Babesiosis is a disease caused by microscopic parasites that infect red blood cells. Many different species (types) of Babesia parasites have been found in animals, only a few of which have been found in people. Babesia microti—which usually infects white-footed mice and other small mammals—is the main species that has been found in people in the United States. Occasional cases caused by other Babesia species have been detected.
The main way is through the bite of an infected tick.
• Babesia microti is spread by Ixodes scapularis ticks, which are commonly called blacklegged ticks or deer ticks. (Although white-tailed deer are the most important food source for the adult stage of the tick, deer are not infected with B. microti.)
• The parasite typically is spread by the young nymph stage of the tick. Nymphs are mostly found during warm months (spring and summer) in areas with woods, brush, or grass.
• Infected people might not recall a tick bite because I. scapularis nymphs are very small (about the size of a poppy seed).
Other possible ways of becoming infected with Babesia include:
• Receipt of a contaminated blood transfusion (no tests have been licensed yet for donor screening); or
• Transmission from an infected mother to her baby during pregnancy or delivery.
Most cases occur in the Northeast and upper Midwest, particularly in parts of New England, New York state, New Jersey, Wisconsin, and Minnesota. In the Northeast, babesiosis occurs in both inland and coastal areas, including off-shore islands, such as Nantucket and Martha’s Vineyard (Massachusetts); Block Island (Rhode Island); and Shelter Island, Fire Island, and eastern Long Island (New York state).
Many people who are infected with Babesia microti feel fine and do not have any symptoms. Some people develop flu-like symptoms, such as fever, chills, sweats, headache, body aches, loss of appetite, nausea, or fatigue. Because Babesia parasites infect red blood cells, babesiosis can cause hemolytic anemia (from the destruction of red blood cells).
Babesiosis can be a severe, life-threatening disease, particularly in people who.
• Do not have a spleen;
• Have a weak immune system for other reasons (such as cancer, lymphoma, or AIDS);
• Have other serious health conditions (such as liver or kidney disease); or
• Are elderly.
Symptoms, if any, can start within a week or so. They usually develop within a few weeks or months, sometimes longer.
See your health care provider.
In symptomatic people, babesiosis usually is diagnosed by examining blood under a microscope and seeing Babesia parasites inside red blood cells. To be sure the diagnosis is correct, your health care provider might have specimens of your blood tested by a specialized reference laboratory (such as at CDC or a health department). In addition, your health care provider might decide to have your blood tested for other possible causes of your symptoms, including other tickborne infections, such as Lyme disease and anaplasmosis.
No. It is not necessary for you to report your case to your health department or CDC.
Reportable conditions are diseases that physicians and/or laboratories must report to their state health department. Babesiosis is reportable in some states but not in others. If babesiosis is reportable in your state, the appropriate authority (a physician, hospital, or laboratory) will submit the necessary information to the health department.
Yes. Effective treatments are available. People who do not have symptoms or signs of babesiosis usually do not need to be treated. The first step is to make sure the diagnosis is correct.
Yes. People can take steps to prevent babesiosis and other tickborne infections. The use of prevention measures is particularly important for people at increased risk for severe babesiosis (for example, people who do not have a spleen). If possible, areas infested with ticks should be avoided, especially during warm months. If such areas cannot be avoided, use protective measures during outdoor activities. See the Prevention and Control page for tips, such as checking yourself daily for ticks. The tiny I. scapularis ticks that spread B. microti usually must stay attached to a person for more than 36–48 hours to be able to transmit the parasite. Daily tick checks can prevent transmission of the parasite.
No vaccine is available to protect people against babesiosis.