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What is West Nile virus?

West Nile virus (WNV) is a mosquito-borne infection that can cause encephalitis or meningitis (an inflammation of the brain and surrounding tissues) in humans. It is believed to have arrived in the eastern US in 1999 or before. You can see current confirmed cases in the US on this Map.


The virus is transmitted to humans by mosquitoes.

It is NOT transmitted from person to person. You cannot get it from touching or kissing a person who has the virus or from a health care worker who has treated someone with it.

Birds are the primary host for the virus (they may also become sick) and mosquitoes are the vectors that transmit it. Humans and livestock are incidentally infected. Horses are at risk of severe disease, too.

Mosquitoes become infected with West Nile virus when they feed on infected birds that carry the virus in their blood. Mosquitoes' salivary glands become infected and those infected mosquitoes can then transmit the virus to humans and other animals while biting them to take blood. During blood feeding, the mosquito injects the virus into the animal or human, where it multiplies and may cause illness.

Crows and some other animals are susceptible to infection with West Nile virus. There is no evidence that a person can get the virus from handling live or dead infected birds or other animals. However, people should avoid bare-handed contact with any dead animals and use gloves or double plastic bags to pick up the carcass and place it in a container.

Because birds are a key link in the transmission of the disease, individuals are encouraged to report dead ravens, crows and blue jays in Michigan. If these dead birds are found on the U-M campus, contact U-M Plant Services at 734-647-2059 for pick up and reporting. In Michigan, you can report a dead bird via the form on the Michigan Department of Natural Resources website.


Most people infected with the West Nile virus have no symptoms of illness.

Approximately 20% of those infected develop a mild illness with fever, headache and body aches, sometimes with skin rash and swollen lymph glands. The incubation period after the bite of an infected mosquito is thought to range from 2 to 14 days, and symptoms generally last 3 to 6 days.

Less than 1% of those infected with the virus will develop severe illness. Symptoms of severe illness may include headache, encephalitis or meningitis, high fever, stiff neck, change in mental status, disorientation, coma, tremors, convulsions, muscle weakness and paralysis. Among those with severe illness, 10% may die, and death is more likely among the elderly.

If you have symptoms of severe illness, call or visit a health care provider as soon as possible for assessment. For information about using UHS, see Nurse Advice by Phone or How to Get Health Care.

Even in areas where mosquitoes carry the virus, very few mosquitoes (much less than 1%) are infected. If a mosquito is infected, less than 1% of people who get bitten and become infected will get severely ill. The chances that you will become severely ill from any one mosquito bite are extremely small.


There is no human vaccine for West Nile infection, but you can reduce the risk of infection in the following ways:

  • Avoid situations where you will be exposed to mosquitoes.
  • Apply insect repellent that contains the active ingredient DEET to exposed skin. Follow the manufacturer's directions for use on the label.
  • Spray clothing with repellents containing permethrin or DEET because mosquitoes may bite through thin clothing. Follow the manufacturer's directions for use on the label.
  • Wear long-sleeved shirts and long pants when outdoors.
  • Maintain window and door screens to keep mosquitoes out of buildings.
  • Empty water from mosquito breeding sites, such as flower pots, pet bowls, clogged rain gutters, swimming pool covers, discarded tires, buckets, barrels, cans and similar sites in which mosquitoes can lay eggs.

More information:

University Health Service:

  • How to Get Health Care: Appointments are required for most medical services, however options are available for urgent concerns.

  • Nurse Advice by Phone is available day and night, which may save a trip to UHS, the ER or an urgent care facility.

Washtenaw County Health Department - phone 734-544-6750

Centers for Disease Control and Prevention