Merck Manual

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Richard D. Pearson

, MD, University of Virginia School of Medicine

Last full review/revision May 2019| Content last modified May 2019
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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 polymerase chain reaction (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 species transmitted by different ticks infect humans in areas of Europe. In Europe, B. divergens is the principle cause of babesiosis, typically 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 red blood cells; thus, Babesia can also be transmitted by blood transfusion, possibly by organ transplantation, and congenitally. A test to screen blood and organ donors for Babesia microti became available in the US in March 2018.

Ixodes ticks infected with Babesia are sometimes coinfected with Borrelia burgdorferi (which causes Lyme disease), Anaplasma phagocytophilum (which causes human granulocytic anaplasmosis [HGA]), Borrelia miyamotoi (which causes an HGA-like illness), or Powassan virus (a flavivirus that causes encephalitis). Thus patients occasionally acquire more than one infection from a tick bite.

Deer tick

Deer tick

Symptoms and Signs

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

When symptomatic, babesiosis usually starts after a 1- to 2-week 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 polymerase chain reaction-based tests

Most patients with babesiosis 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 polymerase chain reaction (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 species. PCR-based assays can help differentiate Babesia from Plasmodium falciparum if blood smear findings are ambiguous, confirm infection in patients with low parasitemia, and identify the Babesia species.


  • Atovaquone plus azithromycin

  • Quinine plus clindamycin

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

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

Quinine 650 mg orally 3 times a day plus clindamycin 600 mg orally 3 times a day 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 orally 3 times a day plus clindamycin 7 to 14 mg/kg orally 3 times a day. Quinine plus clindamycin is considered the standard of care for severely ill patients.

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


To prevent babesiosis, standard tick precautions should be taken by all people in endemic areas. Asplenic patients and patients with AIDS should be particularly cautious.

Tick bite prevention

Preventing tick access to skin includes

  • Staying on paths and trails

  • Tucking trousers into boots or socks

  • Wearing long-sleeved shirts

  • Applying repellents with diethyltoluamide (DEET) to skin surfaces

DEET should be used cautiously in very young children because toxic reactions have been reported. Permethrin on clothing effectively kills ticks. Frequent searches for ticks, particularly in hairy areas and on children, are essential in endemic areas.

Engorged ticks should be removed with care and not crushed between the fingers because crushing the tick may result in disease transmission. The tick’s body should not be grasped or squeezed. Gradual traction on the head with a small forceps dislodges the tick. The point of attachment should be swabbed with alcohol. Petroleum jelly, alcohol, lit matches, and other irritants are not effective ways to remove ticks and should not be used.

No practical means are available to rid entire areas of ticks, but tick populations may be reduced in endemic areas by controlling small-animal populations.

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 older or asplenic patients or in patients with AIDS or other immunocompromising conditions).

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

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

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

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