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The risk of humans acquiring
Lyme disease is a function of the local density of nymphal and adult ticks that are infected with
Lyme disease spirochetes. This in turn, will be related to host-use patterns of ticks and to the densities of both juvenile ticks and their hosts. At a forested site in Dutchess County, NY, we quantified host-use patterns of larval and nymphal Ixodes scapularis Say infesting the 2 dominant vertebrate hosts, white-footed mice and eastern chipmunks, during a 3-yr period. Larval tick burdens were 2-3 times higher on mice than they were on chipmunks, whereas nymphal tick burdens were > 3 times higher on chipmunks than they were on mice. We used multiple regression analysis to examine juvenile tick and host densities as independent variables influencing tick burdens. The density of questing larval ticks was positively correlated with larval tick burdens on mice, whereas the density of questing nymphs was weakly related to nymphal burdens on either host. Effects of the densities of mice and chipmunks on tick burdens were strong in some years, but weak in others. Moreover, the sign of the regression coefficients changed from one year to the next. We argue that these results are inconsistent with a passive encounter model of host selection, and suggest instead that either tick behavior or host responses cause strong biases in the distribution of juvenile ticks on their hosts.