West Nile Virus

West Nile Virus (WNV) 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 WNV is spread

In the majority of cases, WNV is spread by the bite of an infected mosquito, but humans can get WNV 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 WNV 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 WNV in dogs and cats. WNV also causes illness and death in horses, but a WNV vaccine for horses is available. If you are concerned, contact your veterinarian.

Illness and symptoms

Most people who are infected with WNV 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 three to 14 days in humans.

Prevention and Personal Protection

Reducing the number of mosquitoes in your area

In order to reduce the number of mosquitoes around your home and neighborhood, you should decrease the number of places where mosquitoes breed. Mosquitoes reproduce by laying eggs on standing water and they can develop in any standing water that lasts more than four days and even a small bucket with standing water can become home to up to 1,000 mosquitoes. Here are some tips for removing standing water:


  • Throw away or empty tin cans, plastic containers, ceramic pots or other containers.
  • Remove old tires, and turn over wheelbarrows.

Ponds and pools

  • Do not allow water gardens or ornamental ponds to stand still or stagnate.
  • Use mosquito fish in ponds or water gardens.
  • Turn over wading pools when not in use.
  • Change the water in birdbaths at least once a week.
  • Keep swimming pools cleaned and chlorinated. Empty water that collects on swimming pool covers.

Other standing water

  • Fill in puddles. Report large areas of standing water. [Link to form]
  • Clean rain gutters so water flows freely.

Protecting yourself and your family

Here are the best ways to decrease the chances of mosquito bites:

  • Install or repair all window and door screens.
  • Consider staying indoors during peak mosquito biting times, from dusk to dawn.
  • Wear long-sleeved clothing and long pants whenever you are outdoors.
  • Avoid perfumes and colognes when outdoors for long periods of time.
  • Use a mosquito repellent containing 20 to 30 percent DEET (a chemical used as insect repellent) for adults and no more than 10 percent for children when outdoors. Do not use mosquito repellent containing DEET on children under three years of age. Read carefully and follow all directions on the insect repellent package.
  • Hunters should wear gloves when handling and cleaning animals to prevent blood exposure to bare hands. Game meat should be cooked thoroughly.
  • Limit outdoor activities when advised by local officials

The spread of West Nile Virus

West Nile Virus was first detected in the U.S.in 1999 and has quickly spread across the country. It was detected first in Oregonin 2004 in the southeastern countyof Malheur. In MultnomahCounty, the first case of West Nile was found in 2006 in a bird in southeast Portland. Three additional cases were discovered 2008, two in northeast Portland and one in Gresham. There were 16 West Nilevirus cases, all in birds, in 2007.

Alien Invaders: Exotic mosquitoes found in Multnomah County

In spring 2006, Multnomah County Vector Control first found the non-native Japanese or Rock Pool mosquito, Ochlerotatus japonicus, in MultnomahCounty. This Asian species is generally found in Japan, Korea, Taiwan, and South China. It is not known how this species arrived in MultnomahCounty, but the movement of waste tires or freight is suspected as the cause. This invasive species is of public health concern for its capacity to transmit West Nile Virus and Japanese encephalitis. Vector Control has been diligently tracking this mosquito since 2006 and to date, no further mosquitoes have been detected. MultnomahCounty, State of Oregonand the Washington State Department of Health, Zoonotic Disease Program are working together to track and share information on this alien invader.