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Human babesiosis is a tick-transmitted zoonosis associated with two protozoa of the family Piroplasmorida: Babesia microti (in the United States) and B. divergens (in Europe). Recently, infection with an unusual babesia-like piroplasm (designated WA1) was described in a patient from Washington State. We studied four patients in California who were identified as being infected with a similar protozoal parasite. All four patients had undergone splenectomy, two because of trauma and two for other medical reasons. Two of the patients had complicated courses, and one died.

Piroplasm-specific nuclear small-subunit ribosomal DNA was recovered from the blood of the four patients by amplification with the polymerase chain reaction. The genetic sequences were compared with those of other known piroplasm species. Indirect immunofluorescent-antibody testing of serum from the four patients …