Lookback investigations of Babesia microti-seropositive blood donors: seven-year experience in a Babesia-endemic area.

Background: Human babesiosis in the United States is primarily attributable to infection with the intraerythrocytic protozoan parasite, Babesia microti. Transfusion-transmitted Babesia (TTB) is a mounting blood safety concern; approximately 100 US cases of TTB have been reported since 1980. In response, market withdrawal (MW) and/or lookback (LB) has been advocated for cellular components derived from Babesia-positive blood donors.

Study Design And Methods: Immunofluorescence assay (IFA) and selective polymerase chain reaction (PCR) testing of Connecticut donors was conducted from 1999 through 2005. MW/LB was initiated following established procedures on cellular components derived from IFA and/or PCR-positive donors. Recipients of these associated components were offered IFA and PCR testing for B. microti.

Results: A total of 208 seropositive donors were identified, with 474 donations and 656 cellular components subject to MW/LB. Sixty-three recipients were tested for B. microti; eight (12.7%) were IFA and/or PCR positive. A significantly higher proportion of B. microti-positive recipients were identified by LB in 1999 to 2000 (5 of 15, 33.3%) than after implementation of seropositive donor deferral in 2001 (3 of 48, 6.3%). Significant differences in positive LBs were also found when comparing index (50% positive) to previous donations (7.3% positive), and when comparing demonstrably parasitemic to nonparasitemic donors, 33.3 and 2.9%, respectively.

Conclusions: 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.