BACKGROUND : Babesiosis is a tick-borne zoonosis caused by intraerythrocytic protozoa. More than 40 US cases of Babesia microti infection acquired by blood transfusion have been reported. This report describes the identification of a transfusion-associated case of babesiosis and the subsequent identification of the infected blood donor and three other infected recipients of cellular blood components from three other donations by this donor.

STUDY DESIGN AND METHODS : Serum specimens from the donors of blood that had been made into cellular components received by the index recipient and from other recipients of such components from the implicated donor were tested by the indirect fluorescent antibody (IFA) assay for antibodies to B. microti. Whole blood from IFA-positive persons was tested by PCR for B. microti DNA.

RESULTS : IFA testing of serum from 31 of 36 donors implicated a 45-year-old man (titer, 1 in 256), whose donation had been used for RBCs. He likely became infected when bitten by ticks while camping in Minnesota in June 1999 and had donated blood four times thereafter. As demonstrated by PCR, he remained parasitemic for at least 10 months. Of the five other surviving recipients of cellular blood components from the implicated donor, three recipients (one for each of the three other donations) had become infected through either RBC or platelet transfusions.

CONCLUSIONS : Babesiosis should be included in the differential diagnosis of posttransfusion febrile illness, and effective means for preventing transmission by blood transfusion are needed.