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For further information, see CMDT Part 35–05: Babesiosis

Key Features

Essentials of Diagnosis

  • History of tick bite or exposure to ticks

  • Fever, flu-like symptoms, anemia

  • Intraerythrocytic parasites on Giemsa-stained blood smears

  • Positive serologic tests

General Considerations

  • An uncommon intraerythrocytic infection caused mainly by Babesia species and transmitted by Ixodes ticks

  • Most cases in the United States occurs in the coastal northeast, with some cases also in the upper midwest

  • The incidence of the disease appears to be increasing in some areas

    • Caused by Babesia divergens in Europe

    • Caused by Babesia venatorum in China

  • Can be transmitted by blood transfusion, but blood supplies are not screened

  • A survey of a large set of blood samples from endemic regions of the United States identified ~0.4% as potentially infectious for B microti

Clinical Findings

  • Serosurveys suggest that asymptomatic infections are common in endemic areas

  • With B microti infections

    • Symptoms appear 1 to several weeks after a tick bite

    • Parasitemia is evident after 2–4 weeks

  • Typical flu-like illness develops gradually and is characterized by

    • Fever

    • Malaise

    • Fatigue

    • Headache

    • Anorexia

    • Myalgia

  • Other findings may include nausea, vomiting, abdominal pain, arthralgia, sore throat, depression, emotional lability, anemia, thrombocytopenia, elevated transaminases, and splenomegaly

  • B divergens infections

    • Most cases seen in Europe in patients who have had splenectomy

    • These infections progress rapidly with

      • High fever

      • Severe hemolytic anemia

      • Jaundice

      • Hemoglobinuria

      • Acute kidney injury

    • Mortality rates are over 40%


  • Identification of the intraerythrocytic parasite on Giemsa-stained blood smears establishes the diagnosis

  • Repeated smears are often necessary because well under 1% of erythrocytes may be infected, especially early in infection, although parasitemias can exceed 10%

  • Diagnosis can also be made by PCR, which is more sensitive than blood smear

  • An indirect immunofluorescent antibody test for B microti is available from the CDC

    • Antibody is detectable within 2–4 weeks after the onset of symptoms and persists for months

    • A four-fold increase in antibody titer between acute and convalescent sera confirms acute infection



  • Standard therapy for mild to moderate disease is a 7-day course of atovaquone (750 mg orally every 12 hours) plus azithromycin (600 mg orally once daily), which is equally effective and better tolerated than the alternative regimen, a 7-day course of quinine (650 mg orally three times daily) plus clindamycin (600 mg orally three times daily)

  • However, there is more experience using quinine plus clindamycin, and this regimen is recommended by some experts for severe disease

Therapeutic Procedures

  • Exchange transfusion has been used successfully in severely ill asplenic patients and those with parasitemia > 10%



  • Severe complications are most likely to ...

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