AbstractsBabesiosis is an intraerythrocyte parasitic infection, only recently described in infants. The course of the illness varies from mild, self-limited disease to more severe involvement with fever, hemolysis, and thrombocytopenia. Severely affected patients have required exchange transfusions or antimicrobial therapy or both. The present authors describe the clinical manifestations of babesiosis in an infant who apparently acquired the disease via perinatal transmission from her mother.The 4200-gm infant was born after a full-term pregnancy by elective cesarean section because of transverse lie. Her mother was bitten by a tick 1 week before delivery but remained asymptomatic. The infant continued in good health for a time. She was checked frequently for ticks, but no bites were noted.At 1 month of age, the infant became pale and irritable. One day later, she became febrile and was admitted to a local hospital with presumed sepsis. She was subsequently transferred to North Shore University Hospital because of increasing pallor. There is no family history of hemolytic anemia or hemoglobinopathies.Physical examination on admission revealed an alert but pale infant with a rectal temperature of 38.7°C, blood pressure of 98/62 mm Hg, pulse of 200 beats per minute, respiratory rate of 60 breaths per minutes, and weight of 4.8 kg. The liver and spleen were palpable 3 cm below the respective costal margins. There were no hemorrhagic lesions or tick bites.The initial laboratory findings included hemoglobin, 9.3 gm/dl, and leukocyte count, 6500 mm3. The differential count showed 10 per cent segmented neutrophils, 8 per cent immature neutrophils, 60 per cent lymphocytes, 11 per cent atypical lymphocytes, and 11 per cent monocytes. The platelet count was 38,000/mm3, and the reticulocyte count was 3.6 per cent. The total bilirubin concentration was 4.2 mg/dl, with a direct fraction of 0.6 creatinine kinase 282 units/liter (normal, 0–170 units/liter); lactic de-hydrogenase, 894 units/liter (normal, less than 600 units/ liter); alkaline phosphate, 468 units/liter (normal, 109–265 units/liter); serum glutamic oxaloacetic transaminase, 90 units/liter (normal, 14–70 units/liter); and serum glutamic pyruvic transaminase, 90 units/liter (normal 0–54 units/liter). Blood, urine, and cerebrospinal fluid cultures were negative, and stool was guaiac negative.Babesia microti organisms were found in 5 per cent of the erythrocytes on the initial Wright stain of the peripheral smear. The mother's peripheral blood smears at the time of delivery, and 30 and 32 days later, revealed no parasites. Serological studies in both mother and infant showed evidence of babesiosis.After blood, urine, and cerebrospinal fluid cultures were obtained, therapy was begun with ampicillin and pentamicin. Clindamycin phosphate (40 mg/kg/day intravenously in four divided doses) and quinine sulfate (25 mg/kg/day orally in three divided doses) were added after discovery of parasitized red blood cells. Ampicillin and gentamicin were discontinued on the third hospital day. On the fourth hospital day, the hematocrit dropped to 18 per cent, and packed red blood cells were administered. By the fifth hospital day, the platelet count was 102,000/mm3, hepa-tosplenomegaly had been resolved, and the parasitemia was less than 1 per cent. The infant was discharged the next day, to complete a 10-day course of orally administered clindamycin and quinine. She continues to do well 6 months later.