DONOR INFECTIOUS DISEASE TESTING The impact of Babesia microti blood donation screening Laura Tonnetti, Rebecca L. Townsend, Barbara M. Deisting, James M. Haynes, Roger Y. Dodd, and Susan L. Stramer BACKGROUND: Babesia microti, an intraerythrocytic parasite endemic in the Northeast and upper Midwest United States, is responsible for over 200 reported cases of transfusion-transmitted babesiosis (TTB). The American Red Cross has prospectively screened donations in endemic areas for B. microti since 2012. METHODS: Blood donation samples from Massachusetts, Connecticut, Minnesota, and Wisconsin were tested by arrayed fluorescence immunoassay and real-time polymerase chain reaction. Donors with reactive results by any test were deferred and invited to participate in a follow-up study. RESULTS: Screening of 506,540 donations (June 2012–May 2018) yielded 1299 reactives, 177 of which were DNA and antibody positive and 25 DNA positive only. During the same time, 23 unscreened RBC units collected in Connecticut and Massachusetts were involved in TTB cases, making the risk of transmitting the infection from an unscreened donation in these two states 15.6-times greater than from a Babesia-negative unit. B. microti screening in Connecticut and Massachusetts has been associated with a reduction in TTB cases; none reported from blood donors residing in Connecticut since 2016. The positive donor rate has also decreased in Connecticut from 0.67% in 2013 to 0.23% in 2017. Ongoing follow-up testing has shown that only 10% of antibody-positive donors serorevert within 1 year, while 94% of polymerase chain reacton–positive donors become negative within 12 months. CONCLUSIONS: Blood donation screening for B. microti in endemic areas effectively mitigates TTB risk. Screening should be considered for all areas demonstrating ongoing risk defined as clinical cases or positive blood donors including those associated with TTB cases. B abesia microti, the intraerythrocytic parasite responsible for most of the babesiosis cases in the United States, is endemic in the northeastern and upper midwestern United States. B. microti is naturally transmitted by Ioxodes scapularis (i.e., the deer tick or black-legged tick), which often carries multiple pathogens, and is the vector responsible for transmission of the agents of Lyme disease and human granulocytic anaplasmosis. 1–5 The number of Babesia-infected ticks is on the rise, as well as the number of reported cases of babesio- sis, which became a notifiable disease in 2011. 6 The clinical manifestations of babesiosis are generally mild and non- specific(flulike, general malaise), but the infection may occur without symptoms. In the elderly and in individuals who are immunocompromised or are asplenic, the disease can be severe and life threatening. 7 B. microti can be transmitted by blood transfusion, and over 200 cases have been reported in the United States since 1979. 8,9 The majority of cases are from donors resident in seven known B. microti–endemic states (Massachusetts, New York, Connecticut, Michigan, Rhode Island, New Jersey, and Wisconsin), but in recent years, more cases are emerging from neighboring states such as Pennsylvania, New Hampshire, and Maine. 10–12 The American Red Cross has been screening blood donations for B. microti in Connecticut, Massachusetts, Minnesota, and Wisconsin since 2012 under an investigational test protocol, ABBREVIATIONS: AFIA = arrayed fluorescence immunoassay; FDA = Food and Drug Administration; IND = investigational new drug; NAT = nucleic acid test; PCR = polymerase chain reaction; TTB = transfusion-transmitted babesiosis; WP = window period From Scientific Affairs, American Red Cross Holland Laboratory, Rockville, Maryland. Address reprint requests to: Laura Tonnetti, PhD, Scientific Affairs, American Red Cross Holland Laboratory, 15601 Crabbs Branch Way, Rockville, MD 20855; e-mail: laura.tonnetti@ redcross.org Received for publication August 29, 2018; revision received October 3, 2018; and accepted October 3, 2018. doi:10.1111/trf.15043 © 2018 AABB TRANSFUSION 2019;59;593–600 Volume 59, February 2019 TRANSFUSION 593