This patient has asymptomatic babesiosis and should undergo repeat polymerase chain reaction (PCR) assay in 3 months. No treatment is indicated at this time, but the patient should be followed closely for clearance of the parasite. Babesiosis is a protozoal infection that is primarily spread through Ixodes ticks and is most commonly found among residents or travelers to the coastal northeastern United States. Following infection, the parasite lives inside erythrocytes. More than 150 cases of transmission have been documented through transfusion of blood products from asymptomatically infected donors. This probably significantly underestimates the true frequency of this route of transmission because up to 1% of donors from highly endemic regions are seropositive for Babesia microti. Microscopy using Giemsa or Wright staining on a thin blood smear will show trophozoites, appearing as ring forms inside erythrocytes. PCR is the most sensitive method for diagnosing infection, particularly when there is relatively low-level parasitemia, as is the case for asymptomatic infections. Although many Babesia infections resolve spontaneously, an asymptomatic infection that has persisted for more than 3 months after the initial diagnosis is an indication for treatment.

Patients with severe anemia and greater than 10% parasitemia may benefit from exchange transfusion, in which the patient's blood is removed by catheter and replaced with healthy donor blood, but would not be indicated in this patient with asymptomatic disease.

Symptomatic infection is an indication for treatment. Atovaquone combined with azithromycin is the recommended treatment for mild babesiosis. This regimen is equally effective as quinine plus clindamycin, with significantly fewer adverse effects. Severe disease is characterized by significant hemolysis and requires hospitalization for supportive care and monitoring. Quinine plus clindamycin is the preferred therapy for severe disease.