Life Cycle & Transmission

Morphology of  Babesia  in an erythrocyte, as seen in Geimsa staining of a thin blood smear. (Source: Stanford)

Morphology of Babesia in an erythrocyte, as seen in Geimsa staining of a thin blood smear. (Source: Stanford)

Babesiosis is caused by microscopic parasites that infect red blood cells. The parasites are protozoans of the genus Babesia. In order to infect humans, the protozoans require a reservoir host, which is the deer tick (Ixodes scapularis). For more information on disease transmission and the life cycle of deer ticks, please see the “Tick Ecology” section.

Although more than 100 species of Babesia have been reported, only a few species have been known to cause human infection, including B. microti, B. divergens, and B. duncani. However, B. microti is the most common cause of human babesiosis. Babesia species have a complex life cycle that involves asexual reproduction in the red blood cells of their mammalian hosts (humans) and sexual reproduction in their arthropod vector (deer tick) (CDC 2018).

Who gets Babesiosis & where?

The first human case of babesiosis was identified in 1957 near Zagreb, Croatia (Vannier et al. 2008). However, the first known case in Wisconsin wasn’t detected until 1985. Like other tick-borne diseases, babesiosis primarily occurs in the Northeast and Upper Midwest regions of the U.S. Infection is more common in late spring/early summer, when nymphs are most active.


Many people who are infected with Babesia microti do not have any symptoms. Some people develop nonspecific flu-like symptoms, including:

Number of reported cases of babesiosis, by county of residence — 31 states, 2014. Cases are mapped to the patients’ county of residence, which was not necessarily where they became infected. Babesiosis was not a reportable disease in the gray states, and health departments in those states did not notify CDC of cases. (Source: CDC). Click to enlarge.

  • Fever, chills, and sweats

  • Headache and body aches

  • Loss of appetite

  • Nausea

  • Fatigue

Because Babesia parasites infect and destroy red blood cells, babesiosis can cause a special type of anemia called hemolytic anemia. This type of anemia can lead to jaundice (yellowing of the skin) and dark urine. Other complications include low blood pressure, low platelet count, and disseminated intravascular coagulation (DIC), which can lead to blood clots and bleeding (CDC 2018).


In symptomatic people, babesiosis is diagnosed by examining blood specimens under a microscope and seeing Babesia parasites inside red blood cells (CDC 2018).


People who do not have symptoms or signs of babesiosis usually do not need to be treated. Patients who experience symptoms should be given a course of antimicrobial therapy. Two regimens are highly effective: the combination of atovaquone and azithromycin (for mild to moderate disease) and the combination of clindamycin and quinine (for severe disease) (Vannier et al. 2008).

For information on prevention, please see the “Tick Ecology” section.


CDC 2018 (“Babesiosis”):

Vannier et al. 2008 (“Human Babesiosis”):

For additional reading, please visit the “Resources” tab.