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.
How is West Nile virus spread to humans?
West Nile virus is spread to humans through the bite of an infected mosquito.
- 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.
- 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.
The Division of Public Health monitors dead birds for West Nile virus as an early warning sign, which shows that the virus may be present in an area.
- You can call the Dead Bird Reporting Hotline to report a sick or dead bird, and coordinate West Nile virus testing of crows, blue jays, or ravens.
- The hotline is open from May 1 to October 31.
- If you are calling from a Wisconsin area code, please call 800-433-1610.
- If you are calling from a non-Wisconsin area code, please call 608-837-2727.
- You are encouraged to report any sick or dead bird, but corvids (crows, ravens, and blue jays) are of particular interest for West Nile virus, because they are known to get sick and die from a West Nile virus infection.
NOTE: Once a county has one positive dead bird result, testing of other dead birds in that county for West Nile virus will be stopped until the next season begins.
What are the signs and symptoms of West Nile virus?
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:
- Muscle aches
- Joint pain
- Swollen lymph nodes
- Photophobia (sensitivity to light)
Severe signs and symptoms:
- Extreme muscle weakness
- Inflammation of the brain (encephalitis)
- Confusion or disorientation
How is West Nile virus treated?
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.
- West Nile Virus Fact Sheet, P-42104 (multiple languages): Educational fact sheet for the general public on West Nile virus covering signs and symptoms, treatment, and prevention.
- Protecting Your Family From Mosquitoes and Ticks, P-02080 (PDF): A fact sheet with simple steps you can take to protect yourself from mosquitoes.
Mosquito Bites Are Bad: An educational activity book for kids about preventing illnesses spread by mosquitoes.
- Wisconsin Mosquitoes and Mosquito-Borne Diseases: Information on mosquitoes and diseases they spread from our partners at the University of Wisconsin-Madison Medical Entomology Laboratory.
- Midwest Center of Excellence for Vector-Borne Disease Mosquitoes: Information on mosquitoes you may find in the Midwest and prevention 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