Causes and Consequences of Tick (Ixodes scapularis) Burdens on White-Footed Mice (Peromyscus leucopus)

White-footed mice (Peromyscus leucopus) are the principal hosts responsible for infecting ticks (Ixodes scapularis) with the spirochete that causes Lyme disease. In a 3-year field study, we examined the effects of density of mice and ticks on burdens of ticks per mouse and the proportion of the tick population successfully attaching to mouse hosts. We found that burdens of ticks per mouse were lowest in the year (1992) of highest densities of both mice and unattached (host-seeking) ticks. Movements by mice and presumably encounter rates with ticks, were suppressed at high density of mice. However, at high density of mice the population of ticks had a higher success rate in attaching to mice. Persistence of individual mice was positively correlated with burdens of ticks, indicating that heavy infestations of ticks do not reduce fitness of mouse hosts. Fluctuating densities of mice and ticks should strongly affect risk of exposure of humans to Lyme disease.