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What Is Babesiosis?

Babesiosis (buh-bee-zee-OH-sis) is a rare illness that happens when tiny parasites infect red blood cells, usually after a bite from an infected tick.

In the U.S., tick bite-related cases of babesiosis (also called Redwater fever or piroplasmosis) are most common in parts of the Northeast and upper Midwest, usually during warm weather, when ticks are most active. Not all people who get infected have symptoms, but treatment is available for those who do.

What Causes Babesiosis?

Babesiosis is caused by parasites called Babesia microti, which are so small that they can’t be seen without a microscope. The same ticks that spread Lyme disease (Ixodes ticks, also called black-legged or deer ticks) can pick up the parasites by feeding on infected animals, like rodents. Then, they can pass the parasites to humans through a bite. People can have both Lyme disease and babesiosis at the same time.

Rarely, babesiosis can spread through a blood transfusion or pass from a pregnant woman to her fetus.

Babesiosis does not spread person to person.

What Are the Signs & Symptoms of Babesiosis?

After the parasites get into a person's bloodstream, they replicate and infect red blood cells. This doesn’t always cause symptoms, but sometimes leads to:

  • flu-like symptoms such as fever, chills, sweats, headache, body aches, loss of appetite, nausea, and tiredness
  • in people with weak immune systems or some serious health conditions, anemia, low platelet levels, jaundice, organ malfunction, and even death

How Is Babesiosis Diagnosed?

Doctors diagnose babesiosis by asking about a person’s symptoms and doing blood tests to look for the parasites in red blood cells. They also might do other tests to confirm an infection.

How Is Babesiosis Treated?

Babesiosis usually goes away on its own in 1–2 weeks and causes mild symptoms, if any. To treat severe cases, doctors usually give antiparasitic medicines to ease symptoms and kill the parasites. People who get treatment often start feeling better within 48 hours of starting the medicines.

Can Babesiosis Be Prevented?

There’s no vaccine to prevent babesiosis. The best way to prevent it is to avoid tick bites. To protect your family when outdoors:

  • Stay in the middle of the trail, instead of going through high grass or the woods.
  • Wear closed shoes or boots, long-sleeved shirts, and long pants. Tuck pant legs into shoes or boots to prevent ticks from crawling up legs.
  • Use an insect repellent.
  • Consider treating clothing and gear with permethrin to repel ticks. When used properly, permethrin is safe for all ages. But don't use it on clothing or other material a child may suck on or chew.
  • Wear light-colored clothing to help you see ticks more easily.
  • Shower and wash hair after being outside to remove ticks before they attach. Check kids and pets for ticks after they come inside.

If you find a tick, removing it right away can help prevent the tick from passing the parasites or other germs.

What Else Should I Know?

Cases of babesiosis are on the rise in some U.S. states, but it’s still rare, and most people who develop it don't need treatment.

Medically reviewed by: Yamini Durani, MD
Date reviewed: May 2023