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Lyme disease, defined by erythema chronicum migrans and sometimes followed by neurologic, cardiac, or joint involvement, is known to have affected 512 patients in the United States. The
disease seems to occur in three distinct foci: along the northeastern coast, in Wisconsin, and in California and Oregon, a distribution that correlates closely with that of Ixodes dammini in the first two areas and with Ixodes pacificus in the last. The implicated tick, saved by six patients in the Northeast, was identified as nymphal I. dammini. Residence in or travel to endemic areas and history of tick bite may be important clues to diagnosis.