West Nile Virus

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Here’s how it all began: In September 1999, an outbreak of human encephalitis cases and deaths of both wild and exotic birds led the New York City zoo’s veterinary pathologist to suspect that a single causative agent, or pathogen, was responsible. Then—ready for some government acronyms?—The United States Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) Veterinary Services (VS) National Veterinary Services Laboratory (NVSL) isolated and identified the virus as West Nile virus (WNV) from a crow on September 14 and from a horse on October 12 of that year. WNV, an arthropod-borne virus, had never before been detected in the Western Hemisphere: It is now here to stay.

Dr. Timothy Cordes, APHIS Staff Veterinarian, reports that “equine immunity from disease caused by WNV is developed through natural exposure and/or vaccination. Based on the statistics of the last five years, it appears that in most geographic regions the number of equine cases peak in the second year that virus is detected and decline thereafter. This downward trend has been consistent except in rare isolated pockets where horses have not been vaccinated.”

Dr. Cordes predicts that “the anticipated future of West Nile equine encephalomyelitis in the United States should parallel historically the decline seen in other encephalilides, namely Eastern equine encepha­lomyelitis and Western equine encephalomyelitis, against which the horse-owning public now regularly vaccinates with separate vaccines that have no cross-protection for WNV. The significant national decrease in WNV equine cases (compared to the increase in human cases) in 2003 strongly suggests the beneficial outcome of the widespread equine vaccination program in the United States.”

“Veterinary Services predicts the downward trend in equine WNV cases will continue...”

In other words, vaccinations are working, so doesn’t it make sense not to mess with success? Plus, mosquitoes, beware!

“Veterinary Services predicts the downward trend in equine WNV cases will continue in the future,” says Cordes, “with federal and state public health and veterinary programs dedicating valuable resources to surveillance, prevention and control of this disease.” He believes these programs will continue, at least in the near future, to capture public awareness of West Nile and the need to protect the horse population with new available biologics and mosquito mitigation.

First rule: Eliminate potential mosquito breeding areas. Mosquitoes must have standing water for the first stages of development. Get rid of stagnant water where mosquitoes can lay their eggs. If you have more questions, call your state or local health departments. Meanwhile, surveillance from agencies and veterinarians alike will continue to measure the effectiveness of prevention strategies, according to Cordes.

If you’re not conversant about the disease, know that West Nile virus is very serious, and can be fatal. It may be tapering off, but don’t relax. Dr. Gayne Fearneyhough, DVM, and head of diagnostic services for the Texas Veterinary Medical Diagnostics Laboratory, affiliated with the Texas A&M University System, notes that “the equine species appears to be very sensitive to the virus, producing fairly dramatic clinical symptoms, specific to the central nervous system.” Initially, horses may be “wobbly” in the back end, and stumble on turning. An animal that goes down and cannot rise on its own is usually ultimately euthanized. “If clinical symptoms are not severe and you can get them to stay up, you may have a fairly good chance of recovery,” reports this specialist.

Will West Nile virus ever disappear? “This virus has become endemic in wildlife species such as birds, who act as ‘reservoirs.’ Mosquitoes bite birds, and via a blood meal, pick up the virus, then bite another animal and pass it on, explains Dr. Fearneyhough.

Animal health giants Merial, Fort Dodge and Novartis have all developed vaccines available through your veterinarian. “Vaccines are not 100 percent effective, since each horse’s immune system responds individually. With any virus, if you challenge that animal, expose them to enough virus, it can overcome the protection afforded by the vaccine,” says Fearneyhough, but he’s definitely casting his vote in favor of vaccinating.

If you’re on the fence, make the decision to vaccinate after consultation with your veterinarian; hypersensitivity to the vaccine could be one rare reason not to use it. “As the virus has moved through the population, we’ve definitely seen protection afforded by those vaccines,” says Fearneyhough. “If a population of 100 horses is exposed to the mosquitoes carrying the virus, some develop mild symptoms that wouldn’t be recognized. Others will develop clinical symptoms and will recover. Still others will develop severe symptoms and die.” Just as with people, each individual responds differently, and a frank discussion with your veterinarian about risks versus benefits will allow you to make an intelligent decision that could ultimately save your horse’s life, and the lives of the horses in your barn. Vaccinating all horses is certainly a recommendation you’ll likely hear from your consulting veterinarian.