Development and persistence of antibody in persons infected with Babesia microti.

An indirect immunofluorescent test was used to determine antibody titers to Babesia microti in serum samples collected from 16 patients between 1 week and 1 year after the onset of illness. Titers rose to greater than or equal to 1:1,024 during the first few weeks of illness and then fell gradually to 1:16 to 1:256 over the next 5-6 months.