Identifying the reservoir hosts of the Lyme disease spirochete Borrelia burgdorferi in California: the role of the western gray squirrel (Sciurus griseus)

Abstract

We investigated the role of the western gray squirrel (Sciurus griseus) as a reservoir host of the Lyme disease spirochete Borrelia burgdorferi. A survey of 222 western gray squirrels in California showed an overall prevalence of B. burgdorferi infection of 30%, although at a county level, prevalence of infection ranged from 0% to 50% by polymerase chain reaction. Laboratory trials with wild-caught western gray squirrels indicated that squirrels were competent reservoir hosts of the Lyme disease bacterium and infected up to 86% of feeding Ixodes pacificus larvae. Infections were long-lasting (up to 14 months), which demonstrated that western gray squirrels can maintain B. burgdorferi trans-seasonally. Non-native eastern gray squirrels (Sciurus carolinensis) and fox squirrels (Sciurus niger) were infrequently infected with B. burgdorferi.

Figure 1

Left, classification of Californian counties by Borrelia burgdorferi infection prevalence in western gray squirrels (Sciurus griseus). Gray = 0%; yellow = 1–25%; red = 26–50%; white = sample sizes < 3. Right, classification of Californian counties by Lyme disease incidence per 100,000 person-years, based on data for 1997–2006 from the California Department of Public Health. White = < 1; yellow = 1–5; red = > 5. This figure appears in color at www.ajtmh.org.

Figure 2

Relationship between western gray squirrel (Sciurus griseus) infection prevalence and human Lyme disease incidence (cases per 100,000 person-years) in California. Sample sizes varied (from 3 to 71) and can be found in Table 1.