Babesia is a tiny parasite that infects your red blood cells. Infection with Babesia is called babesiosis. The parasitic infection is usually transmitted by a tick bite.

Babesiosis often occurs at the same time as Lyme disease. The tick that carries the Lyme bacteria can also be infected with the Babesia parasite.

The severity of the symptoms of babesiosis can vary. You may have no symptoms at all, or you may have slight flu-like symptoms. Some cases can cause serious, life-threatening complications.

A Babesia infection most often starts with a high fever, chills, muscle or joint aches, and fatigue. Less common symptoms include:

  • severe headache
  • abdominal pain
  • nausea
  • skin bruising
  • yellowing of your skin and eyes
  • mood changes

As the infection progresses, you may develop chest or hip pain, shortness of breath, and drenching sweats.

It’s possible to be infected with Babesia and not have any symptoms. A relapsing high fever is sometimes a sign of undiagnosed babesiosis.

Complications can include:

Babesiosis is caused by infection with a malaria-like parasite of the genus Babesia. The Babesia parasite can also be called Nuttalia.

The parasite grows and reproduces inside the red blood cells of the infected person or animal, often causing intense pain due to the rupture of red blood cells.

There are more than 100 species of the Babesia parasite. In the United States, Babesia microti is the most common strain to infect humans, according to the Centers for Disease Control and Prevention (CDC). Other strains can infect:

  • cattle
  • horses
  • sheep
  • pigs
  • goats
  • dogs

The most common way to contract Babesia is a bite from an infected tick.

Babesia microti parasites live in the gut of the black-legged or deer tick (Ixodes scapularis). The tick attaches to the body of white-footed mice and other small mammals, transmitting the parasite to the rodents’ blood.

After the tick has eaten its meal of the animal’s blood, it falls off and waits to be picked up by another animal.

The white-tailed deer is a common carrier of the deer tick. The deer itself isn’t infected.

After falling off the deer, the tick will typically rest on a blade of grass, a low branch, or leaf litter. If you brush up against it, it can attach to your shoe, sock, or other piece of clothing. The tick then climbs upward, seeking a patch of open skin.

You probably won’t feel the tick bite, and you may not even see it. That’s because most human infections are spread during spring and summer by ticks in the nymph stage. During this stage, the ticks are about the size and color of a poppy seed.

Besides a tick bite, this infection can also pass via contaminated blood transfusions or via transmission from an infected pregnant woman to her fetus. More rarely, it can also be transmitted through an organ transplant.

People with no spleen or a weakened immune system are at greater risk. Babesiosis can be a life-threatening condition for these people. Older adults, especially those with other health problems, are also at higher risk.

The same tick that carries the Babesia parasite can also carry the corkscrew-shaped bacteria responsible for Lyme disease.

A 2016 study found that up to one-fifth of people diagnosed with Lyme were also infected with Babesia. Researchers also found that the babesiosis often went undiagnosed.

According to the CDC, most cases of babesiosis occur in New England, New York, New Jersey, Wisconsin, and Minnesota. These are states where Lyme disease is also prevalent, though Lyme is also prevalent elsewhere.

The symptoms of babesiosis are similar to those of Lyme disease. Coinfection with Lyme and Babesia can cause the symptoms of both to be more severe.

Babesiosis can be difficult to diagnose.

In the early stages, Babesia parasites can be detected by examination of a blood sample under a microscope. Diagnosis by blood smear microscopy requires significant time and expertise. Smears can be negative if there is a very low level of parasitemia in the blood, especially early on in the disease, and they may need to be repeated over several days.

If you or your doctor suspects babesiosis, your doctor can do further testing. They may order an indirect fluorescent antibody test (IFA) on the blood sample. Molecular diagnostics, such as polymerase chain reaction (PCR), may also be used on the blood sample.

Babesia is a parasite and won’t respond to antibiotics alone. Treatment requires antiparasitic drugs, such as those used for malaria. Atovaquone plus azithromycin is used to treat most mild to moderate cases and is usually taken for 7 to 10 days. An alternative regimen is clindamycin plus quinine.

Treatment of severe disease usually consists of azithromycin given intravenously plus oral atovaquone or clindamycin given intravenously plus oral quinine. With severe illness, additional supportive measures may be taken, such as blood transfusions.

It’s possible for relapses to occur after treatment. If you have symptoms again, they must be re-treated. Some people, such as those with weakened immune systems, may need to be treated for longer initially to clear the infection.

Avoiding contact with ticks is the best prevention against both babesiosis and Lyme disease. If you go into wooded and meadow areas where deer are present, take preventive measures:

  • Wear clothing treated with permethrin.
  • Spray repellent containing DEET on your shoes, socks, and exposed areas.
  • Wear long pants and long-sleeved shirts. Tuck your pant legs into your socks to keep ticks out.
  • Inspect your whole body after spending time outdoors. Have a friend look at your back and the backs of your legs, especially behind your knees.
  • Take a shower and use a long-handled brush on areas you can’t see.

A tick must attach to your skin before it can transmit the disease. Attaching usually takes some hours after the tick has come in contact with your skin or clothing. Even if the tick attaches, there is some time before it can transmit the parasite to you. You may have as long as 36 to 48 hours. This gives you time to look for the tick and remove it.

Still, it’s best to be cautious and check for ticks immediately after coming inside. Learn tips for proper tick removal.

Recovery time from babesiosis varies by individual. There is no vaccine against babesiosis. The CDC recommends a 7- to 10-day treatment with atovaquone and azithromycin for nonsevere cases.

Some organizations concerned with the treatment of Lyme disease also specialize in babesiosis. Contact the International Lyme and Associated Diseases Society (ILADS) for information about doctors who specialize in babesiosis.