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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. The lizard host has been proposed as a reason for lower prevalence of Lyme disease in the West. This hypothesis was tested quantitatively by varying the number of lizard hosts. 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.