Clinical Information

Babesiosis is an emergent zoonosis caused by an intraerythrocytic protozoan in the genus Babesia. Babesia microti is responsible for the vast majority of human cases in the United States, with "hot spots" of disease along the Northeast Coast (eg, Martha's Vineyard, Long Island, and Nantucket) and the upper Midwestern states, although the distribution of disease is spreading. In addition, a small number of cases of Babesia duncani and Babesia duncani-like human infection (WA and CA strains) have been reported along Pacific Coast states from Washington to northern California, and Babesia divergens/Babesia divergens-like strains have been isolated from humans in Missouri (MO-1 strain), Kentucky, and Washington. At this time, only Babesia microti is a nationally notifiable disease.

Babesia microti shares a tick vector with Borrelia burgdorferi and Anaplasma phagocytophilum, the causative agents of Lyme disease and human granulocytic anaplasmosis (HGA), respectively. Recent studies suggest that exposure to Babesia microti is quite common in areas endemic for Lyme disease and anaplasmosis, so it is prudent to consider testing for all 3 diseases concurrently. Less commonly, babesiosis may be acquired through blood transfusion, and therefore donor units are tested for this parasite in some endemic areas.

Most patients with babesiosis have a mild illness or are asymptomatic, but some develop a severe illness that may result in death. Patient symptoms may include fever, chills, extreme fatigue, and severe anemia. The most severe cases occur in asplenic individuals and those over 50 years of age. Rare cases of chronic parasitemia, usually in immunocompromised patients, have been described.

The definitive laboratory diagnosis of babesiosis rests on the demonstration of Babesia species characteristic intraerythrocytic parasites in Giemsa-stained thick and thin blood films. This method is capable of detecting (but not differentiating) human-infective Babesia species.

Babesia may closely resemble those of Plasmodium falciparum. The Mayo Clinic real-time PCR assay provides a rapid and sensitive alternative to blood film examination and enables the detection and differentiation of Babesia microti, Babesia duncani/Babesia duncani-like and Babesia divergens/Babesia divergens-like parasites. It does not cross-react with malaria parasites. Finally, antibody testing may be used and is useful for detection of babesiosis, but may be negative in the early phase of illness and cannot distinguish active from past infection.