Babesiosis is infection of red blood cells caused by the single-celled protozoan parasite Babesia. The infection may cause fever, headache, body aches, and fatigue.

Babesiosis is transmitted by the same type of deer ticks (family Ixodidae) that transmits Lyme disease. This infection is common among animals but is relatively uncommon among people. In the United States, Babesia microti infects people on the offshore islands or coastal regions of Massachusetts, Connecticut, New York, and New Jersey. Cases also occur in Wisconsin, Georgia, and California. Other Babesia species infect people in other areas of the world.

Babesia live inside red blood cells and eventually destroy them. Some people, especially healthy people younger than 40, do not have noticeable symptoms. Other people have fever, headache, muscle and joint aches, and fatigue. Anemia may result from the breakdown of red blood cells. The liver and spleen often enlarge.

The risk of severe disease and death is highest for people whose spleen has been removed or who take drugs or have disorders that weaken the immune system (particularly AIDS). In these people, babesiosis may resemble malaria (causing a high fever, anemia, dark urine, jaundice, and kidney failure).

To diagnose babesiosis, a doctor usually examines a blood sample under a microscope.

Usually, no treatment is needed for a mild infection in healthy people with a functioning spleen because the infection typically disappears on its own. People with symptoms are treated with atovaquone plus azithromycin or quinine plus clindamycin. Atovaquone and azithromycin have fewer side effects. In areas where deer ticks are common, people can reduce the risk of getting the infection by taking precautions against ticks (see Bacterial Infections: Preventing Tick Bites).

Last full review/revision March 2007 by Richard D. Pearson, MD