A 65year old male with an 11 year history of chronic myeloid leukemia, well-controlled on dasatinib, presented complaining of a week of fevers, chills, worsening fatigue, and diarrhea, and 1 day of black stools and dark urine. Physical exam was remarkable for pallor and petechiae on dorsum of right hand. Analysis of peripheral blood showed anemia (hemoglobin 8.1g/dL), leukopenia (2.7x10^6/uL), thrombocytopenia (38x10^3/uL), mildly elevated total bilirubin (2.8mg/dL; 0.3 – 1.2mg/dL), elevated aspartate transaminase (135U/L; normal 15 - 41 U/L), markedly elevated C-reactive protein (>160mg/L; normal <7.4mg/dL), and markedly elevated ferritin (8286; normal 23.9 – 336.2ng/mL). Initial blood smear was unrevealing.

The patient’s condition acutely worsened and a bone marrow biopsy was performed to evaluate the cause of pancytopenia, with a clinical differential diagnosis that included transformation of CML to blast crisis, and hemophagocytic lymphohystiocytosis (HLH). Evaluation of the bone marrow aspirate showed no increase in blasts, but did show occasional erythrophagocytosis (panel A), and easily identifiable intra-erythrocyte parasites (panel B). Intracellular parasites were also seen on bone marrow core biopsy (panel C). Review of the initial blood smear demonstrated low-burden parasitemia (panel D). A repeat peripheral blood smear 3 days after presentation demonstrated severe disease, with a parasite burden estimated at 13.2% of erythrocytes infected with many extracellular parasites seen (panel E). Serologic testing for Babesia microti antibodies were positive (titers: IgG >1:256, IgM > 1:320; normal <1:16 and <1:20 respectively).

The patient recovered after red cell exchange transfusion and extended treatment with atovaquone and azithromycin. Babesia parasitemia may be undetectable on presentation, and serial blood smear reviews by a pathologist or trained professional are warranted if suspected, particularly in endemic areas.