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BACKGROUND: Almost all of the reported US tick-borne and transfusion-associated Babesia cases have been caused by Babesia microti, which is endemic in the Northeast and upper Midwest. We investigated a case caused by B. duncani (formerly, the WA1-type parasite), in a 59-year-old California resident with sickle cell disease (HbSS) whose only risk factor for infection was receipt of red blood cell transfusions. CASE REPORT: The patient's case was diagnosed in September 2008: intraerythrocytic parasites were noted on a blood smear, after a several-month history of increasing transfusion requirements. Molecular and indirect fluorescent antibody (IFA) analyses were negative for B. microti but were positive for B. duncani (IFA titer, 1:1024). The complete 18S ribosomal RNA gene of the parasite was amplified from a blood specimen; the DNA sequence was identical to the sequence for the index WA1 parasite isolated in 1991. The patient's case prompted a transfusion investigation: 34 of 38 pertinent blood donors were evaluated, none of whom tested positive by B. microti IFA. The implicated donor-a 67-year-old California resident-had a B. duncani titer of 1:4096; B. duncani also was isolated by inoculating jirds (Mongolian gerbils) with a blood specimen from March 2009, more than 10 months after his index donation in April 2008. The patient's case was diagnosed more than 4 months after the implicated transfusion in May 2008. CONCLUSIONS: This patient had the third documented transfusion case caused by B. duncani. His case underscores the fact that babesiosis can be caused by agents not detected by molecular or serologic analyses for B. microti. [ABSTRACT FROM AUTHOR]