• Also Known As:
  • Babesia parasite infection
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What is babesiosis?

Babesiosis is a blood infection caused by the parasite, Babesia. In the U.S., it is primarily spread to people by bites from infected blacklegged ticks, also called deer ticks (Ixodes scapularis). You may not notice the tick bite responsible for babesiosis because very small, young nymph ticks are the most likely to spread it. After infection, the parasite invades red blood cells, eventually causing them to burst. While rare, babesiosis has been transmitted by blood transfusions.

Babesiosis usually causes non-specific flu-like symptoms (e.g., fever, fatigue, chills, sweats, and headache), or none at all. However, it can cause life-threatening complications in some individuals. Because the parasite infects and destroys red blood cells, it can lead to a serious condition called hemolytic anemia. Those at highest risk include people with weakened immune systems, with underlying health conditions like kidney disease, the elderly, or individuals lacking a spleen. The spleen helps to clear infected red blood cells.

While there are more than 100 species of the Babesia parasite, only a handful of them infect people. Babesia microti causes most U.S. cases. In 2017, the Centers for Disease Control and Prevention (CDC) received notification of 1,994 confirmed cases in the U.S. Most cases occurred in the Northeast, Mid-Atlantic and Upper Midwest where the disease is well established (endemic).

There have been a handful of babesiosis cases on the West Coast. These are caused by a different parasite species, Babesia duncani. Different species of Babesia are found in Europe and Asia, and symptoms of infection may vary from the typical presentation. Babesiosis likely occurs worldwide, but little is known about the frequency of babesiosis in areas where malaria also exists. That’s because both parasites infect red blood cells and the infections have similar, non-specific symptoms, making it very difficult to differentiate if the infection is caused by Plasmodium species (malaria) or Babesia parasites.

Borrelia burgdorferi (the bacteria that cause Lyme disease) and Babesia are transmitted by the same species of tick. About 20% of people with babesiosis also have Lyme disease. Those individuals have symptoms that are more severe and last longer than individuals with either infection alone.

Humans are not an essential host in the Babesia lifecycle and do not usually spread the infection to others. The parasite relies on rodents, primarily the white-footed mouse, and Ixodes ticks to complete its development. Human-to-human transmission of the infection is very uncommon.

Although rare, it is possible for babesiosis to be transmitted by blood transfusion because the vast majority of people with B. microti infections are asymptomatic and never diagnosed, so the donor is allowed to donate blood without knowing that the infection may be present. The first U.S. case of transfusion-transmitted babesiosis was reported in 1980. Since then, more than 200 cases of transfusion-associated infections have been documented.

Currently, the blood donor history questionnaire is used to identify and indefinitely defer individuals who have had a history of babesiosis. In 2018, the U.S. Food and Drug Administration approved the first donor screening test for B. microti and issued draft recommendations for a regional risk-based approach to donor screening in affected states.

Transmission may also occur from an infected mother to her baby during pregnancy (congenital infection), though this is even more rare than transmission through blood transfusion.


About Babesiosis

Signs and Symptoms

Babesiosis signs and symptoms can range from non-existent to severe.

Most people will have no signs or symptoms of a babesiosis. When symptoms do occur, they usually begin within a few weeks or months after infection. Sometimes, however, symptoms may first appear or reappear (relapse) many months after exposure, especially in people with weakened immune systems.

Typical signs and symptoms include:

  • Fever, chills, sweats
  • Muscle aches
  • Joint pain
  • Headache
  • Loss of appetite
  • Nausea
  • Fatigue

These symptoms can last for months in some untreated individuals.

Since babesiosis affects red blood cells, it can also cause signs and symptoms of hemolytic anemia, a disorder where red blood cells are destroyed faster than they are made. The severity depends on the extent of the infection. In addition to those listed above, signs and symptoms related to the anemia caused by destruction of the red blood cells may include:

  • Pale skin
  • Dizziness
  • Yellow eyes and skin (jaundice)
  • Dark urine
  • Enlarged spleen
  • Enlarged liver

In some cases, babesiosis can cause life-threatening complications, especially in individuals who:

  • Do not have a spleen
  • Have weakened immune systems
  • Have underlying health conditions (such as liver or kidney disease)
  • Are older than 50 years of age

In the U.S., babesiosis has been fatal in fewer than 10 percent of people diagnosed with the disease. Most of those deaths were in elderly people or in individuals without a spleen.

