West Nile Virus

West Nile is a viral infection that can affect humans, horses, and many types of birds. It is carried by mosquitoes, which become carriers by feeding on infected birds.

In 2018, there were two human cases of West Nile virus in two Oregon counties: Harney and Clackamas. The virus was found in one bird, 58 mosquito pools — samples of about 50 mosquitoes each — and two horses. In 2017, seven humans, 92 mosquito pools, five horses and one bird tested positive for West Nile in Oregon.

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.

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

The virus is endemic in Oregon and it is important to vaccinate your horses. Initial vaccination requires two doses given three to six weeks apart. Immunity may not be achieved for up to six weeks after the second dose. Some horses may require a third dose to stimulate proper immunity. Following the initial series, annual boosters—given two to four weeks before the mosquito season begins—will provide adequate protection. 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 suspected cases to state officials.

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.
  • Clean bird baths and change water in them at least weekly, and more often in the summer.
  • 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.

Updated: 2019-05-20 07:00:00

Sources: Oregon Health Authority, State Public Health Veterinarian, State Veterinarian, Oregon Department of Agriculture