An Overview of Babesiosis

A rare infectious disease spread by deer ticks

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Babesiosis is a rare infectious disease caused by microorganisms called protozoa. There are over 100 species of Babesia and the disease is quite common in animals, including deer, mice, horses, cats, and dogs. Only two species of Babesia are known to infect humans. While the disease has been found worldwide, most documented cases to date have been diagnosed in the Northeastern United States, a few Northwestern states, and throughout Europe.

babesiosis diagnosis
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Babesiosis may not cause symptoms. The majority of people who are infected with Babesia either don't feel sick (asymptomatic) or have only mild symptoms.

Common Symptoms

If a person does feel unwell, at first they may have nonspecific "flu-like" symptoms such as:

  • A general sick feeling (malaise)
  • Feeling tired or rundown (fatigue)
  • Loss of appetite and nausea
  • Fever, chills, and "cold sweats" (diaphoresis)
  • Joint paint

If a person gets babesiosis from a tick, symptoms usually show up within a few weeks of the bite. Sometimes, it may be longer—up to nine weeks.

Since ticks carrying diseases like babesiosis or Lyme disease infect when they are still very small and hard to see, it's not unusual for a person to find out they have a tick-borne illness and not remember ever having a tick bite.

Some people will get very sick after being bitten by a tick carrying Babesia. The more severe cases usually occur in people who are elderly, don't have a spleen (either because it was surgically removed or because they were born without one), or don't have a good immune system (as the result of a chronic disease that affects the immune system, like HIV, or because they are taking a medication or receiving a drug therapy, such as chemo, that suppresses their immune system).

Babesiosis is also more common and can be more serious, in people who already have another tick-borne disease, such as Lyme. As many as 20 percent of people who have Lyme are also found to be infected with Babesia.

Severe Symptoms

More severe cases of Babesiosis can include:

  • Hemolytic anemia
  • Enlarged spleen, if the patient still has one
  • Renal failure
  • Abnormal yellowing of the skin and eyes (jaundice)
  • Liver failure
  • Protein in urine
  • Congestive heart failure
  • Blood clots caused by a condition called disseminated intravascular coagulation (DIC)
  • Rarely, the infection can lead to Adult Respiratory Distress Syndrome (ARDS)

In these severe cases, babesiosis can last for months, if not years, and potentially be fatal if not treated.

Most otherwise-healthy people who become infected with Babesia may not even know since they don't feel sick. If they do get sick from babesiosis, symptoms are generally mild.

Whether they have no symptoms, just a few symptoms, or severe symptoms, the illness caused by Babesia is often compared to malaria. In fact, babesiosis is sometimes described as being a "malaria-like" disease. This is because the pathogens that cause malaria and babesiosis both make a person sick by getting inside the red blood cells (erythrocytes) and destroying them (hemolysis).


Of the 100-some species of Babesia parasite, only two seem to infect humans: B. microti and B. divergens. For people who live in the United States, babesiosis is usually caused by B. microti. In Europe, babesiosis is caused by B. divergens or B. bovis, which causes a much more severe disease than is typical in the U.S.

A person becomes infected with Babesia after being bitten by a tick carrying the parasite. The type of tick that most commonly causes babesiosis, Lyme, and other tick-borne illnesses is the deer tick.

In the U.S., these ticks are mostly found (endemic) to New England and some states in the mid-to-northwest, including Washington, Minnesota, and California. Babesiosis cases occur more frequently on several islands off the East Coast U.S. including Martha's Vineyard, Long Island, Fire Island, and Nantucket Island.

Despite their name, deer ticks are not only found on deer. They may attach themselves to many wild and domestic animals, humans included. For example, a person may get a tick on them because it hitched a ride on their dog or from participating in outdoor activities like hiking or gardening in tall grass.

After a tick attaches itself to the skin and bites, it begins feeding on the blood supply of the host. If the tick is infected with the Babesia parasite, the microorganism enters the bloodstream of the host as the tick feeds. A tick infected with B. microti needs to stay attached for 36–48 hours to transmit Babesia to a host.

