West Nile

West Nile Route of TransmissionWhat is West Nile Virus?

West Nile virus is a virus most commonly spread by infected mosquitoes. West Nile virus can cause febrile illness, encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord).

How do People Get Infected with West Nile Virus?

Most people get infected with West Nile virus by the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds. Infected mosquitoes can then spread the virus to humans and other animals.

In a very small number of cases, West Nile virus has been spread through blood transfusions, organ transplants, and from mother to baby during pregnancy, delivery, or breastfeeding.


The chances of someone becoming infected with West Nile virus is low and most people do not have symptoms. Those who do become ill typically with:

  • Fever
  • Headache
  • Body aches
  • Swollen lymph glands that last a few days
  • Nervous system involvement may occur in a small percentage of people.

Symptoms may begin 3 to 15 days after a bite by an infected mosquito. Older adults and people with weak immune systems have a higher chance of severe disease caused by the virus.

There is no specific medication to treat West Nile virus infection other than to help with the symptoms. If you think you may have a West Nile virus infection, contact your health care provider.

While people typically become ill from the West Nile virus from from June through October, most people become ill during August and September.

Who is at Risk for Infection with West Nile Virus?

Anyone living in an area where West Nile virus is present in mosquitoes can get infected. West Nile virus has been detected in all lower 48 states (not in Hawaii or Alaska). Outbreaks have been occurring every summer since 1999. The risk of infection is highest for people who work outside or participate in outdoor activities because of greater exposure to mosquitoes.

How Can People Reduce the Chance of Getting Infected?

The most effective way to avoid West Nile virus disease is to prevent mosquito bites:

  1. Use insect repellents when you go outdoors. Repellents containing DEET, picaridin, IR3535, and some oil of lemon eucalyptus and para-menthane-diol products provide longer-lasting protection.
  2. Wear long sleeves and pants from dusk through dawn when many mosquitoes are most active.
  3. Install or repair screens on windows and doors. If you have it, use your air conditioning.
  4. Help reduce the number of mosquitoes around your home. Empty standing water from containers such as flowerpots, gutters, buckets, pool covers, pet water dishes, discarded tires, and birdbaths.
  5. Clean and chlorinate swimming pools, outdoor saunas, and hot tubs; drain water from pool covers.
  6. Trim tall grass, weeds, and vines since mosquitoes use these areas to rest during hot daylight hours.

Useful Resource:

Wisconsin Department of Health Other Mosquito diseases

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