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Babesia microti is a parasite that infects red blood cells (RBCs) in mammals. It is transmitted to humans by tick bites, transfusion, organ transplantation, and congenital acquisition. Although the Babesia natural history and seroprevalence in donors have been well described, gaps in knowledge relevant to transfusion remain.Mice were infected with dilutions of parasitized blood to address the minimal infectious dose and the kinetics of parasitemia by quantitative polymerase chain reaction (qPCR) and of antibodies by enzyme immunoassay.In immunocompetent DBA/2 mice infected with 100 parasitized RBCs (pRBCs) and in immunodeficient NSG mice infected with 63 pRBCs, parasitemia was detectable in five of five mice each. Peak parasitemia up to 2 × 10The mice in this study were highly susceptible to Babesia infection using as few as 1 to 2 log pRBCs and maintained chronic parasitemia. If the infectious dose in human transfusion recipients is comparably low, a highly sensitive assay targeting parasite RNA may safeguard the blood supply, particularly before antibody detection.

Babesiosis is a malaria-like illness caused by infection by members of the genus Babesia, a group of tick-borne intraerythrocytic protozoan parasites.1 Babesia microti is responsible for the overwhelming majority of human Babesia infections reported in the United States, where it is endemic in parts of the Northeast and upper Midwest. The parasite is primarily transmitted to humans through exposure to Ixodes scapularis (“deer ticks”) in endemic areas. However, B. microti is also readily transmissible by blood transfusion, and transfusion-transmitted babesiosis (TTB) is increasingly recognized as posing a risk to the blood supply.2,3 While B. microti infection results in asymptomatic or mild clinical findings in most immunocompetent hosts, infection in selected patient subsets, notably those who are immunosuppressed, asplenic, and/or at extremes of age, may lead to severe or even fatal disease.3,4 Overrepresentation of transfusion recipients among these high-risk groups accounts for the relatively high mortality ascribed to TTB.4,5 Currently, the only mandated strategy for TTB mitigation in use is a question regarding history of babesiosis posed directly to the potential donor before donation. The failure of this approach is evidenced by more than 150 cases of TTB that have been reported since 1979 with at least 12 fatalities since 2005.3 Despite being acknowledged as the foremost infectious risk to the US blood supply at present,5 there are as yet no validated, US Food and Drug Administration (FDA)-approved and commercially available tests for B. microti screening in blood donors. We report the development of a high-throughput enzyme immunoassay (EIA) that detects antibodies to B. microti, and B. microti seroreactivity was determined with this EIA in samples from New York Blood Center (NYBC) blood donors collected over a 4-month period in 2012. This pilot study was used to optimize the cutoff of the EIA and to validate the EIA and confirmatory algorithms before an FDA licensure trial launched in 2013.

BACKGROUND The tick-borne pathogen Babesia microti has become recognized as the leading infectious risk associated with blood transfusion in the United States, yet no Food and Drug Administration–licensed screening tests are currently available to mitigate this risk. The aim of this study was to evaluate the performance of an investigational enzyme immunoassay (EIA) for B. microti as a screening test applied to endemic and nonendemic blood donor populations. STUDY DESIGN AND METHODS The study aimed to test 20,000 blood donors from areas of the United States considered endemic for B. microti and 10,000 donors from a nonendemic area with the investigational B. microti EIA. Repeat-reactive samples were retested by polymerase chain reaction (PCR), blood smear, immunofluorescent assay (IFA), and immunoblot assay. In parallel, serum samples from symptomatic patients with confirmed babesiosis were tested by EIA, IFA, and immunoblot assays. RESULTS A total of 38 of 13,757 (0.28%) of the donors from New York, 7 of 4583 (0.15%) from Minnesota, and 11 of 8363 (0.13%) from New Mexico were found repeat reactive by EIA. Nine of the 56 EIA repeat-reactive donors (eight from New York and one from Minnesota) were positive by PCR. The specificity of the assay in a nonendemic population was 99.93%. Among IFA-positive clinical babesiosis patients, the sensitivity of the assay was 91.1%. CONCLUSION The B. microti EIA detected PCR-positive, potentially infectious blood donors in an endemic population and exhibited high specificity among uninfected and unexposed individuals. The EIA promises to provide an effective tool for blood donor screening for B. microti in a format amenable to high-throughput and cost-effective screening.