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Babesiosis: About

Babesiosis is an illness spread by ticks. Babesiosis is caused by a tiny parasite called Babesia that infects and destroys red blood cells. The Babesia parasite is spread by the deer tick, also known as the black-legged tick.

Babesiosis is most common in the Northeastern U.S. (especially parts of New England, New York, and New Jersey) and upper Midwest (especially Wisconsin and Minnesota) and peaks during the warm months when ticks are most active. Anyone can get babesiosis, but it is more severe in the elderly and in those who have a weakened immune system.

People who spend more time outdoors are at higher risk of being bitten by an infected tick. Ticks can be found in areas with woods, brush, or tall grass. In Wisconsin, ticks are most active from May to September, but it is important to use caution year-round to prevent tick bites.


Adult tick with black spot on back rests on a leaf

Babesia is spread to humans through the bite of an infected deer tick.
  • A tick must be attached for at least 36-48 hours to spread the parasite to a person. It is important to remove ticks as soon as they are found to help prevent illness.
  • Most humans are infected by immature ticks, called nymphs.
    A pair of runner shoes walking through tall grasses
    • Nymphs are very small, about the size of a poppy seed. They are difficult to see, and most people may not feel their bite.
    • Nymphs are most active during the spring and early summer.
  • Adult ticks also spread Babesia.
    • Adults are much larger than nymphs, and are more likely to be found and removed before the parasite is spread to the person.
    • Adult ticks are most active during the cooler months.
  • Ticks can attach to any part of the body but are often found in hard-to-see areas, such as:
    • Behind the knees
    • Armpits
    • Scalp
    • In and around the ears
    • Inside the belly button
    • Groin
  • Although it is rare, a person can also get babesiosis from infected blood products used in transfusions. To help prevent this from happening, potential blood donors who have ever been diagnosed with babesiosis are unable to donate blood.

Babesiosis is preventable and treatable. Visit our Tick Bite Prevention page to learn how to prevent tick bites, and how to properly remove a tick if you are bitten.

Symptoms can show up one to four weeks after being bitten by an infected tick.

It is possible to not have any symptoms and have babesiosis. People who are elderly, have a weak immune system, do not have a spleen, or have other serious health conditions can have more severe symptoms.

Common signs and symptoms include:

  • Flu-like symptoms (fever, chills, headache, body aches, loss of appetite, nausea, tiredness)
  • Anemia (not having enough red blood cells)
  • Low blood platelets (blood cannot clot properly)
  • Jaundice (liver complications)
  • Spleen and liver enlargement

Severe Symptoms:

  • Difficulty breathing
  • Heart attack
  • Kidney failure
  • Liver disease
  • Confusion
  • Death

People who do not have symptoms do not need to be treated. Antibiotics used together with certain drugs that treat malaria have been found to work well in most patients. Very severe cases may require a blood transfusion.

DHS Resources
Centers for Disease Control and Prevention (CDC) Resources
Partner Resources
Babesiosis is preventable and treatable. Visit our Tick Bite Prevention page to learn how to protect yourself from illnesses spread by ticks.

Questions about illnesses spread by ticks? Contact us!
Phone: 608-267-9003 | Fax: 608-261-4976

Last revised June 29, 2020