MSD Manual

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Chelsea Marie

, PhD, University of Virginia;

William A. Petri, Jr

, MD, PhD, University of Virginia School of Medicine

Reviewed/Revised Mar 2023
Topic Resources

Babesiosis is infection of red blood cells caused by the protozoa Babesia.

Babesia are protozoa (single-cell infectious organisms) that live inside red blood cells and eventually destroy them. Babesiosis is transmitted by the same type of deer ticks (family Ixodidae) that transmits Lyme disease Lyme Disease Lyme disease is a tick-transmitted infection caused by Borrelia species, primarily by Borrelia burgdorferi and sometimes by Borrelia mayonii in the United States. These... read more Lyme Disease . These ticks may be infected with the protozoa that cause babesiosis, the bacteria that cause Lyme disease or those that cause other tick-borne diseases such as anaplasmosis Ehrlichiosis and Anaplasmosis Ehrlichiosis and anaplasmosis are tick-borne bacterial infections that cause fever, chills, muscle aches, headache, a general feeling of illness (malaise), and sometimes a rash. Symptoms of... read more Ehrlichiosis and Anaplasmosis and relapsing fever, and the virus that causes encephalitis.

Babesiosis is common among animals but is relatively uncommon among people. In 2019, there were 2,418 reported cases of babesiosis in the United States from the states where the deer ticks are known to be (see Centers for Disease Control and Prevention: Babesiosis Data & Statistics). Babesia microti infects people on the offshore islands or coastal regions of Massachusetts, Rhode Island, Connecticut, New York (including eastern Long Island and Shelter Island), and New Jersey. Cases also occur in Wisconsin and Minnesota. Different Babesia species infect people in Missouri, Washington, and California and in other areas of the world.

Symptoms of Babesiosis

Some people with babesiosis, especially healthy people younger than 40, do not have noticeable symptoms. Infants have a range of mild to severe illness.

Symptoms of babesiosis usually start about 1 to 2 weeks after people are bitten. People may have fever, headache, muscle and joint aches, and fatigue. In healthy people, symptoms usually resolve after a week. Breakdown of red blood cells (hemolysis) may cause anemia Overview of Anemia Anemia is a condition in which the number of red blood cells is low. Red blood cells contain hemoglobin, a protein that enables them to carry oxygen from the lungs and deliver it to all parts... read more (called hemolytic anemia) and jaundice Jaundice in Adults In jaundice, the skin and whites of the eyes look yellow. Jaundice occurs when there is too much bilirubin (a yellow pigment) in the blood—a condition called hyperbilirubinemia. (See also Overview... read more Jaundice in Adults (the skin and the whites of the eyes turn yellow) in severe cases. The liver and spleen often enlarge.

Diagnosis of Babesiosis

  • Examination of a sample of blood

  • Blood tests

Doctors may suspect babesiosis in people who have typical symptoms and hemolytic anemia and who live in or have traveled to an area where the infection is common. Often, people do not remember a tick bite.

To diagnose babesiosis, a doctor usually examines a blood sample under a microscope to look for Babesia. To help identify the protozoa, a blood sample may be tested for Babesia's genetic material (DNA) or for antibodies to Babesia. (Antibodies Antibodies One of the body's lines of defense ( immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more Antibodies are proteins produced by the immune system to help defend the body against a particular attack, including that by parasites.)

Treatment of Babesiosis

  • When treatment is needed, atovaquone plus azithromycin or quinine plus clindamycin

Usually, no treatment is needed for a mild case of babesiosis in healthy people with a functioning spleen because babesiosis typically disappears on its own.

People with symptoms are usually treated with atovaquone (a medication typically used to treat protozoa infections) plus azithromycin (an antibiotic). Quinine (used to treat malaria) plus clindamycin (an antibiotic) is used to treat severe disease. Atovaquone plus azithromycin has fewer side effects and is as effective as quinine plus clindamycin in people with mild to moderate babesiosis.

Prevention of Babesiosis

In areas where deer ticks are common, people can reduce the risk of getting babesiosis by taking precautions against ticks.

People can reduce their chances of picking up or being bitten by a tick by doing the following:

  • Staying on paths and trails when walking in wooded areas

  • Walking in the center of trails to avoid brushing up against bushes and weeds

  • Not sitting on the ground or on stone walls

  • Wearing long-sleeved shirts

  • Wearing long pants and tucking them into boots or socks

  • Wearing light-colored clothing, which makes ticks easier to see

  • Applying an insect repellent containing diethyltoluamide (DEET) to the skin

  • Applying an insect repellent containing permethrin to clothing or wearing clothing commercially pretreated with permethrin

To remove a tick, people should use fine-pointed tweezers to grasp the tick by the head or mouthparts right where it enters the skin and should gradually pull the tick straight off. The tick's body should not be grasped or squeezed. Petroleum jelly, alcohol, lit matches, or any other irritants should not be used.

Did You Know...

  • For a tick bite, use tweezers to grasp the head and mouthparts and remove gradually. Do not grasp the tick's body and do not try to remove using petroleum jelly, alcohol, or a match.

People who have had babesiosis should not donate blood or probably organs to prevent transmission. In the United States, screening of blood and organ donors is now done in states where the incidence of infection is relatively high.


Deer ticks

More Information

The following English-language resource may be useful. Please note that The Manual is not responsible for the content of this resource.

  • Centers for Disease Control and Prevention: Babesiosis

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