The panic surrounding West Nile virus has quieted down, but the virus is still a cause for concern among horse owners. This crafty virus crossed the Atlantic in 1999 and has made a rapid progress westward. Over 9,000 equine West Nile cases were reported last year, including 431 Illinois cases. With a new vaccine available and an increased vigilance against mosquitoes, veterinarians and horse owners alike hope these numbers will be lower in 2003.

Mosquitoes are the pests that carry and spread West Nile virus. Several different species can pick up the virus by feeding on infected birds. “Birds are the amplifying host. The virus is present in their bloodstream for long periods, and many birds are unaffected by the virus. Migratory birds may be how the virus spread to new areas,” says Dr. Brett Tennent-Brown, the equine veterinarian handling most of the West Nile cases at the University of Illinois Veterinary Teaching Hospital in Urbana.

West Nile virus causes an encephalitis, or inflammation of the brain. It damages the brain and the spinal cord, leading to neurological signs such as tremors, circling, difficulty walking, unusual stance, and muscle twitching. This disease is similar to eastern equine encephalitis, western equine encephalitis, and Venezuelan equine encephalitis. However, West Nile virus is from a different family of viruses; even if your horse has been vaccinated for these other diseases, it is not safe from West Nile.

There is no cure or specific treatment for horses diagnosed with a West Nile virus infection. Horses are given supportive care to help them on the road to recovery. Nevertheless, around 35 percent of horses diagnosed with West Nile virus during the last 3 years died or were euthanized. It is easier and less expensive to prevent than to treat West Nile virus infections.

The good news is that a vaccine is now available. It was first conditionally approved in August 2001. It is now fully licensed and available through veterinarians. “The West Nile vaccine is a killed-virus vaccine requiring two shots given 3 to 6 weeks apart, followed by an annual booster,” notes Dr. Tennent-Brown. “Keep in mind that horses are not immune to the virus until 2 weeks after the second shot.”

The vaccine is not licensed for use in horses under one year of age. Several research projects are under way to develop vaccine schedules for young horses. The consensus is that it is safe to vaccinate foals beginning at 6 to 8 weeks of age. If the mare had been previously vaccinated, a booster can be given to the foal at 3 to 4 months of age. Data on the vaccine shows it to be highly effective and very safe. There have been local tissue reactions reported, but no other serious side effects. Failure of the vaccine is usually attributed to infection prior to vaccination or infection before full immunity is achieved.

“Mosquito control is also very important in helping prevent West Nile virus infections,” says Dr. Tennent-Brown. “Reducing the potential exposure to mosquitoes is the first step. Standing water is the breeding ground for these pests. They can reproduce within 5 days and need only a small amount of water.”

Mosquitoes are most active and populous at dusk and dawn. Extra care should be taken to avoid turning out horses at these times. Keeping horses away from standing water and spraying horses with mosquito repellents during these hours is also beneficial.

Make sure all water sources are clean. It is important that no organic matter is present. Try to cut back vegetation around water and near the barn. Sheets, leg covers, and facemasks can also be sprayed with mosquito repellents. It is also a good idea to spray barn stalls, aisle ways, and the lower branches of shade trees.

Placing fans in the barn can reduce exposure to mosquitoes. Dr. Tennent-Brown recommends putting screens on barn doors and stalls—so long as care is taken not to trap mosquitoes inside.

Consult your local veterinarian about the prevalence of West Nile virus in your area. For further discussion of prevention, diagnoses, and treatment of this disease speak with your veterinarian.