Clinical Information

Babesiosis is a tick-transmitted zoonosis caused by intraerythrocytic protozoa in the genus Babesia. Babesia microti is responsible for the vast majority of human cases in the United States, with most cases occurring along the Northeast Coast and the upper Midwestern states. A small number of cases of Babesia duncani human infection have also been reported along Pacific Coast states from Washington to northern California, and Babesia divergens/B divergens-like strains have been detected in humans in Missouri (MO-1 strain), Kentucky, and Washington. In Europe, B divergens and Babesia venatorum are the primary causes of human babesiosis.

Humans most commonly acquire infection through the bite of an infected tick. The most common tick vectors in the United States are Ixodes scapularis and Ixodes pacificus, while Ixodes ricinus and other ticks transmit the parasite in Europe and Asia. Less commonly, babesiosis may be acquired through blood transfusion and across the placenta from the mother to the fetus.

Most patients with babesiosis are asymptomatic or have only a self-limited, mild, flu-like illness, 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.

Babesiosis is conventionally diagnosed through microscopic examination of Giemsa-stained thick and thin peripheral blood films looking for characteristic intraerythrocytic Babesia parasites. This method is relatively rapid, widely available, and capable of detecting (but not differentiating) human-infective Babesia species. It is also necessary for calculating the percentage of parasitemia, which is used to predict prognosis, guide patient management, and monitor response to treatment. However, microscopic examination requires skilled microscopists and may be challenging in the setting of low parasitemia or prior drug therapy. Also, Babesia species may closely resemble those of Plasmodium falciparum.

The Mayo Clinic real-time polymerase chain reaction assay provides a rapid and more sensitive alternative to blood film examination for detection and differentiation of B microti, B duncani, and B divergens/B divergens-like parasites. It does not cross-react with malaria parasites.