First reported by Victor Babes in 1888, Babesia microti is an intracellular parasite that infects red blood cells, and its related infectious disease is known as babesiosis. The vector of the disease is the tick of the Ixodes species, also the vector of Borrelia burgdorferi , the bacterium that causes Lyme disease. For this reason, coinfections of babesiosis and Lyme disease are common.

The B. microti life cycle is separated into two stages: the asexual and sexual stages. Within the asexual stage, the tick infects the white-footed mouse, Peromyscus leucopus, with sporozoites (the infective agent of the parasite). These sporozoites enter the mouse’s red blood cells and begin reproducing. Some sporozoites enter the sexual stage and mature into gametes. When another tick infects the mouse, these gametes are transmitted and can produce more sporozoites. Whenever humans come in contact with these infected ticks, the sporozoites infect the blood and enter the asexual stage. Inside a human’s red blood cells, the parasite takes on its cyclical form and trophozoite rings are seen on the Giemsa-stained blood smear, similar to the appearance of the malaria parasite, Plasmodium falciparum. However, in babesiosis, these trophozoites can pair up into a tetrad structure to form a “Maltese-cross”.

Infections commonly occur within the months of May to September. Most infections are without symptoms; however immunocompromised patients (such as the very young, elderly, and splenectomised patients) can exhibit symptoms ranging from mild to severe after one to eight weeks of exposure. Mild infections can mirror viral infections, whereas severe infections often lead to death. Other symptoms associated with the hemolytic effects of the parasite include malaise, fever, chills, sweating, and myalgia. Hemolytic anemia is present in severe cases with jaundice, hemoglobinuria, and shortness of breath. Severe complications that can occur include respiratory, liver or renal failure, as well as disseminated intravascular coagulation. Quinine sulfate and clindamycin is used in the treatment of babesiosis.