May also be called: Redwater Fever; Piroplasmosis

Babesiosis (buh-bee-zee-OH-sis) is a rare illness caused by infection with microscopic parasites that infect red blood cells.

More to Know

Babesiosis is caused by parasites of the genus Babesia. There are more than 100 species of Babesia, but most infections in humans in the United States are caused by Babesia microti. The parasites are spread by Ixodes ticks (also called black-legged or deer ticks), which are the same ticks that spread Lyme disease. Typically, ticks will feed on infected rodents or cows and then feed on humans later, transmitting the parasites in their saliva. In rare cases, babesiosis can be transmitted through a blood transfusion or passed from a pregnant woman to her fetus.

Once they are in a person's bloodstream, the parasites replicate and infect red blood cells. This can cause flu-like symptoms such as fever, chills, headache, body aches, loss of appetite, nausea, and fatigue. Many people with babesiosis have no symptoms, but in those with weakened immune systems or other serious health conditions, the disease can lead to anemia, jaundice, organ malfunction, and even death.

In the United States, babesiosis is most common in the Northeast and upper Midwest, particularly in the summer months, when ticks are most active. Treatment for severe cases usually involves antimicrobial medications that lessen symptoms and eliminate the parasites.

Keep in Mind

Although babesiosis can be dangerous, it's quite rare, and most people who develop it don't need treatment. Babesiosis usually goes away on its own in about 1 or 2 weeks. People who do require treatment usually see improvement within 48 hours of starting antimicrobial therapy. The disease usually is completely cured within 3 months.

All A to Z dictionary entries are regularly reviewed by KidsHealth medical experts.