In August 1966 a stained blood film from a California man suspected of having malaria revealed the presence of erythrocytic parasites of unusual morphology. The patient, who had had a splenectomy, had a clinical illness resembling malaria, was treated with chloroquine, and recovered. An epidemiologic history provided no evidence of exposure to malaria.
Animal inoculations with blood taken about 90 days after the onset of illness demonstrated no organisms. The indirect fluorescant-antibody test of the patient's serum was negative to 10 Plasmodium antigens, but Babesia antibodies were demonstrated in both complement-fixation and tube latex-agglutination tests. The serologic evidence for diagnosis of babesiosis was supported by the distinct morphologic appearance of the mature parasites. This is the second documented case of babesiosis in man in the world, both of which occurred in splenectomized persons.