In very rare instances, Babesia has been transmitted human-to-human via blood transfusion.


If a person doesn't have symptoms and doesn't remember being bitten by a tick, they may not realize they have been infected with Babesia. People who only feel a little sick may think they just have a case of the flu.

Anyone who lives in or visits an area where deer ticks are common and where cases of Babesia infection have been reported, especially if they work or play outside, are at risk for tick-borne illness. Babesiosis occurs in both men and women, of all ages and races. While it's considered to be a rare infection with only 400-some cases reported in medical literature, since many people are asymptomatic, the true number of cases could be much higher.

The only way to know for sure if someone has been infected with Babesia is to test their blood for the parasite. If a person goes to the hospital or their doctor's office because they feel sick or because they were bitten by a tick and are concerned about tick-borne infections, there are a number of ways to diagnose babesiosis:

Diagnostic Tests

  • Blood tests to check for low blood cell counts (anemia, thrombocytopenia, leukopenia)
  • Antibody tests to look for Babesia antibodies in the blood
  • Microscopic analysis of the red blood cells to look for Babesia parasites
  • Tests to rule out other infections that cause similar symptoms or confirm co-occurring Lyme infection
  • Other blood tests to check the function of major organs (liver)
  • Urine test (urinalysis) to assess kidney function

Other tests may be needed if the patient is very ill or has other health conditions.


A person with Babesiosis who doesn't have symptoms or feel sick probably won't go to the doctor. Usually, this is not a problem: in mild cases, the infection will resolve on its own (spontaneously) and not cause any longterm complications or health issues.

For people who do get sick, have conditions that compromise their immune system (HIV), or don't have a spleen (asplenic), treatment for babesiosis usually begins as soon as they are diagnosed. The first-line treatment is antibiotic therapy. Depending on the patient, several different antibiotics can be used, but the most commonly prescribed to treat Babesia infection are clindamycin and quinine.

Antibiotics are given for every diagnosed case of babesiosis even if the person doesn't feel sick. Alternative antibiotic regimens, such as azithromycin, can be prescribed for patients who can't take the two most commonly prescribed medications.

To treat the initial illness and prevent complications, treatment may need to be more aggressive or go on longer in patients who don't have a spleen, are elderly, and are immunocompromised.

In rare cases, if a person still has the parasite in their blood even after taking antibiotics as prescribed, they may need a blood transfusion—but this usually only happens when patients don't have a spleen.

Babesiosis is treatable, even curable, but the best strategy is prevention. People who live in or are planning to visit areas where deer ticks are common can take several steps to help avoid tick bites that could potentially transmit Babesia.

Preventing Babesiosis

  • Dress in long-sleeved clothing, pants, and socks (tuck pants into socks or boots).
  • Wear clothing that is light-colored, which makes ticks easier to see.
  • When hiking, stay on cleared paths and avoid areas of tall grass.
  • Check your pets for ticks and make sure they get flea and tick prevention medication regularly.
  • Check yourself and family for ticks after working and playing outside; promptly remove ticks if you find any.
  • Use different types of tick-repellent, which can be applied to clothes and bare skin.

If you start to feel sick within a few weeks or months after a tick bite, seek medical attention.

A Word From Verywell

Babesiosis is treatable, even curable, once diagnosed. The best strategy is prevention. If you're living in or visiting an area where deer ticks are common or Babesiosis cases have been reported (throughout the Northeastern U.S. and some mid-and-northwestern states), be sure to wear light-colored, long-sleeved clothes when working or playing outside, use tick-repellents on skin and clothes, and check yourself, your family, and pets for ticks.

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

By Abby Norman
Abby Norman is a freelance science writer and medical editor. She is also the author of "Ask Me About My Uterus: A Quest to Make Doctors Believe in Women's Pain."