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West Nile Virus in Horses

Healthy horse after having been given the treatment for West Nile Virus  

The West Nile Virus (WNV):

West Nile Virus, which is carried by mosquitoes, came to the US from Europe, Asia, Africa or the South Pacific in 1999. The virus attacks the nervous system of horses, birds, and humans, beginning by damaging cells within the brain, then moving down the spine. This differentiates WNV from viruses such as Eastern Equine Encephalitis and Western Equine Encephalitis that attack the nervous system and travel up the spinal cord to the brain.

West Nile Virus infects horses, birds and humans, but only birds are able to concentrate enough of the virus within their bodies that they can spread an infection to other birds, horses, or humans. This means horses don’t transmit West Nile Virus infection directly to other horses, and horses can’t normally transmit the virus directly to people.

West Nile Virus Symptoms in horses

Horses with West Nile Virus lose their balance and become ataxic, usually with more problems in the hind limbs than in the front limbs. Their muscles can be either flaccid or stiff. Horses develop fevers, some become hyper, some become depressed, some are blind, and most have a head tilt. Problems usually develop bilaterally, which means they occur on both sides of the body rather than on just one side. Although 30% of sick horses die, those that do not die may return to normal and have no residual problems. In the few years since West Nile Virus has been in the US, it has infected more than 22,000 horses.

Prevention of West Nile Virus

Fortunately there is a vaccine that helps prevent West Nile Virus infection. The vaccine is injected into the horse’s muscles and it stimulates the horse’s immune system to attack West Nile Virus. If a mosquito with West Nile Virus bites, the horse’s white blood cells are primed to attack the virus before it can damage the nervous system. While some vaccines contain live, weakened material that grows within the animal’s body, West Nile Virus Innovator is a vaccine without live components. This is called a killed vaccine. Killed vaccines have the advantage of stimulating the immune system without ever being able to actually cause an infection.

Horses are given two injections of West Nile Virus Innovator vaccine between 3 and 6 weeks apart. Ideally the first injection is prior to mosquito season. After the first two injections, a single vaccine injection is given annually.  Over 8 million vaccine doses have been given, and the adverse reaction rate has been 0.008%. In some cases, adverse reactions have been seen with pregnant horses, so it is best to discuss the use of this vaccine with your veterinarian if your horse is pregnant.

The best method of prevention though is to avoid mosquito bites. To repel mosquitoes that carry West Nile Virus, use Flys-Off Fly Repellent Ointment.

Watch a video about the West Nile Virus and how it is spread (the human story).

Find more West Nile Virus stats, reports, cases, and maps.  

The articles here were answered by a variety of pharmacists and veterinarians
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  • fever
  • ataxia
  • head tilt
  • blindness
  • stiff or flaccid muscles
      In the few years since West Nile Virus has been in the US, it has infected more than 22,000 horses.

    West Nile Virus (WNV), which is carried by mosquitoes, kills 1/3 of the horses it infects.

    Recommended medications for West Nile Virus in horses
  • West Nile Virus Innovator
  • Flys-Off Fly Repellent Ointment
    This information is for educational purposes only and is intended to be a supplement to, and not a substitute for, the expertise and professional judgment of your veterinarian. The information is NOT to be used for diagnosis or treatment of your pet. You should always consult your own veterinarian for specific advice concerning the treatment of your pet.

    The information about medications is not intended to cover all possible uses, directions, precautions, warnings, allergic reactions, drug interactions or adverse effects, nor should it be construed to indicate that use of a particular drug is safe, appropriate or effective for your pet. It is not a substitute for a veterinary exam, and it does not replace the need for services provided by your veterinarian.

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