B. microti prevalence followed expected geographical patterns. Screening was feasible with a performance comparable or superior to other infectious disease blood donor screening assays.

Foci of statistically higher B. microti seroprevalence among blood donors were observed; however, B. microti transfusion transmission risk exists for blood collected throughout Connecticut and portions of Massachusetts. Similarly, a seasonal peak was identified; nevertheless, seropositive donations were found year-round. Thus, geographic and/or seasonal exclusion methods are insufficient to fully safeguard the blood supply from Babesia transmission. Steps should be taken to reduce risk of transfusion-transmitted B. microti, perhaps through implementation of year-round, regional testing.

Recipients of components from B. microti-positive donors were infected via transfusion, with index donations from parasitemic donors posing the greatest transmission risk. This report of B. microti transmission detected through LB, coupled with ongoing TTB cases, indicates that interventions are needed to reduce transmission of B. microti to US blood recipients.

BACKGROUND Babesia infection is caused by intraerythrocytic tickborne parasites. Cases of transfusion-transmitted babesiosis have been increasingly recognized. To date, no Babesia test has been licensed for screening US blood donors. We conducted a longitudinal study to assess the course and markers of Babesia infection among seropositive donors identified in a seroprevalence study. STUDY DESIGN AND METHODS Eligible donors had B. microti indirect fluorescent antibody (IFA) titers ≥1:64. Enrollees were monitored up to 3 years, by IFA and three methods for evidence of parasitemia: B. microti nested PCR analysis (at two laboratories), hamster inoculation, and blood-smear examination. RESULTS Among 115 eligible donors, 84 (73%) enrolled. Eighteen enrollees (21%) had evidence of parasitemia for 30 total specimens (17% of 181), which were collected in 9 different months and tested positive by various approaches: PCR (25 specimens/16 persons), hamster inoculation (13 specimens/8 persons), and blood smear (1 specimen positive by all three approaches). Overall, 14 persons had ≥1 specimen with positive PCR results at both laboratories (12 persons) and/or had parasitologically confirmed infection (8 persons). Three of nine persons who had >1 specimen with evidence of parasitemia had nonconsecutive positives. Several enrollees likely had been infected ≥1 year when their last positive specimen was collected. The final three specimens for seven persons tested negative by all study methods, including IFA. CONCLUSION Seropositive blood donors can have protracted low-level parasitemia that is variably and intermittently detected by parasitologic and molecular methods. Donor-screening algorithms should include serologic testing and not solely rely on molecular testing.

We prospectively identified several real-time PCR-positive blood donors, including an IFA-negative real-time PCR-positive donor, in an area highly endemic for B. microti. These results suggest the need to include nucleic acid testing in planned mitigation strategies for B. microti.