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Babesiosis is an infection of red blood cells caused by the single-celled parasite, Babesia microti, which is spread to humans by a tick bite.


Babesiosis is a rare, tick-transmitted disease that is caused most often by the single-celled parasite Babesia microti. By 1995, fewer than 500 cases of babesiosis had been reported in the United States. The disease occurs primarily in New England and New York, especially on the coastal islands. However, cases have occurred in other parts of the United States. Because of tick activity, the risk for babesiosis is highest during June and July.

Ticks are small, blood-sucking arachnids. Although some ticks carry diseasecausing organisms, most do not. Babesia microti is spread to humans through the bite of the tick Ixodes scapularis (also called Ixodes dammini). Ixodes scapularis, called the "blacklegged deer tick," usually feeds on deer and mice. A tick picks up the parasites by feeding on an infected mouse and then passes them on by biting a new host, possibly a human. To pass on the parasites, the tick must be attached to the skin for 36-48 hours. Once in the bloodstream, Babesia microti enters a red blood cell, reproduces by cell division, and destroys the cell, causing anemia. Humans infected with Babesia microti produce antibodies that can be helpful in diagnosing the infection.

Causes and symptoms

Babesia microti live and divide within red blood cells, destroying the cells and causing anemia. The majority of people who are infected have no visible symptoms. In those who become ill, symptoms appear one to six weeks following the tick bite. Because the ticks are small, many patients have no recollection of a tick bite. The symptoms are flu-like and include tiredness, loss of appetite, fever, drenching sweats, and muscle pain. Nausea, vomiting, headache, shaking chills, blood in the urine, and depression can occur.

Persons who are over 40 years old, have had their spleen removed (splenectomized), and/or have a serious disease (cancer, AIDS, etc.) are at a greater risk for severe babesiosis. In severe cases of babesiosis, up to 85% of the blood cells can be infected. This causes a serious, possibly fatal, blood deficiency.


Babesiosis can be diagnosed by examining a blood sample microscopically and detecting the presence of Babesia microti within the blood cells. The blood can also be checked for the presence of antibodies to the parasite.


In serious cases, babesiosis is treated with a combination of clindamycin (Cleocin) and quinine. Clindamycin is given by injection and quinine is given orally three to four times a day for four to seven days. To reduce the number of parasites in the blood, severely ill patients have been treated with blood transfusions.


Otherwise healthy patients will recover completely. Babesiosis may last several months without treatment and is a severe, potentially fatal disease in splenectomized patients.


The only prevention for babesiosis is to minimize exposure to ticks by staying on trails when walking through the woods, avoiding tall grasses, wearing long sleeves and tucking pant legs into socks, wearing insect repellent, and checking for ticks after an outing. Remove a tick as soon as possible by grasping the tick with tweezers and gently pulling. Splenectomized people should avoid northeastern coastal regions during the tick season.



Mayo Clinic Online. March 5, 1998.


Anemia A below normal number of red blood cells in the bloodstream.

Parasite An organism that lives upon or within another organism.


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What are Babesia?

What Happens When People Get Babesiosis?

How Do People Prevent Babesiosis?


Babesiosis is a rare disease carried by ticks infected with the Babesia parasite. It most often affects cows, horses, sheep, dogs, and cats, but it can be transmitted to people through tick bites.


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Tickborne diseases

Watch out for that tick! Ticks carry many different diseases, including babesiosis (ba-bee-ze-O-sis), Lyme disease, and Rocky Mountain spotted fever.

What are Babesia?

Babesia (ba-BEE-ze-a) are protozoa, or one-celled organisms, that often live as parasites* infecting cows, horses, sheep, goats, dogs, cats, and other animals. Ticks pick up Babesia when they feed on infected animals. The protozoa then multiply in the tick, and when the tick bites a person or another animal, the protozoa travel from the tick into the new host, where they begin multiplying again.

* parasites
are creatures that live in and feed on the bodies of other organisms. The animal or plant harboring the parasite is called its host.

What Happens When People Get Babesiosis?

The Babesia parasite invades the bodys red blood cells and can destroy them. If left untreated, babesiosis may destroy red blood cells faster than the body can replace them.


Doctors believe that many cases of babesiosis do not cause any symptoms. But in some cases, symptoms may start one to four weeks after the tick bite occurs, and symptoms may last for several weeks or months. People with babesiosis may experience fever, chills, sweating, fatigue, and anemia. Its symptoms are similar to those of malaria.

Diagnosis and treatment

Doctors diagnose babesiosis by examining blood under a microscope. If they detect the parasite, they will prescribe medications to fight the infection and to rid the body of the parasite. Babesiosis is usually curable, although repeated courses of treatment may be necessary.

Most patients recover with few, if any, lasting effects. The most serious and sometimes fatal cases are found in elderly people, in pregnant women, in people who have had their spleens* removed, or in people with immune deficiencies.

* spleen
is an organ near the stomach that helps the body fight infections.

How Do People Prevent Babesiosis?

The best methods of prevention are the use of insect repellant and proper clothing. People in tick-infested areas should wear long-sleeve shirts and long pants tucked into socks or boots. That can help to keep ticks from reaching the skin. If ticks do attach to the skin of people or pets, the ticks should be removed with tweezers.

See also

Lyme Disease


Rocky Mountain Spotted Fever



The U.S. National Institute of Allergy and Infectious Diseases has information about babesiosis and other tickborne diseases at its website.