Transfusion-associated babesiosis: shouldn't we be ticked off?

H uman babesiosis in the United States is attributable almost exclusively to infection with the intraerythro-cytic protozoan parasite Babesia microti. The primary mechanism of parasite transmission to humans is by the bite of an infected deer/black-legged tick, Ixodes scapularis, the same tick that serves as the vector for Lyme borreliosis, human granulocytic anaplasmosis, and several other tick-borne diseases (1). Babesiosis is often described as an emerging infectious disease in the United States. The first documented case of clinical disease caused by B. microti was reported on Nantucket Island, Massachusetts, in 1969 (2). Since then, hundreds to thousands of babesiosis cases have been described, leading the Council of State and Territorial Epidemiologists to recommend that it be designated as a nationally notifiable disease beginning in January 2011 (3). Geographically , B. microti is limited primarily to endemic areas in the Northeast (Connecticut, Massachusetts, New Jersey, New York, and Rhode Island) and upper Midwest (Minnesota and Wisconsin); another species, B. duncani, is reported on rare occasions in California and Washington. Infections with B. microti produce a spectrum of disease , ranging from asymptomatic, self-resolving infections to severe, life-threatening illnesses that are often dictated by the host's immune status. Common signs and symptoms of babesiosis include fever, headache, chills, drenching sweats, myalgia, malaise, and hemolytic anemia. More severe cases frequently occur in immunocompromised populations , including neonates and infants, elderly persons, and asplenic patients. Complications of babesiosis may include acute respiratory distress, severe hemolysis, disseminated intravascular coagulation, renal dysfunction, hepatic compromise, myocardial infarction, and death. Concomitant with the emergence of babesiosis as a public health concern has been a proliferation of cases of transfusion-associated babesiosis. The first documented case of transfusion-transmitted Babesia was reported in Boston in 1979 (4). Subsequently, cases of babesiosis associated with blood transfusion have rapidly increased. For many years, based on largely anecdotal information, the number of transfusion-associated cases was estimated to be approximately 70. Determining an accurate tally of cases was deemed difficult because many transfusion cases were not reported in an organized manner or were not considered novel enough to justify publication. In this issue, Her-waldt and colleagues (5) have compiled an exhaustive list of known cases (n ϭ 162) of transfusion-associated babe-siosis from 1979 to 2009, which has important implications for public health and transfusion medicine. In the absence of a feasible and approved method for screening the blood supply, B. microti has become the infectious …