With serious cases, complications of babesiosis may include:


The goals of diagnostic testing are to identify a Babesia infection or to rule out babesiosis as the cause of symptoms. Additional laboratory tests are often performed to detect complications such as anemia and organ dysfunction.

Babesiosis is hard to diagnose because there are no specific symptoms identifying it and because many individuals have no symptoms. Babesiosis should be suspected in regions where the disease is concentrated, such as the Northeast U.S. or the Upper Midwest.

Laboratory Tests


  • Peripheral blood smear—babesiosis is usually diagnosed by blood smear. An examination of a blood smear may be done when a complete blood count (CBC) shows abnormal results. A laboratory scientist will examine your blood sample under the microscope to look for Babesia parasites inside red blood cells. Successful diagnosis of babesiosis by blood smear often depends on the individual’s degree of infection. Sometimes multiple smears need to be examined. In some instances, samples may be sent to a reference laboratory that employs lab professionals with special training and experience in examining blood smears for parasites.The presence of the Babesia parasites in your infected red blood cells confirms a babesiosis diagnosis. However, Babesia parasites can be difficult to distinguish from other parasites in a blood smear.A negative blood smear does not rule out babesiosis. You can have a Babesia infection while lacking visible parasites in a blood smear. Results may need to be confirmed by additional testing.
  • Antibody testing (indirect fluorescent antibody test, IFA)—This blood test looks for antibodies produced in response to a B. microti infection. It may be used if blood smear results are inconclusive. This test has high sensitivity and specificity in Babesia detection. The antibody level (titer) rises in about 2-4 weeks after infection and wanes at 6-12 months.A negative result for B. microti antibodies typically means that you most likely have never been infected with the parasite. The test detects antibodies in 88 to 96 percent of infected people, so false-negative test results (people who have a negative test result despite being infected) are possible.This test only detects B. microti. It is possible to have babesiosis due to infection with other Babesia species. For instance, people who may have been exposed on the West Coast and test negative for B. microti should be given a separate test for Babesia duncani antibodies.

    A high level (titer) of immunoglobulin M (IgM) antibodies to Babesia indicates a current infection, while a positive but lower level IgM antibody level typically indicates a past infection. An increased immunoglobin G (IgG) antibody to Babesia indicates that you were infected in the past.

  • Molecular testing—polymerase chain reaction (PCR) tests may also be used to identify B. microti. A positive result means that Babesia was detected in your blood. A negative result does not rule out that Babesia was present in small numbers that the test could not detect.

General tests

  • Complete blood count (CBC)—certain CBC findings may be consistent with, but not specific for, babesiosis. These include:
    • Decreased red blood cells, hemoglobin and hematocrit
    • Decreased platelet numbers (thrombocytopenia)
    • Normal or slightly decreased white blood cell count
  • Comprehensive metabolic panel (CMP)—the tests included in this panel are used to determine general health status and organ function. Certain results may be consistent with, but not specific for, babesiosis. These include:
    • Increased creatinine level
    • Increased blood urea nitrogen (BUN) level
    • Increased liver enzymes, including alanine aminotransferase (ALT) and aspartate aminotransferase (AST)
    • Increased lactate dehydrogenase (LD) levels are consistent with a more severe infection and more parasites in your blood.

If a urinalysis is performed, results may be positive for protein and/or blood in the urine.


The main way to prevent babesiosis is avoiding tick bites. As with all tickborne diseases, these measures include:

  • Avoiding overgrown grass or brush
  • Wearing long pants, long-sleeved shirts, and socks when outdoors
  • Tucking pant legs into socks
  • Applying insect repellent
  • Showering soon after being outdoors
  • Checking your entire body for ticks after time outdoors


In the U.S., most people with babesiosis recover without treatment. Treatment is typically only required if someone has signs and symptoms. However, treatment should be individualized and based on whether it is thought a person can clear the infection without treatment or is at risk for relapse (unsuccessful treatment) or severe babesiosis.

Individuals who are treated usually take a combination of two prescription medications for 7-10 days. In severe cases, patients may receive supportive care such as blood transfusions or dialysis. For more on treatment, see the resources under Related Content.

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