Influence of elevation on Ixodes pacificus (Acari: Ixodidae) nymph seasonality, abundance, and Borrelia infection prevalence in the Sierra Nevada foothills.

The western blacklegged tick, Ixodes pacificus Cooley and Kohls, is a significant public health concern due to its capacity to vector Borrelia burgdorferi sensu stricto (Spirochaetales: Spirochaetaceae), the causative agent of Lyme disease. To determine the influence of elevation on the seasonality and abundance of Ixodes pacificus nymphs in the Sierra Nevada foothills of California, we used a standardized flagging procedure to sample nymphs from rocks and logs at 16 sites during 2018 to 2020. We sampled a total of 4,720 rocks and logs during 168 surveillance events and collected 1,469 I. pacificus nymphs. Nymphal abundance was similar on rock and log substrates. Tick abundance and seasonality varied by elevation, with lower elevation sites exhibiting higher nymphal abundance and earlier seasonal activity. Nymphal activity started later and persisted longer into the summer months at higher elevation sites. Nymphal abundance on rocks and logs was not predictive of adult abundance the following year, as estimated by drag sampling for adult ticks along hiking trails within the nymph sampling areas. Overall, 4.9% and 1.4% of the tested nymphs were infected with Borrelia burgdorferi s.l. and B. miyamotoi, respectively. We found no relationship between elevation and B. burgdorferi s.l. infection prevalence, although fewer nymphs were tested from higher elevation sites. These findings advanced our understanding of I. pacificus nymphal abundance and distribution and helped improve assessment of public health risks associated with tick exposure in the Sierra Nevada foothills.