West Nile Virus

West Nile is a viral infection first isolated in 1937 in Uganda. West Nile virus can affect humans, horses, and many types of birds. It is carried by mosquitoes, which become carriers by feeding on infected birds. The first case in Oregon was reported in Malheur County in August 2004.

Please consider vaccination or a booster for your horses. The best time to do this is before the start of mosquito season in your area.

2013 activity


Four horses have tested positive, including two new cases in Klamath and Umatilla Counties. The second equine to test positive for WNV in 2013 is a horse from Ontario (Malheur Co.). The affected horse is a 2 year old quarterhorse stallion who was not vaccinated. The horse is recovering. A horse from Union County was the first Oregon equine to test positive for the virus this year.


A bird from Ontario (Malheur County) tested positive for West Nile virus, the first one of the 2013 season.

Positive Mosquito Pools

  • Malheur County 61
  • Baker County 13    
  • Jackson County 1 
  • Klamath County 3
  • Morrow County 2
  • Umatilla County 2
  • Union County 2

Human Cases

According to Oregon Public Health Division officials, there have been 14 human cases of West Nile virus in Oregon residents: 10 from Malheur County, 2 in Harney County, and 1 each in Linn and Benton Counties.

Symptoms of the virus in equines

The virus causes an inflammation of the brain. Equines who contract the virus can experience lethargy, lack of coordination, stumbling, confusion, fever, stiffness, muscle twitching, depression, and weakness in the legs. About one-third of infected horses die.

Protect your horses through immunization

Since the virus is now endemic in Oregon, it is important to vaccinate your horses. The vaccine requires two doses given three to six weeks apart. Immunity may not be achieved until up to six weeks after the second dose, and some horses may require a third vaccination.

An annual booster should be given prior to the start of the mosquito season in your area. Consult your veterinarian for more information on immunizing your horses against West Nile Virus.

If you suspect your horse is infected with West Nile Virus

Contact your veterinarian for an examination. Veterinarians are required to report horses with the appropriate clinical signs. Contact the Oregon Dept. of Agriculture at (503) 986-4760 before submitting any samples for testing.

West Nile Virus in humans

Humans can only get the virus from the bite of an infected mosquito; the disease does not spread from other animals to humans, or from person to person. The illness causes no symptoms in 80 percent of those who are infected. Of those who develop an infection, most are mild, with fever and flu-like symptoms. About one in 150 develops severe illness, including coma, convulsions, paralysis and vision loss. The most severe cases, as well as most deaths, occur in those over 55 who have conditions such as diabetes or hypertension or chronic illness.

Minimize the threat of exposure to West Nile Virus

  • In addition to vaccinating your horses, the best way to minimize the threat of West Nile for your and your horses is to control mosquito populations and prevent exposure to them.
  • Eliminate sources of standing water that can serve as a breeding ground for mosquitoes, including children's wading pools, old tires, buckets, and other containers.
  • Change water in bird baths weekly.
  • Consider avoiding outdoor activities from dusk until dawn or take extra care to use repellent and protective clothing during evening and early morning.
  • When possible, wear long sleeves, long pants and socks when outdoors.
  • Treating clothes with repellents containing permethrin or DEET will provide extra protection, since mosquitoes may bite through thin clothing. Do not apply repellents containing permethrin or DEET directly to skin. Here are some tips for using DEET safely.

Veterinarians: Submit samples of possible cases for testing

The Center for Disease Control and Prevention has given the Oregon Department of Human Services a grant to be used to test equines or bird (or other animals) suspected of being infected with WNV. There is no charge for this testing. Please use the sample submission form below.

Published: March 11, 2009;    Updated: September 30, 2013

Filed Under: Zoonotic Diseases, Seasonal Issues, Equine

Sources: News reports, Oregon Health Authority Acute and Communicable Disease and Prevention, State Public Health Veterinarian, State Veterinarian, ODA