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.