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By Richard D. Pearson, MD, Emeritus Professor of Medicine, University of Virginia School of Medicine

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Babesiosis is infection with Babesia protozoa. Infections can be asymptomatic or cause a malaria-like illness with fever and hemolytic anemia. Disease is most severe in asplenic patients, the elderly, and patients with AIDS. Diagnosis is by identification of Babesia in a peripheral blood smear, serologic test, or PCR. Treatment, when needed, is with azithromycin plus atovaquone or with quinine plus clindamycin.

Endemic areas in the US include the islands and the mainland bordering Nantucket Sound in Massachusetts, Rhode Island, eastern Long Island and Shelter Island in New York, coastal Connecticut, and New Jersey, as well as foci in Wisconsin and Minnesota in the upper Midwest. Babesia duncani has been isolated from patients in Washington and California. A currently unnamed strain designated MO-1 has been reported in patients in Missouri. Other Babesia sp transmitted by different ticks infect humans in areas of Europe. In Europe, B. divergens is the principle cause of babesiosis in patients who have had a splenectomy.


In the US, Babesia microti is the most common cause of babesiosis in humans. Rodents are the principal natural reservoir, and deer ticks of the family Ixodidae are the usual vectors. Larval ticks become infected while feeding on an infected rodent, then transform into nymphs that transmit the parasite to another animal or to a human. Adult ticks ordinarily feed on deer but may also transmit the parasite to humans. Babesia enter RBCs, mature, and then divide asexually. Infected erythrocytes eventually rupture and release organisms that invade other RBCs; thus, Babesia can also be transmitted by blood transfusion, possibly by organ transplantation, and congenitally. Currently, no tests to screen for Babesia in blood donors are available.

Ixodes ticks infected with Babesia are sometimes coinfected with Borrelia burgdorferi (which causes Lyme disease), Anaplasma phagocytophilum (which causes human granulocytic anaplasmosis [HGA]), or Borrelia miyamotoi (which causes an HGA-like illness).

Deer tick.

Symptoms and Signs

Asymptomatic infection may persist for months to years and remain subclinical throughout its course in otherwise healthy people, especially those < 40 yr.

When symptomatic, the illness usually starts after a 1- to 2-wk incubation period with malaise, fatigue, chills, fever, headache, myalgia, and arthralgia, which may last for weeks. Hepatosplenomegaly with jaundice, mild to moderately severe hemolytic anemia, mild neutropenia, and thrombocytopenia may occur.

Babesiosis is sometimes fatal, particularly in the elderly, asplenic patients, and patients with AIDS. In such patients, babesiosis may resemble falciparum malaria, with high fever, hemolytic anemia, hemoglobinuria, jaundice, and renal failure. Splenectomy may cause previously acquired asymptomatic parasitemia to become symptomatic.


  • Light microscopy of blood smears

  • Serologic and PCR-based tests

Most patients do not remember a tick bite, but they may reside in or report a history of travel to an endemic region.

Babesiosis is usually diagnosed by finding Babesia in blood smears, but differentiation from Plasmodium species can be difficult. Tetrad forms (the so-called Maltese cross formation), although not common, are unique to Babesia and helpful diagnostically.

Serologic and PCR-based tests are available. Antibody detection by indirect fluorescent antibody (IFA) testing using B. microti antigens can be helpful in patients with low-level parasitemia but may be falsely negative in those infected with other Babesia sp.


  • Atovaquone plus azithromycin

  • Quinine plus clindamycin

Asymptomatic patients require no treatment, but therapy is indicated for patients with persistent high fever, rapidly increasing parasitemia, and falling Hct.

The combination of atovaquone and azithromycin given for 7 to 10 d has fewer adverse effects than traditional therapy with quinine plus clindamycin. Adult dosage is atovaquone 750 mg po q 12 h and azithromycin 500 to 1000 mg po the first day followed by a daily dose of 250 to1000 mg. In children > 5 kg, dosage is atovaquone 20 mg/kg po bid plus azithromycin 10 mg/kg po once, then 5 mg/kg/day for 7 to 10 days.

Quinine 650 mg po tid plus clindamycin 600 mg po tid or 300 to 600 mg IV 4 times a day for 7 to 10 days can also be used. Pediatric dosage is quinine 10 mg/kg po tid plus clindamycin 7 to 14 mg/kg po tid. Quinine plus clindamycin is considered the standard of care for severely ill patients.

Exchange transfusion has been used in hypotensive patients with high parasitemia.


Standard tick precautions (see Tick Bite Prevention) should be taken by all people in endemic areas. Asplenic patients and patients with AIDS should be particularly cautious.

Key Points

  • Endemic areas of babesiosis in the US include the coast and islands of southern New England and New Jersey as well as parts of the upper Midwest.

  • Babesiosis ranges from a mild, asymptomatic infection to a severe, life-threatening illness (mainly in the elderly and asplenic or immunosuppressed patients).

  • Symptoms resemble those of malaria, with prolonged fever, headache, myalgias, and sometimes jaundice.

  • Diagnose using light microscopy of blood smears and sometimes PCR-based tests.

  • Treat symptomatic patients with atovaquone plus azithromycin or, if symptoms are severe, quinine plus clindamycin.

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