Variation in the density of questing Ixodes pacificus (Acari:Ixodidae) nymphs infected with Borrelia burgdorferi at different spatial scales in California.

The density of, and prevalence of infection with Borrelia burgdorferi in, Ixodes pacificus nymphs as well as the density of infected nymphs were compared at 12 properties at a small rural community at high risk for Lyme disease (CHR) and at 12 areas at the University of California Hopland Research and Extension Center (HREC), Mendocino County, California. The mean infection prevalence and density of infected nymphs were 1.7% (range, 0-4.2%) and 0.10 infected nymphs per 100 m2 (range, 0-0.23 per 100 m2) at the HREC, and 12.4% (range, 3.9-41.3%) and 1.83 infected nymphs per 100 m2 (range, 0.29-22.17 per 100 m2) at the CHR. Thus, the mean density of infected nymphs differed 18-fold between CHR and HREC and 76-fold between properties at the CHR. Also, there was up to 10-fold variation in infection prevalence and 16-fold variation in density of infected nymphs between discrete areas within properties at the CHR. The high densities of infected nymphs recorded at the CHR suggest that, despite the low statewide incidence of Lyme disease, the medical community should be alerted that Lyme disease can be highly endemic in rural areas of northwestern California. The prevalence of spirochetal infection was higher for nymphs collected in southern/western, as compared to northern/eastern, exposures at both HREC and CHR. Infection prevalence and nymphal density were negatively associated at the HREC, whereas they tended to be associated positively at the CHR. A positive association was observed between nymphal density and density of infected nymphs when data from CHR and HREC were combined, and when data from the CHR were considered alone, but not for data from the HREC alone.