When left untreated, silent babesial infection may persist for months or even years. Although treatment with clindamycin and quinine reduces the duration of parasitemia, infection may still persist and recrudesce and side effects are common. Improved treatments are needed.

To determine whether a unique group of clinical and laboratory manifestations characterize certain major deer tick-transmitted human pathogens in North America, we compared the symptoms, short-term complications, and laboratory test results of New England residents who became ill due to > or =1 of these pathogens. Patients completed a uniformly structured questionnaire and submitted blood samples for serologic and polymerase chain reaction (PCR) testing after developing symptoms of Lyme disease, human babesiosis, or human granulocytic ehrlichiosis (HGE). Complete blood count with thin blood smear, PCR, and immunoglobulin M antibody tests helped differentiate the acute manifestations of these diseases. Physicians should consider use of tests designed to diagnose babesiosis and HGE in patients with Lyme disease who experience a prolonged flulike illness that fails to respond to appropriate antiborrelial therapy.

Human infection due to Babesia microti has been regarded as infrequent and a condition primarily affecting the elderly or immunocompromised. To determine whether risk in endemic sites may be increasing relative to that of Borrelia burgdorferi and to define its age-related clinical spectrum, we carried out a 10-year community-based serosurvey and case finding study on Block Island, Rhode Island. Less intensive observations were conducted in nearby sites. Incidence of babesial infection on Block Island increased during the early 1990s, reaching a level about three-fourths that of borrelial infection. The sera of approximately one-tenth of Block Island residents reacted against babesial antigen, a seroprevalence similar to those on Prudence Island and in southeastern Connecticut. Although the number and duration of babesial symptoms in people older than 50 years of age approximated those in people 20 to 49 years of age, more older adults were admitted to hospital than younger adults. Few Babesia-infected children were hospitalized. Babesial incidence at endemic sites in southern New England appears to have risen during the 1990s to a level approaching that due to borreliosis.