A spatially-explicit model of acarological risk of exposure to Borrelia burgdorferi-infected Ixodes pacificus nymphs in northwestern California based on woodland type, temperature, and water vapor

In the far-western United States the nymphal stage of the western black-legged tick Ixodes pacificus has been implicated as the primary vector to humans of Borrelia burgdorferi sensu stricto (hereinafter referred to as B burgdorferi) the causative agent of Lyme borreliosis in North America In the present study we sought to determine if infection prevalence with B burgdorferi in I pacificus nymphs and the density of Infected nymphs differ between dense-woodland types within Mendocino County California and to develop and evaluate a spatially-explicit model for density of Infected nymphs in dense woodlands within this high-Incidence area for Lyme borreliosis In total 4 9% (264) of 5431 1 pacificus nymphs tested for the presence of B burgdorferi were infected Among the 78 sampling sites infection prevalence ranged from 0% to 22% and density of infected nymphs from 0 to 2 04 per 100 m(2) Infection prevalence was highest in woodlands dominated by hardwoods (62%) and lowest for redwood (1 9%) and coastal pine (0%) Density of infected nymphs also was higher in hardwood-dominated woodlands than in conifer-dominated ones that included redwood or pine Our spatial risk model which yielded an overall accuracy of 85% indicated that warmer areas with less variation between maximum and minimum monthly water vapor in the air were more lil ely to include woodlands with elevated acarological risk of exposure to infected nymphs We found that 37% of dense woodlands in the county were predicted to pose an elevated risk of exposure to infected nymphs and that 94% of the dense-woodland areas that were predicted to harbor elevated densities of infected nymphs were located on privately-owned land Published by Elsevier GmbH