Serosurvey for human babesiosis in New York.

Healthy individuals at high risk of exposure to ticks were surveyed to determine the prevalence and incidence of asymptomatic babesiosis on Shelter Island, New York, during a single transmission season. Paired sera obtained in June and October 1978 were tested for antibodies to Babesia microti by indirect immunofluorescence. Point prevalence values of 4.4% in June and 6.9% in October were obtained. Six of 102 persons tested in both months showed at least a fourfold rise in titer of antibodies to B. microti (an incidence of 5.9% for the season). None of 300 serum specimens obtained from the New York City metropolitan area had significant titers to B. microti. Of the six persons who seroconverted, four gave a history of tick bite during the transmission season; three of the four persons claimed to recognize the tick as an Ixodes dammini, the vector for B. microti.