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State veterinarian urges horse owners to vaccinate against West Nile virus

Washington tops U.S. in 2008 with 41 horses testing positive for the virus

OLYMPIA — Horse owners should take advantage of the availability of vaccines to prevent West Nile virus—a potentially fatal disease in horses—particularly since Washington led the nation last year with 41 horses testing positive for the disease.

West Nile virus (WNV) is spread by mosquitoes that have fed on an infected bird. The disease can sicken people, horses, many types of birds and other animals. It is not spread from horses to other animals.

WNV cases were confirmed last year for 26 horses in Yakima County; 10 horses in Grant County; four horses in Benton County; and one horse in Kittitas County. In previous years, WNV horse cases have been detected in Western Washington as well.

Although the first confirmed case of WNV in a horse last year wasn't reported until Aug. 1, there is no guarantee it will take that long for it to show up this year, according to State Veterinarian Dr. Leonard Eldridge.

"Nearly all of horses that acquired West Nile virus last year did not have current vaccinations," Eldridge said. "It's a shame that unvaccinated horses continue to be one of our best sources warning that the virus is active in a local community."

Eldridge said that older, sick or weak horses have less chance of survival if infected. WNV is fatal in about one-third of all horses that show clinical signs, although most horses do not become ill and show no symptoms at all. Those that do become ill display loss of coordination, loss of appetite, confusion, fever, stiffness and muscle weakness, particularly in the hindquarters.

"Vaccinating your horse or getting the proper booster shots is the best way to help protect your animal and prevent greater expense from treating your horse after the disease is acquired," Eldridge said. "A spring dose of vaccine, even in previously vaccinated horses, is necessary to maintain protective immunity in most horses."

Horse owners should consult with their veterinarians for vaccination recommendations and WNV control measures. The vaccine requires two doses the first year of vaccination two to four weeks apart. Immunity will not be achieved until three to five weeks after the second vaccination, so it's important to avoid waiting until mosquito season is in full swing. An annual booster dose should be administered prior to the start of the mosquito season.

The state veterinarian also recommends that horse owners take measures to reduce mosquito populations. Precautions include removing standing water from yards and barns and changing water in troughs or bird baths that could be a source of mosquito breeding.

Veterinarians who learn of potential WNV cases in horses or other animals should contact the State Veterinarian’s Office at (360) 902-1881.

State and local health, mosquito control districts, other state agencies and volunteers work together on environmental monitoring and prevention measures for the virus. Additional information on WNV can be found at the state Department of Health Web site at www.doh.wa.gov/WNV.

Note: The U.S. Department of Agriculture's Animal and Plant Health Inspection Service maintains a Web site showing West Nile virus distribution maps, 1999 - 2008.. Annual trends in U.S. distribution of equine West Nile virus cases.

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