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It is possible that the main title of the report Babesiosis is not the name you expected. Please check the synonyms listing to find the alternate name(s) and disorder subdivision(s) covered by this report.


  • Piriplasmosis
  • Redwater Fever

Disorder Subdivisions

  • None

General Discussion

Babesiosis is a rare infectious disease caused by single-celled microorganisms (protozoa) belonging to the Babesia family. It is believed that the Babesia protozoa are usually carried and transmitted by ticks (vectors). Babesiosis occurs primarily in animals; however, in rare cases, babesiosis infection may occur in humans. Certain Babesia species are known to cause babesiosis infection in humans (e.g., Babesia microti), and the deer tick is a known vector. Human babesiosis infection may cause fever, chills, headache, nausea, vomiting, and/or muscle aches (myalgia). Symptoms may be mild in otherwise healthy people; in addition, some infected individuals may exhibit no symptoms (asymptomatic). However, a severe form of babesiosis, which may be life-threatening if untreated, may occur in individuals who have had their spleens removed (splenectomized) or who have an impaired immune system. A different form of babesiosis has been reported in Europe that is associated with a more severe expression of symptoms.


Most people with babesiosis do not experience any symptoms (asymptomatic) or experience only mild symptoms. However, in some cases, babesiosis can cause severe complications. These severe cases usually only occur in individuals greater than 50 years of age, individuals who have impaired immune systems (immunocompromised), or individuals who have had their spleen removed (splenectomized).

The symptoms associated with babesiosis usually develop approximately one to four weeks (incubation period) after exposure to the parasite. Symptoms vary greatly from case to case. The initial symptoms may include fever, a general feeling of ill health (malaise), fatigue, and loss of appetite. Additional early symptoms include joint pain (arthralgia), muscle pain (myalgia), chills, sweats, and headaches.

Affected individuals may also have additional symptoms including nausea, vomiting, and/or abdominal pain. In some cases, an abnormally large liver and or spleen (hepatosplenomegaly) may be present. Immune compromised individuals with severe babesiosis may have impaired kidney function and an abnormal yellow discoloration to their skin, mucous membranes, and whites of the eyes (jaundice).

Laboratory examination of blood samples from affected individuals may reveal abnormally low levels of red blood cells (hemolytic anemia) due to their destruction by the parasite. Additional laboratory findings may include abnormally low levels of platelets (thrombocytopenia) and white blood cells (leukopenia).

In 20-25 percent of human babesiosis cases, affected individuals also have Lyme disease. Individuals who have simultaneous infection with both diseases usually experience more severe expression of symptoms and a longer duration of those symptoms. Individuals with babesiosis may also have another infectious disease known as ehrlichiosis. In rare cases, an individual can be simultaneously affected by all three diseases.

In rare cases, this infection may be responsible for a respiratory condition known as adult respiratory distress syndrome (ARDS). (For more information on this disorder, choose "Adult Respiratory Distress" as your search term in the Rare Disease Database.)


Babesiosis is caused by single-celled microorganisms (protozoa) from the genus Babesia. These microorganisms are parasites that invade red blood cells (erythrocytes).

There are more than 100 species of Babesia. In most cases, the two species of Babesia that cause disease in humans (pathogenic) are Babesia microti and Babesia divergens. The species involved varies depending upon specific geographic location.

The major cause of babesiosis in the northeastern United States is infection by B. microti. In California and Washington a new Babesia parasite, called WA-1, is believed to be responsible for the disorder. In Europe, B. divergens and B. bovis are usually responsible for babesiosis.

Babesia protozoa such as B. microti are transmitted to humans through the bite of infected ticks. The ticks serve as "vectors," the term for any organism that is infected with and later transmits a particular disease agent (e.g., bacterium or virus) to another organism, which may then become infected. The deer tick (Ixodes dammini or scapularis) is the most common vector that transmits babesiosis.

In extremely rare cases, babesiosis may be transmitted following a blood transfusion with blood that is contaminated with the microorganism.

Affected Populations

Babesiosis is a rare infectious disease that affects males and females in equal numbers. It can affect individuals of any age although it is more likely to occur in people over 50 years of age, people who have weakened immune systems, and people who have had their spleens removed.

In the United States, most cases are confined to the northeastern coast in the states of New York, Massachusetts, and Connecticut. The infected tick is present in greater numbers in these areas (endemic). Islands off the northeastern coast including Long Island, Martha's Vineyard, Nantucket, and Block Island are also areas where the tick may be found. Cases of babesiosis have also been reported in Washington, California, Georgia, New Jersey and Wisconsin. A more serious form of babesiosis occurs in Europe.

Approximately 200 cases of babesiosis were reported in the United States during the 1980s. More than 450 cases have been reported in the medical literature. However, because some affected individuals may develop few associated symptoms and findings, the disorder may often remain unrecognized and may therefore be underdiagnosed, making it difficult to determine the true frequency of babesiosis in the general population. The disease has been diagnosed with greater frequency during the 1990s.

Standard Therapies


The diagnosis of babesiosis is made based upon a thorough clinical evaluation, a detailed patient history (e.g., recent tick bite), characteristic findings, and specialized tests such as the examination of blood smears that screen for the parasite inside red blood cells (erythrocytes). The diagnosis may also be confirmed by antibody testing (indirect immunofluorescent assay).


In most healthy people, babesiosis usually resolves spontaneously and causes few or no symptoms. People with an impaired immune system may require treatment with drugs such as clindamycin, quinine, and/or other antiparasitic or antibiotic drugs. Clindamycin and quinine are the drugs most commonly used to treat individuals with severe symptoms of babesiosis.

Treatment with two different drugs, atovaquone and azithromycin, has been used in cases where clindamycin and quinine were ineffective. Individuals who have had their spleens removed who have severe cases of babesiosis may be treated with blood transfusions.


Individuals who will be exposed to areas with high numbers of tick vectors for the Babesia parasites (e.g., fields, wooded or marsh areas, etc.) should consider taking certain steps to prevent infection. Such steps should include wearing long-sleeved shirts, long pants, and hats; wearing light-colored clothing to make ticks more visible; using appropriate tick repellents; and carefully checking clothing and skin (particularly the scalp and the back of the neck) after being in such locations.

Investigational Therapies

Research on tropical diseases is ongoing. For more information about these disorders, contact the World Health Organization (WHO) listed in the Resources below.

Information on current clinical trials is posted on the Internet at All studies receiving U.S. government funding, and some supported by private industry, are posted on this government web site.

For information about clinical trials being conducted at the NIH Clinical Center in Bethesda, MD, contact the NIH Patient Recruitment Office:

Tollfree: (800) 411-1222

TTY: (866) 411-1010


For information about clinical trials sponsored by private sources, contact:



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Lyme Disease Foundation

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Centers for Disease Control and Prevention

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World Health Organization (WHO)

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Genetic and Rare Diseases (GARD) Information Center

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Tel: (301)251-4925

Fax: (301)251-4911

Tel: (888)205-2311

TDD: (888)205-3223


For a Complete Report

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