Babesia are malaria-like protozoans that parasitize and reproduce within mammalian red blood cells. They have a complex life cycle involving several different stages and physical forms and are maintained in nature primarily via exchange between Ixodes ticks and various mammals. The first Babesia species was discovered in 1888 by Victor Babes, a Romanian pathologist in whose honor the organisms were subsequently named. Over 100 distinct species have since been identified within the Babesia genus, though only a few of these are currently known to be human pathogens.

Babesiosis has long been recognized as a disease of cattle and other domesticated animals, but the first human case was not described until 1957, when a young Croatian farmer contracted the illness and died some days later of renal insufficiency. In the late 1960s, the first North American cases appeared on Nantucket Island, and the disease is now recognized as an emerging and occasionally serious zoonosis in the United States.

Babesiosis has been reported in North and South America, Europe, and southern and eastern Asia. In the United States, the primary agent of human babesiosis is Babesia microti, which is transmitted by the bite of Ixodes scapularis, the same tick species that vectors Lyme disease. Cases of babesiosis caused by B. microti occur in southern New England and the northern Midwest. Additional cases of babesiosis caused by other species of Babesia occur primarily in the western U.S.; cases from Missouri and Kentucky have also been reported.

Clinically, babesiosis appears to have a wide spectrum of disease severity. Most patients experience a viral-like illness that can last weeks to months but which usually resolves fully. A significant minority of patients are entirely asymptomatic. In patients with a complicating condition, such as underlying immunosuppression, the disease course can be severe and potentially fatal. Some species of Babesia, such as B. divergens, appear to be more virulent than others.

Although primarily transmitted by tick bite, babesiosis can also be acquired via blood transfusion and maternal-fetal transmission.

Signs and Symptoms

In immunocompetent patients, symptoms of babesiosis usually begin one to six weeks after inoculation and are non-specific. Typical early manifestations include intermittent fevers accompanied by fatigue and malaise, headache, chills, and myalgias. Nausea, vomiting, reduced appetite, and depression can also occur. Some patients will develop enlarged livers or spleens. The usual disease course lasts weeks to several months, but some patients take even longer to fully recover. Coinfection with Lyme disease or anaplasmosis may complicate the clinical presentation and predispose the patient to more severe disease.

At the greatest risk for severe babesiosis are the elderly, asplenetic patients, patients with HIV or malignancies, and patients on immunosuppressive medications. In these populations, the disease course is longer and the fatality rate is in the neighborhood of 20%, even with proper antibabesial therapy. The most common serious complication of babesiosis is acute respiratory failure, but heart failure, liver and renal failure, disseminated intravascular coagulation, and coma are also well-recognized severe manifestations of babesiosis.


The fact that the early symptoms of babesiosis are largely non-specific makes diagnosis difficult. Nevertheless, physicians encountering a patient from an endemic area who presents with fever and a viral-like illness, especially in the summer months, should be alert to the possibility that babesiosis may be responsible for the patient’s symptoms.

While the physical exam is usually unremarkable, conventional blood tests can produce a pattern that suggests the diagnosis. Because the Babesia organisms cause lysis of red blood cells, patients will frequently develop hemolytic anemia, as well as lymphopenia and thrombocytopenia. Elevated serum lactate dehydrogenase levels are also common, and hyperbilirubinemia and an elevated erythrocyte sedimentation rate may also be present.

If babesiosis is suspected, microscopic examination of blood smears should be pursued. Giemsa or Wright stains are typically used. In early illness, the infection rate of erythrocytes can be less than 1%, so multiple smears over a period of days may be needed to confirm the diagnosis. Babesial DNA can also be detected by polymerase chain reaction (PCR) in cases where smears are negative but the diagnosis is still suspected.

Immunofluorescence (IFA) of IgM and IgG antibodies is sometimes employed to confirm a babesiosis diagnosis. However, antibodies to Babesia organisms can remain high for months or years after clinical resolution of illness, so the test is not a reliable indicator of active infection.

Centers for Disease Control & Prevention, 2009.

Cunha BA., 2008.