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Key Features

  • Babesia microti is a malaria-like protozoan that infects humans bitten by infected Ixodes scapularis (deer tick)

  • After inoculation, the protozoan penetrates erythrocytes and starts an asynchronous cycle that causes hemolysis

  • In the United States, the majority of cases occur in the Northeast and upper Midwest from May to October

  • Infection is a transfusion-transmissible disease

  • Incubation period is 1–4 weeks after tick bite, or 1–9 weeks after blood transfusion

Clinical Findings

  • Approximately half of infected children are asymptomatic

  • Symptoms are nonspecific and most commonly include

    • Sustained or cyclic fever up to 40.9°C

    • Shaking chills

    • Sweats

  • Other associated nonspecific symptoms include

    • Malaise

    • Fatigue

    • Anorexia

    • Arthralgias

    • Myalgias

    • Headache

  • Physical examination findings are usually minimal but may include

    • Hepatosplenomegaly

    • Jaundice

    • Dark urine

  • Disease is usually self-limited, causing symptoms for 1–2 weeks with fatigue that may persist for months

  • Hemolytic anemia and thrombocytopenia are common


  • Babesial parasites are identified in blood by microscopic evaluation of thin or thick blood smears or by polymerase chain reaction (PCR) amplification of babesial DNA

  • Babesia parasites are intraerythrocytic organisms that resemble Plasmodium falciparum ring forms

  • The tetrad form (Maltese cross), if visualized, is pathognomonic

  • Specific serologic tests are also available through the Centers for Disease Control and Prevention (CDC)


  • Azithromycin (10 mg/kg up to 500 mg on the first day, followed by 5 mg/kg up to 250 mg/day) in combination with atovaquone (20 mg/kg, up to 750 mg, twice a day) for 7–10 days

    • Treatment of choice for mild to moderate disease

    • Causes fewer adverse side effects than other regimens.

  • Clindamycin (10 mg/kg, up to 600 mg, every 8 hours) in combination with quinine (8 mg/kg, up to 650 mg, every 8 hours) is standard of care for severely ill patients

  • Longer courses of treatment may be needed in immunocompromised patients

  • Partial or complete RBC exchange transfusion is indicated for persons with

    • Severe babesiosis, as indicated by high-grade parasitemia (≥ 10%)

    • Significant hemolysis

    • Renal, hepatic, or pulmonary compromise

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