Dorothy Wallace

A current model for Ixodes scapularis was revised and extended to represent the temperature dependent life cycle and host structure for Ixodes pacificus, the vector of the Lyme disease pathogen Borrelia burgdorferi in the Western United States. Parameters were adjusted to reflect data collected in Mendocino, CA. The hosts for I. pacificus include the Western Fence Lizard, which is not only an incompetent host for B. burgdorferi, but some studies claim that it eliminates it in ticks that feed on this lizard, making it an example of a so-called refractory host. The lizard host has been proposed as a reason for lower prevalence of Lyme disease in the West than in the northeast. This hypothesis was tested quantitatively by varying the number of lizard hosts. When refractory host densities are comparable to those reported for Mendocino County, CA, resulting disease risk is lowered roughly in proportion to the total questing infected nymph population, and that proportion corresponds to the ratio of Lyme disease prevalence in Mendocino to that of the northeast. Model results suggest that the presence of refractory hosts produces a dilution effect for B. burgdorferi infection and is likely to be the reason for observed lower prevalence of Lyme disease in the Western United States.