 76 counties were characterized as suitable for detecting Bbss-infected host-seeking nymphs by at least one of our models. Conclusions Our modeling effort highlights potentially suitable habitat for encountering Bbss-infected I. pacificus nymphs in Pacific coast states, but it does not differentiate magnitudes of risk within or among counties. As a next step to improving acarological risk assessment in the western US, we highlight counties where enhanced tick surveillance is warranted.