West Nile Virus Vaccine in Horses

The image of a cowboy riding off into the sunset on a palomino pony after a long hard day of rescuing a damsel in distress has reached the point of a cliché.. A cowboy’s partner will forever be his trusty horse, and as long as his horse is healthy, he can continue riding off into the sunset. But what happens when a cowboy’s horse is infected by a deadly virus? Will there be a happy ending to that story?

West Nile virus is defined as zoonotic, which means it can be transferred between animals and humans. The virus is mosquito-borne and spreads through intermediate hosts like blue jays and black birds. For this reason, the virus is more common in the summer or fall when birds are migrating from the north. Both humans and horses can be infected by West Nile; however, they are both considered “dead-end” hosts, meaning they cannot transmit the disease to others.

West Nile virus first infected horses in 1999 with a case in New York. Since then, the United States has seen more cases of West Nile in horses as well as humans. The year 2012 was the most deadly for humans with 286 deaths nationwide.

Dr. Leslie Easterwood, clinical assistant professor at the Texas A&M College of Veterinary Medicine & Biomedical Sciences, explains the symptoms horses experience when they are infected by the virus. “West Nile virus causes an encephalitis type syndrome with fever, general depression, and dull mental attitude,” she said. “Horses may be unstable or show neurologic signs and can have a characteristic twitching of muscles along their neck and shoulders.”

A vaccine for the virus is available for horses, and veterinarians highly recommend it to horse owners.

“Vaccination has significantly decreased the incidence of West Nile disease in horses in the face of a steady case number in humans, where we have no vaccine available,” said Easterwood. “Horses are recommended to be vaccinated twice yearly in areas of the country where mosquitos are present all year.”

As a horse owner it is important to recognize the signs of the virus and to have a veterinarian ready to call. The sooner the symptoms are identified and the horse receives treatment, the better chance the horse has in surviving. According to Easterwood, one third of all equine West Nile cases generally survive with proper intervention.

“If an owner sees neurologic signs of wobbly stance, unsteady movement, depression, not eating, with or without muscle twitching, they should call their veterinarian right away,” explained Easterwood, even if the horse has been vaccinated.

Although the West Nile vaccine has proven effective, Easterwood reminds horse owners that no vaccine is 100 percent protective.

Pet Talk is a service of the College of Veterinary Medicine & Biomedical Sciences, Texas A&M University.