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West Nile virus

West Nile virus (WNV) is a viral disease that was commonly found in Africa, Europe, the Middle East and West Asia. It first appeared in New York in 1999 and quickly spread throughout North America. It was first detected in Wisconsin in 2001. WNV has been documented in over 250 species of birds and several species of mammals, including humans.


WNV is spread through the bite of infected mosquitos. The virus is maintained and replicates in certain species of birds. A mosquito becomes infected with the virus when it takes a blood meal from a bird with a circulating virus in its blood and then transmits the virus to another bird or mammal, including humans.


Many species of birds can be infected with WNV without showing signs. However, corvid species (blue jays, crows, and ravens), some raptors (hawks and owls), and species of birds that have never been exposed to the virus can die from the infection. The infection can cause inflammation of the brain and heart. Signs of infection in birds may not appear until the last stage of the disease when the brain becomes inflamed. These signs may include tremors, head tilting, lethargy, loss of coordination, inability to fly or walk properly or trouble standing upright. Often, birds die suddenly after exhibiting no signs at all.


Currently, the effects of WNV on bird populations in Wisconsin and the rest of North America are unknown. Some simple precautions can be taken to help reduce to spread of the virus by reducing or eliminating mosquito breeding habitats.

  • Eliminate stagnant water from your property.
  • Regularly clean birdbaths to remove the stagnant water and mosquito larvae.
  • Empty water-filled containers where mosquitoes may breed.

If you find a group of dead birds, contact the DNR Wildlife Switchboard by emailing or calling 608-267-0866. You will need to leave a message for the switchboard staff to return your call. In your message, please include the number of animals, the species (such as crow), if they were sick or dead, the specific location where you saw them, including the county and your contact information.


Humans are susceptible to WNV, however, typically only a small number of people exposed to the virus become infected. People who do become infected with WNV typically either have no symptoms or mild, flu-like disease. In some cases, usually among the elderly, WNV causes a serious disease that affects the brain tissue and can be fatal.

People should take precautions in areas where WNV activity is high. Reducing exposure to mosquitoes is the best method of control. More information about reducing exposure to WNV can be found on the Wisconsin Department of Health website [exit DNR].