Babesiosis, a tickborne zoonotic disease caused by intraerythrocytic protozoa of the genus babesia, is characterized by nonimmune hemolytic anemia that resolves with antimicrobial treatment and clearance of parasitemia. The development of warm-antibody autoimmune hemolytic anemia (also known as warm autoimmune hemolytic anemia [WAHA]) in patients with babesiosis has not previously been well described.


After the observation of sporadic cases of WAHA that occurred after treatment of patients for babesiosis, we conducted a retrospective cohort study of all the patients with babesiosis who were cared for at our center from January 2009 through June 2016. Data on covariates of interest were extracted from the medical records, including any hematologic complications that occurred within 3 months after the diagnosis and treatment of babesiosis.


A total of 86 patients received a diagnosis of babesiosis during the 7.5-year study period; 18 of these patients were asplenic. WAHA developed in 6 patients 2 to 4 weeks after the diagnosis of babesiosis, by which time all the patients had had clinical and laboratory responses to antimicrobial treatment of babesiosis, including clearance of Babesia microti parasitemia. All 6 patients were asplenic (P<0.001) and had positive direct antiglobulin tests for IgG and complement component 3; warm autoantibodies were identified in all these patients. No alternative explanation for clinical hemolysis was found. WAHA required immunosuppressive treatment in 4 of the 6 patients.


We documented post-babesiosis WAHA in patients who did not have a history of autoimmunity; asplenic patients appeared to be particularly at risk.

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We thank Tania Torres for her assistance in the blood-bank review of the cases presented and Sam Telford III, for his critical input and review of an earlier version of the manuscript.

Source Information

From the Divisions of Infectious Diseases (A.E.W., M.W.M., K.D., S.V., J.H.M., F.M.M.), Transfusion Medicine (W.J.S.), and Hematology (M.O.A.), Brigham and Women’s Hospital, Harvard Medical School (A.E.W., M.W.M., W.J.S., M.O.A., J.H.M., F.M.M.), and Dana–Farber Cancer Institute (A.E.W., M.O.A., F.M.M.) — all in Boston.

Address reprint requests to Dr. Woolley at the Division of Infectious Diseases, Brigham and Women’s Hospital, 75 Francis St., Boston, MA 02115, or at .