Two Cases of Transfusion-Transmitted Anaplasma phagocytophilum

Anaplasma phagocytophilum, the causative agent of human granulocytic anaplasmosis, is an obligate intracellular bacterium most commonly acquired from tick bites. High seroprevalence rates in endemic regions suggest that transfusion transmission of A phagocytophilum would be a common event; however, only 2 cases have previously been reported. The exact cause of this discrepancy is not known. Whole blood leukocyte-reduction methods used by many blood centers are thought to reduce the risk of transfusion transmission of many pathogens, including A phagocytophilum. We report 2 additional cases of transfusion-transmitted A phagocytophilum in which leukocyte reduction of all transfused units failed to prevent microbial transmission.