Geographical distribution patterns and habitat suitability models for presence of host-seeking ixodid ticks in dense woodlands of Mendocino County, California

Abstract

We used drag sampling to examine the geographical distribution patterns of ixodid ticks engaging in open (non-nidicolous) host-seeking behavior in dense woodland habitats of the climatically and ecologically diverse Mendocino County in north coastal California. The findings based on this sampling methodology reflect risk of human exposure to host-seeking ticks rather than the true distribution of the ticks. Drag sampling in 78 sites yielded 7,860 nymphal or adult Ixodes pacificus Cooley & Kohls, 220 Dermacentor occidentalis Marx, 150 Ixodes spinipalpis Hadwen & Nuttall, 15 Hemaphysalis leporispalustris (Packard), 12 Ixodes angustus Neumann, 12 Ixodes auritulus Neumann, and a single Dermacentor variabilis (Say). I. pacificus, which is the primary vector of the Lyme disease spirochete Borrelia burgdorferi to humans in California, occurred in all 78 sites examined. D. occidentalis, another tick species commonly biting humans in California, and H. leporispalustris typically were encountered in oak-associated woodlands in the central or eastern parts of the county. In contrast, three species of Ixodes ticks (I. angustus, I. auritulus, and I. spinipalpis) most commonly were found questing openly in woodlands with redwood present in the western part of the county. I. angustus and I. spinipalpis are occasional human biters and known experimental vectors of B. burgdorferi. Our study represents the first collection of large numbers of openly host-seeking I. spinipalpis ticks. Univariate tests of associations between presence of ticks (D. occidentalis, H. leporispalustris, I. angustus, I. auritulus, or I. spinipalpis) and environmental geographical information systems-remote sensing (GIS/ RS)-based data indicated that elevation, number of growing degree-days, and tasseled cap brightness, greenness, and wetness are especially useful predictors of presence of openly hostseeking ticks. Combinations of the above-mentioned GIS/RS-based data yielded significant logistic regression models for habitat suitability of host-seeking ticks for all five above-mentioned species. The model equations were used to create spatial surfaces of predicted presence of suitable habitat for openly host-seeking ticks in Mendocino County dense woodlands.