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About Powassan Virus

Powassan virus is an illness spread by ticks. It spreads to humans by the bite of an infected deer tick, also known as a black-legged tick.

Powassan virus is rare in Wisconsin. The first case in the state was identified in 2003. Since then, most cases of Powassan virus have been in northern Wisconsin. Nationally, Powassan virus cases are most common in the northeast and Great Lakes regions.

Anyone can get Powassan virus. People who spend more time outdoors are at a higher risk of getting bitten by an infected tick. Ticks are commonly found in areas with woods, brush, or tall grass. Ticks are most active from May to November, but it’s important to use caution year-round.

Powassan virus cases in Wisconsin map

Protect yourself with Tick Bite Prevention

Powassan 101

Powassan virus is spread to people through the bite of an infected deer tick. Ticks get the virus by biting an infected mammal, usually small rodents.

To prevent getting sick, it’s important to remove ticks as soon as you find them. It isn’t known exactly how long an infected tick must be attached to a person to spread Powassan virus. It’s likely less than 12 hours, and could be as little as 15 minutes.

Most people are infected by immature ticks, called nymphs. Nymphs are very small, about the size of a poppy seed. They’re hard to see. Most people don’t feel their bites. This makes them harder to remove right away. Nymphs are most active during the spring and early summer.

Adult tick with black spot on back rests on a leaf

Adult ticks also spread Powassan virus. Adults are much larger than nymphs. They’re more likely to be found and removed before the virus spreads to the person. Adult ticks are most active during the cooler months in early spring and fall.

Ticks can attach to any part of the body. They often are found in hard-to-see areas, such as:

  • Behind the knees.
  • In the armpits.
  • On the scalp.
  • In and around the ears.
  • Inside the belly button.
  • On the groin.

Powassan virus is preventable and treatable. Learn about Tick Bite Prevention and how to properly remove a tick if you’re bitten.

Symptoms can show up one week to one month after being bitten by an infected tick. Many people who become infected with Powassan virus don’t have any symptoms.

Common early signs and symptoms include:

  • Fever
  • Headache
  • Nausea
  • Vomiting
  • Muscle weakness
  • Stiff neck

Severe illness can include:

  • Confusion
  • Loss of coordination
  • Paralysis
  • Speech difficulty
  • Seizures
  • Memory loss
  • Encephalitis (inflammation of the brain)
  • Meningitis (inflammation of the membranes around the brain and spinal cord)

See your doctor right away if you have any of these symptoms after being outdoors, even if you don’t remember being bitten by a tick.

Powassan virus can cause serious illness. Powassan virus causes death in about 10% of severe cases. Half of those who survive severe disease have permanent or long-term neurologic symptoms. These can include recurring headaches, memory problems, and muscle weakness or loss.

Currently, there isn’t a treatment or vaccine for Powassan virus. Over-the-counter pain relievers may help with symptoms. In severe cases, patients may need supportive treatment in a hospital. If you believe you or a family member may have Powassan virus, contact a doctor right away.

Resources from the Wisconsin Department of Health Services (DHS)

Resources from the CDC (Centers for Disease Control and Prevention)

Partner resources

  • Information from the University of Wisconsin-Madison Medical Entomology Laboratory—Wisconsin Ticks and Tick-borne Diseases
  • Information on ticks found in the Midwest, surveillance resources, and tick biology and development from the Midwest Center of Excellence for Vector-Borne Diseases—Ticks

Contact us

Questions about illnesses spread by ticks? We’re here to help.

Bureau of Communicable Diseases
Phone: 608-267-9003
Fax: 608-261-4976

Last revised January 3, 2023