Human babesiosis: reservoir in infection on Nantucket Island.

Examination of blood films from six species of rodents and lagomorphs on Nantucket Island disclosed infections with Babesia microti in all of five Microtus pennsylvanicus (field mice) and 31 of 39 Peromyscus leucopus (white-footed or deer mice). Six human cases of clinical babesiosis have recently been diagnosed on the island.