West Nile Virus

West Nile Virus Information for Horse Owners

West Nile Virus (WNV) is a virus that can infect many different types of animals but causes sickness only in a small number of species. In North America, humans, horses, and especially members of the crow family (crows, ravens, magpies, blue and gray jays) are at most risk for serious illness. The main reservoir for the virus is thought to be wild waterfowl. The WNV is spread from bird to bird, and to other animals, by the bites of mosquitoes. The horse is a dead-end host for WNV - the virus does not spread from horse to horse, or to other animals, including humans.

WNV first appeared in North America in 1999 and has spread steadily across the country over the past three years. The virus was detected in birds in Manitoba in early July 2002, and now is found throughout Manitoba. Cases of WNV in horses in Manitoba have been confirmed. 

What to Look for in Your Horse

Horses and other members of the equine species become infected following being bitten by a mosquito carrying WNV. The symptoms relate to the central nervous system, because the virus causes an encephalomyelitis or brain and spinal cord infection. Symptoms include listlessness, depression, loss of appetite, stumbling and incoordination, weakness of limbs, muscle twitching, partial paralysis, inability to swallow, head pressing, recumbency, convulsions and death. Fever may or may not be present. The symptoms will progress from the very early, mild symptoms through to recumbency and death in a period of five to seven days.

These symptoms are easily confused with other nervous system disorders in the horse. Diseases to be considered when making a diagnosis include rabies, sleeping sickness, equine viral rhinopneumonitis, equine protozoal myeloencephalitis, and tetanus.

All ages of horses are susceptible to the disease, although very young or very old horses, or horses already compromised by some other disease condition, are more likely to be affected.

Treatment Options

WNV is a serious disease, with a case fatality rate approaching 35 - 40 %. In many cases, the affected horse is humanely euthanised after becoming recumbent and exhibiting convulsions. However, if symptoms are identified early, and supportive therapy is instituted, the success rate can be improved. These treatments may include fluid therapy, non-steroidal anti inflammatory drugs, antibiotics, diuretics, and excellent nursing care.

Diagnosis

When a horse owner identifies any of the symptoms outlined, a veterinarian should be consulted to make a differential diagnosis, to collect samples for testing and to begin treatment. Blood samples are collected from the horse to test for the presence of WNV antibodies. Other tests will also be run to rule out other potential causes of the symptoms. A second blood sample, 10 - 14 days later is collected to confirm the diagnosis. If your horse dies, the preferred option is to have the animal subjected to a post mortem at the provincial veterinary laboratory. In any event, the horse's brain and portions of the spinal cord should be collected for analysis. The brain will be tested for rabies as well as for WNV.

Prevention

There are two vaccines available for WNV in horses, and these are available through your veterinarian. The veterinarian must keep detailed records on what horses are vaccinated (breed, age, reproductive status, etc.), lot numbers, results and reactions. A blood sample before vaccination may be recommended especially if the horse could be exported. Some countries require a negative WNV blood test and the vaccination could hinder evaluation for previous infection.

The vaccine is a killed virus vaccine. It requires two doses in the initial series to build up protective levels of antibodies. The two doses are administered at an interval of 3-4 weeks, with protective immunity developing in 2-4 weeks after the second vaccination. Horse owners must realize that their horse will not be protected form WNV infection for at least five weeks, and more likely eight weeks, after the vaccination series is begun.

Consult with your veterinarian to set up a health program for your horse. WNV is only one of many diseases to be considered in a vaccination program. Consider all the factors -risk, cost, mosquito levels, time of year, before embarking on a WNV vaccination program.

Other preventative measures center on mosquito control. Where possible, stable horses at night in a barn with mosquito netting in place. Use insect sprays and repellants. Where permitted, burn smudges to reduce mosquito activity near your horses. Reduce the amount of tall vegetation around corrals and barns. And get rid of all sources of standing, stagnant water.

Final Comments

West Nile Virus is an emerging disease, and it can be a serious, fatal disease in the horse.  The risk to your horse of contracting WNV is low. If you see signs of a central nervous system disorder in your horse, call for veterinary assistance to rule out the more likely diseases such as rabies, and to institute a treatment regimen.

Links:

• West Nile Virus (CFIA)

• West Nile Virus (Manitoba Health)