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Babesia microti

Babesiosis is a tickborne zoonotic disease that is caused primarily by the intraerythrocytic parasite Babesia in the United States. The first case of human babesiosis acquired in the United States was identified in 1969 on Nantucket Island, Massachusetts. Since then, Babesia microti has become endemic in New England and regions of the upper Midwest. A small number of cases caused by infection with Babesia duncani have been detected in Pacific Coast states from Washington to northern California. Babesia divergens has been detected in humans in Missouri, Kentucky, and Washington.

The primary animal reservoir for Babesia microti is the white-footed mouse, Peromyscus leucopus. Humans most commonly become infected after being bitten by a Black legged tick, Ixodes scapularis, that has obtained a blood meal from an infected mouse. Human transmission can also occur through blood transfusions, transplantation of organs from infected donors, or congenital (mother-to-child) transmission. Babesia sporozoites enter erythrocytes and are then disseminated throughout the reticuloendothelial system. Parasitemia typically lasts from 2 to 7 months but may persist form more than 2 years.

The clinical manifestations of B. microti infection range from asymptomatic infection to severe disease. Common symptoms include fatigue, malaise, weakness, fever, nausea, anorexia, myalgias, abdominal pain, and diarrhea. Severe complications can occur including hemolysis, thrombocytopenia, renal failure, acute respiratory distress syndrome, shock, and death. The risk of severe infection is particularly high among patients who do not have a functional spleen or are immunocompromised.

Most asymptomatic persons do not require treatment. Symptomatic patients can be treated using a combination of antimicrobial medications, such as azithromycin and atovaquone. Red blood cell exchange might be necessary for patients with severe hemolytic anemia.

Abnormal laboratory test results, such as thrombocytopenia, hemolytic anemia, and elevated aminotransferase and serum alkaline phosphatase levels, may occur, but none of these results are highly sensitive for the diagnosis of babesiosis. Urinalysis may reveal proteinuria and hemoglobinuria.

Babesiosis is usually diagnosed by observing the organisms in infected RBC on Giemsa-stained peripheral blood smears. An indirect IgG antibody test may also be useful if the parasitemia is too low to detect microscopically or if the infection has cleared naturally or following treatment.

If the diagnosis of babesiosis is being considered, a pathologist review of a blood smear should be requested. In symptomatic patients with acute infection, Babesia parasites typically can be seen inside erythrocytes. Sometimes it can be difficult to distinguish Babesia from Plasmodium parasites. Patients with documented infections have usually had titers ranging from 1:320 to 1:2560. Reference value is <1:64. Real-time polymerase chain reaction assay is a highly sensitive method to detect Babesia DNA and to differentiate B. microti, B. duncani, and B. divergens.

Babesia is transmissible via blood transfusion, and persons who acquire babesiosis through contaminated blood have been shown to have significantly worse health outcomes and a higher risk for death than do those who acquire the disease from a tick bite. The incubation period for transfusion-related babesiosis ranges from 11 to 176 days (median, 37 days).

Obtaining a donor history is largely ineffective in preventing transfusion-transmitted babesiosis, because 80% of infected persons are asymptomatic and may remain parasitemic for several months. In addition, babesia species survive in red cells that are stored in either liquid or frozen form. In regions in which babesia is endemic, approximately 1 to 2% of blood donors have laboratory evidence of current or past infection, with the prevalence reaching 10% in some hyperendemic areas.

In 2019, the U.S. Food and Drug Administration (FDA) recommended screening blood donations for Babesia in states where residents were considered to be at high risk for Babesia infection. As a result, FDA recommended blood donation screening in the following 15 states or jurisdictions: Connecticut, Delaware, Maine, Maryland, Massachusetts, Minnesota, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, Virginia, Wisconsin, and the District of Columbia.

FDA recommends year-round regional nucleic acid testing (NAT) for blood collected in Connecticut, Delaware, Maine, Maryland, Massachusetts, Minnesota, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, Virginia, Wisconsin and Washington, D.C. Donors that test positive must be deferred for at least 2 years.

As an alternative to testing, FDA guidance allows for use of pathogen reduction technology (PRT) for platelets and plasma using an FDA-approved pathogen reduction device. 


Vannier E, Krause PJ: Human Babesiosis. N Engl J Med. 2012 Jun 21;366(25):2397-2407

Manian FA et al. Case 27-2016 — A 71-Year-Old Woman with Müllerian Carcinoma, Fever, Fatigue, and Myalgias. N Engl J Med 2016; 375:981-991September 8, 2016DOI: 10.1056/NEJMcpc1607091

Moritz ED et al. Screening for Babesia microti in the U.S. Blood Supply. N Engl J Med. 2016;375:2236-45

Swanson M, Pickrel A, Williamson J, Montgomery S. Trends in Reported Babesiosis Cases — United States, 2011–2019. MMWR Morb Mortal Wkly Rep 2023;72:273–277. DOI:

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