INFECTION WITH A BABESIA-LIKE ORGANISM IN NORTHERN CALIFORNIA

Background. Human babesiosis is a tick-transmitted zoonosis associated with two protozoa of the family Piroplasmorida: Babesia microti (in t he United States) and B. divergens (in Europe). Recently, infection wi th an unusual babesia-like piroplasm (designated WA1) was described in a patient from Washington State. We studied four patients in Californ ia who were identified as being infected with a similar protozoal para site. All four patients had undergone splenectomy, two because of trau ma and two for other medical reasons. Two of the patients had complica ted courses, and one died. Methods. Piroplasm-specific nuclear small-s ubunit ribosomal DNA was recovered from the blood of the four patients by amplification with the polymerase chain reaction. The genetic sequ ences were compared with those of other known piroplasm species. Indir ect immunofluorescent-antibody testing of serum from the four patients and from other potentially exposed persons was performed with WA1 and babesia antigens. Results. Genetic sequence analysis showed that the organisms from all four patients were nearly identical. Phylogenic ana lysis showed that this strain is more closely related to a known canin e pathogen (B. gibsoni) and to theileria species than to some members of the genus babesia. Serum from three of the patients was reactive to WA1 but not to B. microti antigen. Serologic testing showed WA1-antib ody seroprevalence rates of 16 percent (8 of 51 persons at risk) and 3 .5 percent (4 of 115) in two geographically distinct areas of northern California. Conclusions. A newly identified babesia-like organism cau ses infections in humans in the western United States. The clinical sp ectrum associated with infection with this protozoan ranges from asymp tomatic infection or influenza-like illness to fulminant, fatal diseas e.