The impact of the spirochete Borrelia burgdorferi on white-footed mice: Implications for the ecology of Lyme disease

Background/Question/Methods

Parasitic infection can have diverse direct and indirect effects on host phenotype. The implications of such effects are magnified for zoonotic diseases, where changes in an animal host’s immunocompetence and intra- or interspecific interactions lead to altered disease ecology and may cascade into altered risk of human disease. Lyme disease afflicts thousands of people in northeastern US every year and is caused by the spirochete Borrelia burgdorferi. The bacterium is transmitted by blacklegged ticks that have fed on infected small mammals. In particular, white-footed mice (Peromyscus maniculatus) are known to be important components of the disease ecology of B. burgdorferi.  Previous work has indicated that space use by white-footed mice has important implications for tick encounter rates and therefore pathogen transmission, as well as incidental predation on invasive gypsy moths and on the eggs of native ground-nesting birds. Using a bait vaccine against B. burgdorferi, we immunized three field populations of white-footed mice against the pathogen within a Lyme-disease-endemic zone.  We examined the influence of vaccination on white-footed mouse immune function, and on indexes of space use, including activity levels and foraging behavior.

Results/Conclusions

We found that the vaccine induces elevated white blood cell counts in mice, and has subtle but potentially important effects on mouse behavior. Vaccination reduced wheel-running speed in lab settings, but did not lead to changes in field activity levels measured by track plates. In addition, mice on vaccinated grids showed less risk-sensitivity to predation in their foraging behaviors. Reduced activity suggests reduced space use by white-footed mice and decreased encounter rates with ticks and incidental prey items. In contrast, increased risky behavior suggests increased space use particularly in sites with low protective cover. The results of the experiment, therefore, suggest that the impact of vaccination on community ecology will depend on whether parasites and prey of mice are more or less likely to be encountered in the riskier sites frequented by vaccinated mice.