West Nile virus is a disease that mostly affects birds and is spread by mosquitoes. Mosquitoes spread the virus by biting infected birds and then passing it on to other birds, animals, or people. Most people do not become very sick, but in some cases, West Nile virus can cause encephalitis, or swelling of the brain.

How West Nile virus is spread

In the majority of cases, West Nile virus is spread by the bite of an infected mosquito, but humans can get the virus in any of the following ways:

  • From the bite of a mosquito that is carrying the virus.
  • Through blood transfusions or organ donations from an infected donor. (You cannot get West Nile virus from donating your blood.)
  • From pregnant mothers to unborn children.
  • Through breast milk (but mothers are still encouraged to breastfeed their babies).

There is no evidence that a person can get the virus by handling live or dead infected birds. However, it is important to wear gloves when handling any dead animal and wash hands thoroughly with soap and hot water after removing gloves. There is a very low infection rate of West Nile virus in dogs and cats. The virus also causes illness and death in horses, but a vaccine for horses is available. If you are concerned, contact your veterinarian.

Illness and symptoms

Most people who are infected with West Nile virus do not have any symptoms or get sick at all. Less than one percent of people who get bitten and become infected get severely ill. Some people who are infected may develop a mild illness, with fever and body aches. The elderly are at greatest risk for serious illness. See your health provider if you experience symptoms.

Symptoms, when they occur, include:

  • Fever
  • Headache
  • Nausea
  • Body aches
  • Other symptoms

In rare occasions, the disease may progress to encephalitis that may cause severe headache, confusion, weakness, or dizziness. The time between a mosquito bite and onset of illness ranges from 3 to 14 days in humans.

Prevention and Personal Protection

You can help prevent the spread of West Nile Virus by reducing the number of mosquitoes around your home and neighborhood and protecting yourself from mosquito bites.

Report Dead Birds

Some birds, especially corvids such as crows and jays, are known to get sick and die from West Nile virus. Wild bird die-offs can indicate the presence of West Nile virus in our community. Multnomah County Vector Control checks dead birds for West Nile seasonally between May 15 and September 30.

To report a dead bird outside of these dates:

  • Portland Audubon Society | 503-292-9453, general@audubonportland.org
  • Oregon Department of Fish and Wildlife, Sauvie Island Wildlife Area | 503-621-3488

More Information

Centers for Disease Control & Prevention:


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Multnomah County Vector Control
5235 N Columbia Blvd
Portland, Oregon