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Texas Agriculture Archive

November 1, 2002

Horse owners urged
to vaccinate now for WNV

By Lana Robinson
Field Editor

Although animal health officials have urged proper vaccination of horses as the best protection against West Nile Virus, only half the state's horses have been innoculated.

On Oct. 23, Rob Daily, director of the Equine Business Unit for Fort Dodge Animal Health, makers of the vaccine, reported that approximately 750,000 doses of the WNV equine vaccine had been sold in Texas.

"The horse population in Texas is right at 600,000. If you divide the number of doses sold by two, since the horse requires two shots, that means that only half of the horses in the state have been vaccinated," he said.

West Nile Virus is an arbovirus (short for arthropodborne virus) that causes encephalitis (inflammation of the brain). Arboviruses, including West Nile Virus, are transmitted by blood-feeding insects such as mosquitoes.

The most common sign in horses is weakness, usually in the hindquarters. Weakness may be indicated by a widened stance, stumbling, leaning to one side and toe dragging. In extreme cases, paralysis may follow. Fever is sometimes evident, as are depression and fearfulness. Approximately 40 percent of cases of West Nile viral encephalitis in horses proved fatal during the 1999 outbreak in the U.S. Many horses that do not die must be euthanized.

Through Oct. 18, 709 horses in Texas had tested positive for West Nile Virus.

"Horse owners need to vaccinate ahead of the disease," said Daily. "It's endemic in Texas. It's a proactive vaccination program. Unfortunately, many horses have died unnecessarily because people waited to see if it (WNV) would come into their area."

Nationally, the number of equine case reports continues to surge, with 9,036 cases in 36 states reported through Oct. 6 to USDA/ APHIS by state health officials. WNV has now been associated with illness and death in several other mammal species, including humans, squirrel, wolf, four dogs in Louisiana and one dog in Illinois (which leads the country in number of human cases), and mountain goat and sheep in Nebraska, which leads the country in numbers of equine cases with 1,008 through Oct. 15.

According to Daily, to properly vaccinate a horse, it takes two shots at least three weeks prior to being exposed.

"If the animal receives only one shot or it contracted the virus just days after receiving the vaccination, it may not offer protection as it takes time for the immune system to recognize the vaccine and mount a response," he said. "It's hard to evaluate how effective a vaccine is in the face of an outbreak like this. But even a very efficacious vaccine sometimes, due to the individual horse's immune system, may not work. But that's not the fault of the vaccine, but rather the horse's immune system."

There is no minimum age requirement for the WNV vaccine. Fort Dodge Animal Health (FDAH) recommends that veterinarians follow the same protocol as for encephalomyelitis vaccines (EEE,WEE). There is no specific recommendation for pregnant mares. There were a handful (6) of pregnant mares included in the safety study with no adverse reactions. There have been no adverse reactions reported in pregnant mares, thus far. Although, there is not enough information to make a claim for use. There is no specific recommendation and not enough information to make a claim regarding nursing mares. Veterinarians should use their own discretion. FDAH has no particular recommendation for stallions.

USDA has clarified their policy and FDAH has been able to move their planning forward. FDAH expects to conduct challenge studies (in Iowa) within the next 6 months to 1 year.

According to Carla Everett, information officer for the Texas Animal Health Commission, the agency is recommending a booster shot at six months for those with horses in an area challenged by mosquitoes or where WNV has been diagnosed. TAHC also advises owners to further protect horses with insect repellents, and if possible, house the animals at night in stalls with screens.

Unless rabies is suspected, WNV testing for horses is usually conducted at the Texas Veterinary Medical Diagnostic Laboratory (TVMDL) at College Station. WNV testing for humans, birds and mosquitoes is conducted by the Texas Department of Health (TDH). Direct WNV inquiries to 1-888-883-9997. For online WNV statistics, visit: www.tdh.state.tx.us/zoonosis/diseases/Arboviral/westNile /statistics. If you have further questions, contact the Texas Animal Health Commission toll free at 1-800-550-8242; or Fort Dodge Animal Health Customer Service , 1-800-685-5656.