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Babesia infection can range from subclinical to severe. Symptoms, if any, usually develop within a few weeks or months after exposure but may first appear or recur many months later, particularly in persons who are or become immunosuppressed.

Clinically manifest Babesia infection is characterized by the presence of hemolytic anemia and nonspecific flu-like symptoms (e.g., fever, chills, body aches, weakness, fatigue). Some patients have splenomegaly, hepatomegaly, or jaundice.

Risk factors for severe babesiosis include asplenia, advanced age, and other causes of impaired immune function (e.g., HIV, malignancy, corticosteroid therapy). Some immunosuppressive therapies or conditions may affect the clinical manifestations (e.g., the patient might be afebrile). Severe cases can be associated with marked thrombocytopenia, disseminated intravascular coagulation, hemodynamic instability, acute respiratory distress, myocardial infarction, renal failure, hepatic compromise, altered mental status, and death.


Diagnosis of babesiosis requires a high index of suspicion, in part because the clinical manifestations are nonspecific. For acutely ill patients, the findings on routine laboratory testing frequently include hemolytic anemia and thrombocytopenia. Additional findings may include proteinuria, hemoglobinuria, and elevated levels of liver enzymes, blood urea nitrogen, and creatinine.

If the diagnosis of babesiosis is being considered, manual (non-automated) review of blood smears should be requested explicitly. In symptomatic patients with acute infection, Babesia parasites typically can be detected by light-microscopic examination of blood smears, although multiple smears may need to be examined. Sometimes it can be difficult to distinguish between Babesia and Plasmodium (especially P. falciparum) parasites and even between parasites and artifacts (such as stain or platelet debris). Consider having a reference laboratory confirm the diagnosis—by blood-smear examination and, if indicated, by other means, such as molecular and/or serologic methods tailored to the setting/species.

More on DPDx: Laboratory Diagnosis


Most asymptomatic persons do not require treatment. Treatment decisions should be individualized, especially for patients who have (or are at risk for) severe or relapsing infection.

For ill patients, babesiosis usually is treated for at least 7-10 days with a combination of two prescription medications — typically either:

  • Atovaquone PLUS azithromycin; OR
  • Clindamycin PLUS quinine (this combination is the standard of care for severely ill patients).

The typical daily doses for adults are provided in the table below.

Drug Adult dosage (usually treat for at least 710 days)
Atovaquone 750 mg orally twice a day
along with
Azithromycin On the first day, give a total dose in the range of 500–1000 mg orally; on subsequent days, give a total daily dose in the range of 250–1000 mg
Clindamycin 600 mg orally 3 times a day
300–600 mg intravenously 4 times a day
along with
Quinine 650 mg orally 3 times a day


Some patients—including those with severe illness—might require or benefit from supportive care, such as:

  • Antipyretics;
  • Vasopressors (if the blood pressure is low and unstable);
  • Blood transfusions;
  • Exchange transfusions (in which portions of a patient’s blood or blood cells are replaced with transfused blood components);
  • Mechanical ventilation; or
  • Dialysis



Atovaquone is in pregnancy category C. Data on the use of atovaquone in pregnant women are limited, and the risk to the embryo-fetus is unknown. Because data are available about safe administration of quinine plus clindamycin during pregnancy, this drug combination, rather than atovaquone (plus azithromycin), generally is recommended for treatment of symptomatic babesiosis during pregnancy, unless the preferred medications are not available or tolerated.

It is not known whether atovaquone is excreted in breast milk. Atovaquone should be used with caution in women breastfeeding infants who weigh <5 kg.

Atovaquone has been used safely in children who weigh > 5 kg.


Azithromycin is in pregnancy category B. Data on the use of azithromycin in pregnant women are limited. Azithromycin may be used during pregnancy in those patients who will clearly benefit from the drug.

According to a case report, azithromycin was excreted in breast milk and the nursing infant did not have adverse effects. Azithromycin should be used with caution in breastfeeding women, although the risk to the exposed infant probably is low.

In controlled clinical trials for various bacterial infections, oral azithromycin has been safely administered to pediatric patients aged 6 months to 16 years. Anecdotal cases of babesiosis in children, including infants, have been safely treated with azithromycin plus atovaquone.


Clindamycin is in pregnancy category B. Data on the use of clindamycin in pregnant women are limited, although no congenital anomalies have been reported. Clindamycin may be used during pregnancy in those patients who will clearly benefit from the drug.

Pregnancy Category B: Either animal-reproduction studies have not demonstrated a fetal risk but there are no controlled studies in pregnant women or animal-reproduction studies have shown an adverse effect (other than a decrease in fertility) that was not confirmed in controlled studies in women in the first trimester (and there is no evidence of a risk in later trimesters).

Clindamycin is excreted in breast milk. The American Academy of Pediatrics classifies clindamycin as usually compatible with breastfeeding.

The parenteral form of clindamycin contains benzyl alcohol, which has been associated with a fatal “gasping syndrome” in premature infants.

Prevention of Transfusion-associated Babesiosis

Before giving blood, potential blood donors are asked if they have ever been diagnosed with babesiosis. If the answer is “yes,” they are indefinitely deferred from donating blood. If you have a patient who has (or had) laboratory evidence of Babesia infection, advise the patient to refrain indefinitely from donating blood. If the patient recently donated blood, alert the appropriate blood collection agencies and public health authorities (i.e., local or state health department). In March 2018, FDA approved the first B. microti screening tests for U.S. blood donors, who can feel fine despite being infected.

Podcast: Babesiosis for Health Care Providers (3:15)

More on: Babesia in the Blood Supply

Page last reviewed: October 30, 2019