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West Nile Virus and Horses

West Nile virus

West Nile virus is a mosquito-borne disease of birds that was first detected in the U.S. in 1999. It has steadily moved west and late in 2002 the disease was confirmed in 2 birds and 2 horses in Washington State. The Centers for Disease Control and Prevention have predicted that the West Coast will probably be hit hard with West Nile Virus in 2004.

This virus is spread by mosquitoes when they feed on infected birds. There are at least 6 known species of mosquitoes in our region that can transmit this virus. Our mosquito season generally runs from March through October, peaking in August and September.

Although the virus does not affect most humans and horses, it should be considered a serious threat. The virus normally induces mild symptoms, but it can cause encephalitis, an inflammation of the brain, or lining of the brain and spinal cord. The virus can affect all equines, including horses, mules, and donkeys.

Symptoms in Horses

Most horses are not affected by the virus and do not show signs of illness. Look for two or more of these symptoms: loss of appetite, fever, difficulty walking (stumbling, knuckling over, falling), head tilt, muscle tremors and weakness, depression, hypersensitivity, paralysis, and convulsions. The mortality rate in horses that contract West Nile virus is about 30%. Approximately 40% of the horses that recover will exhibit some residual affects of the disease, such as neurological or behavioral abnormalities.

Vaccinating Horses

There are two vaccines available to provide protection against West Nile virus for horses, mules and donkeys. In addition to Fort Dodge Animal Health’s West Nile Innovator approved by USDA in 2002, a new vectored modified live vaccine was recently approved. The new product, using recombinant DNA technology, Recombitek Equine West

Nile Virus vaccine is produced by Merial. The cost of these two vaccines is very comparable. These vaccines must be obtained from veterinarians and must be administered by or under the supervision of a veterinarian with a valid veterinarian/client/ patient relationship. The vaccination requires two initial doses, ideally three to four weeks apart. Depending on the vaccine used, immunity may not be achieved for up to six weeks after the second dose. An annual booster is required prior to the mosquito season.

Horses vaccinated against other mosquito- borne diseases, such as Western Equine Encephalomyelitis (WEE), are not protected against West Nile virus. However, there are new combination vaccines that include West Nile virus with Equine Encephalomyelitis strains.

Previously Vaccinated Horses:

  1. If your horse was fully vaccinated last year, follow-up with a booster a few weeks to a month prior to the mosquito season this year.

Unvaccinated Horses:

  1. Vaccinate all previously unvaccinated adult horses and all young horses under one year-of-age in March/April with a two-dose initial series, three to four weeks Talk to your veterinarian about your best options for vaccinating pregnant mares.
  1. Young horses under one year-of-age may need a third shot approximately eight weeks after the initial two-dose

Other Protection for Horses

Vaccination of horses is the primary protection against West Nile virus. However, other steps can be taken to minimize the threat of West Nile virus. Recent research suggests that keeping horses in stalls at night could reduce their risk of

infection. Stables should have well- maintained screens. Using fans in stables can also reduce the ability of mosquitoes to feed on horses.

Additional precautions include treating the horse area with an insecticide and/or using repellents on horses. Repellents require good coverage over the entire horse and frequent re- applications. Consult your veterinarian about their use.

Reporting Suspected Cases in Horses

Horse owners are encouraged to contact a veterinarian if their animals display signs of illness including loss of appetite, fever, tremors, weakness, or inability to stand. Veterinarians are required to report all cases of equine encephalitis. Any suspected cases of West Nile virus in horses and other equines must be reported to the Washington State Department of Agriculture (WSDA).

The mosquito goes through four separate and distinct stages in its life cycle: egg, larva, pupa,

and adult. Each of these stages can be recognized by its’ special appearance. The time it takes for the egg, larva, and pupa stages to develop depends on temperature and species characteristics. Depending on the environment, some species can go through their entire life cycle in as little as four days or as long as one month.

Reducing Mosquito Populations

Be aware that mosquitoes can breed in any water that stands for four to five days. This includes buckets, barrels, cans, tarp hay covers, ponds, bird baths, etc. Be sure to check containers in “out-of-the-way” places.

Prevent stagnant pools of water. Discard or drill holes in the bottom of old tires and other containers around your farm where water can accumulate and serve as a breeding ground for mosquitoes.

Change the water at least every week in stock tanks, bird baths, etc. Aerate water that is not regularly changed. Add larvae eating fish to ponds and stock tanks.

Protecting Yourself from Mosquito Bites

Apply insect repellent containing DEET (N, N- diethyl-m-toluamide). The more DEET a repellent contains the longer it can protect you from mosquito bites. Choose a repellent that provides protection for the length of time that you will be outdoors.

  • Wear long-sleeved shirts and long pants whenever you are in mosquito-infested Spray clothing with repellents containing permethrin or DEET as mosquitoes may bite through thin clothing.
  • Stay indoors at dawn, dusk, and in the early evening, which are the peak mosquito biting


Obtaining More Information

Washington State Department of Health
1-866-78VIRUS (1-866-788-4787)

Washington State Department of Health

Washington Department of Agriculture

Centers for Disease Control & Prevention

Animal & Plant Health Inspection Service