Babesiosis: Symptoms, Risks and Treatment

Quick answer

Babesiosis is an infection of the red blood cells caused by tiny Babesia parasites, spread mainly by blacklegged (deer) ticks in the Northeast and upper Midwest. Many people have no symptoms; others get fever, chills, sweats and fatigue. It can be serious for people without a spleen, with weak immune systems, or over 50. Effective treatment exists.

What is babesiosis?

Babesiosis is caused by Babesia, a parasite rather than a bacterium or virus. Many Babesia species infect animals, but only a few infect people. CDC notes that Babesia microti, the main species found in people in the U.S., usually infects white-footed mice and other small mammals. Ticks pick up the parasite from these animals and can pass it on when they bite.

Because the same blacklegged tick can carry the Lyme bacterium, babesiosis is one of the best-known Lyme disease co-infections. A small New York study of 52 adults with an early Lyme rash, published in CDC’s journal, found convincing evidence of babesiosis in four of them (about 8%).

Where babesiosis occurs

CDC says most U.S. cases occur in the Northeast and upper Midwest. A CDC report in MMWR (2023) looked at cases reported from 2011 to 2019:

• 37 states reported 16,456 cases in total.
• New York, Massachusetts and Connecticut reported the most cases.
• Cases rose significantly in Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island and Vermont.
• CDC now considers babesiosis endemic (regularly found) in Maine, New Hampshire and Vermont, which were not previously seen that way.

Learn about the tick that spreads it on our blacklegged tick page, or compare it with others on types of ticks.

Babesia symptoms

CDC says many people with babesiosis feel fine and never have symptoms. When symptoms do appear, they are flu-like:

• Fever, chills and sweats
• Headache and body aches
• Loss of appetite and nausea
• Fatigue and weakness

Because the parasites attack red blood cells, babesiosis can cause hemolytic anemia, where red blood cells are destroyed faster than the body can replace them. CDC’s clinician page adds that some people have an enlarged spleen or liver, or yellowing of the skin and eyes (jaundice).

Timing: Symptoms may start about a week after infection, but often develop over a few weeks or months. In people with weakened immune systems, they can first appear or come back many months later. Tick bites are often not noticed, so mention any time in tick habitat to your provider.

Who is at higher risk of severe illness

CDC says babesiosis can be serious or even life-threatening for people who:

• Do not have a spleen
• Have a weakened immune system, for example from cancer, lymphoma or HIV
• Have other serious health problems, such as liver or kidney disease
• Are older than 50

CDC’s clinician guidance also lists certain immune-suppressing medicines, heart failure, and newborns exposed through transfusion, pregnancy or a tick bite. In severe cases, complications can include problems with blood clotting, breathing, the kidneys and the liver.

Babesiosis and blood transfusions

Babesiosis can spread through a transfused blood product that contains the parasite, CDC says. People who carry the parasite without symptoms may not know they have it. The 2023 MMWR report noted that transfusion-acquired babesiosis has been linked to worse outcomes than infection from a tick bite, and that the FDA recommends screening blood donations for Babesia in a group of higher-risk states and Washington, D.C. The authors recommended adding the three newly endemic states.

If you have had babesiosis, ask your provider or blood center about blood donation rules.

How babesiosis is diagnosed

CDC says diagnosis takes a high level of suspicion, because the symptoms are not specific. Tests may include:

• Blood smear: a trained lab worker looks at a thin layer of blood under a microscope for parasites inside red blood cells. CDC advises asking for a manual review; several smears may be needed.
• Reference lab testing: CDC recommends considering confirmation at a reference lab, such as a health department or CDC, using smears and, if needed, molecular (PCR) or antibody tests.
• Blood counts and other labs: ill patients often have hemolytic anemia and low platelets, and may have raised liver enzymes or kidney values.

The IDSA/AAN/ACR Lyme guideline notes that signs of red blood cell breakdown, such as anemia with raised bilirubin or LDH, especially suggest babesiosis in someone with Lyme disease.

Babesiosis treatment

We don’t give doses or treatment advice; your provider will decide. Here is what CDC’s clinician guidance describes:

• No symptoms: most people do not need treatment.
• Ill but not severely: usually a combination of two medicines, most often atovaquone with azithromycin. Clindamycin with quinine is an alternative, and is generally preferred in pregnancy.
• Severe illness: hospital care with the same medicine pairs, plus supportive care when needed. In some very severe cases, an exchange transfusion is used.
• Weakened immune systems: treatment is usually longer, with repeated blood smears to track the parasite.

Doxycycline, often used for Lyme disease, is not on CDC’s list of babesiosis treatments. If you are being treated for Lyme and still have a fever, tell your provider. CDC notes that tiredness can linger for weeks after treatment, but relapse is uncommon in people with healthy immune systems.

Preventing babesiosis

CDC’s advice: avoid tick-infested areas when you can; in tick habitat, use repellent and wear socks, long sleeves and pants; and check yourself and your pets for ticks before going inside. Our prevention guide covers the details. For other infections this tick can carry, see anaplasmosis, Powassan virus and Borrelia miyamotoi.

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