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West Nile Virus: About

West Nile virus is an illness spread by the bite of an infected mosquito. In Wisconsin, it is spread by Culex species mosquitoes.

West Nile virus cases occur throughout the U.S. In Wisconsin, West Nile virus was first found in wild birds in 2001, and the first human infections were reported in 2002.

Anyone can get West Nile virus, but people who spend more time outdoors are at a higher risk of being bitten by an infected mosquito. Mosquitoes are usually most active in Wisconsin from May to September.

The mosquitoes that spread West Nile virus can be found in areas near standing water, which they need to breed. It is important to remove standing water sources, such as containers, leaves, and yard debris, to reduce breeding habitats in your yard.

For more tips, please visit our Mosquito Bite Prevention page.

West Nile virus cases map

West Nile virus is spread to humans through the bite of an infected mosquito.

A mosquito on a skin.
  • Mosquitoes get infected with West Nile virus by feeding on infected birds.
    • After feeding on a bird that has West Nile virus, the virus may end up inside of the mosquito.
    • Once it has the virus, a mosquito can spread the virus to other birds and mammals, including humans, when they take another blood meal.
  • The mosquitoes that spread West Nile virus are active during the summer and into the fall, or anytime during warm weather months.
  • The risk of West Nile virus is present anytime mosquitoes are active, but it is highest during the months of August and September.
  • Few mosquitoes actually carry the virus, but it is important to take prevention measures when spending time outside.
  • West Nile virus cannot be spread person to person or directly from a bird to a person.
  • It is possible to become infected with West Nile virus through a blood transfusion or organ transplant from infected donors, but this is very rare.

Dead Bird Reporting

As of January 2020, the Division of Public Health no longer collects dead birds for West Nile virus testing. Due to this change, the Dead Bird Reporting Hotline (800-433-1610) has been disconnected.

What should I do if I find a dead bird?

Please refer to the Wisconsin Department of Natural Resources Wildlife Health Program for instructions on what to do if you find a dead bird. If you are told to dispose of the bird’s carcass, don’t handle it with your bare hands. Use gloves or an inverted plastic bag to place the carcass in a garbage bag, which can then be placed in your regular trash.

Symptoms can start 3–14 days after being bitten by an infected mosquito.

About 80% of people infected with West Nile virus never develop symptoms, while about 20% of people may experience mild illness. Less than 1% of people infected will become severely ill. In rare cases, the infection may be fatal, especially in the elderly and people with other medical conditions. Past infection with West Nile virus can make you immune to the virus in the future.

Mild signs and symptoms:

  • Fever
  • Headache
  • Muscle aches
  • Joint pain
  • Rash
  • Swollen lymph nodes
  • Photophobia (sensitivity to light)
  • Nausea
  • Vomiting

Severe signs and symptoms:

  • Extreme muscle weakness
  • Inflammation of the brain (encephalitis)
  • Confusion or disorientation
  • Paralysis
  • Coma
  • Death

There is currently no available treatment or vaccine for West Nile virus. Over-the-counter pain relievers may be given to relieve mild symptoms. In severe cases, patients may need to be hospitalized to receive supportive treatment. If you believe you or a family member may have West Nile virus, contact your doctor.

DHS resources
CDC resources

Mosquito Bites Are Bad: An educational activity book for kids about preventing illnesses spread by mosquitoes.

Partner resources
West Nile virus is preventable. Visit our Mosquito Bite Prevention page to learn how to protect yourself from illnesses spread by mosquitoes.

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

Last revised June 15, 2022