What is babesiosis?

Updated: Apr 01, 2021
  • Author: Rachel E Strength, MD; Chief Editor: Michael Stuart Bronze, MD  more...
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Babesiosis is a tick-borne, malaria-like illness caused by species of the intraerythrocytic protozoan Babesia. Humans are incidental hosts for Babesia when bitten by nymph or adult ticks. Babesia infection is most commonly seen in the north midwestern and northeastern United States. It can also be found throughout the world in certain parts of Europe, Asia, Africa, and South America. [1]

Human babesiosis is a zoonotic infection in which ticks transmit Babesia organisms from a vertebrate reservoir to humans [1, 2] ; humans are typically dead-end hosts. In the United States, most infections are caused by Babesia microti, a species commonly found in mice. Other species known to infect humans include B. duncani, B. divergens, B. venatorum, and B. crassa. [3]  

Babesia species and organisms of the closely related genus Theileria parasitize the erythrocytes of wild and domestic animals.These parasites are members of the order Piroplasmida, named for the pear-shaped forms found within infected red blood cells (RBCs). [4]

Over 2000 cases of babesiosis were reported in the US in 2018. [5]  In healthy individuals, most infections are asymptomatic. Several groups of patients become symptomatic, and, within these subpopulations, significant morbidity and mortality occur. The disease most severely affects patients who are elderly, immunocompromised, or asplenic. [3]

Babesiosis can be difficult to diagnose. Although the index of suspicion should be high in areas endemic for Babesia infection, patients with babesiosis have few, if any, localizing signs to suggest the disease. Confirmation of the diagnosis depends on the degree of parasitemia and the expertise and experience of laboratory personnel. [4, 6]

Most patients infected by B. microti who are otherwise healthy appear to have a mild illness and typically recover without specific chemotherapy.  Asymptomatic patients do not necessarily require treatment [7] ; the decision to treat should be an individualized one. For symptomatic cases, treatment is recommended. In addition, patients should be advised to take precautions against tick exposure and to refrain from donating blood until 2 years from the time of a reactive nucleic acid test result for Babesia. [6, 8]

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