California battles West Nile virus


About 30 percent of clinical cases either die or must be euthanized.

SACRAMENTO, CALIF.—At least 119 horses have died this year in the Golden State as a result of West Nile virus; 271 confirmed cases plagued the state at presstime, according to the Animal Health and Safety Services of California. The state experienced just one confirmed case in 2003.

"West Nile has moved westward across the United States since 1999, and it as been a result of migratory bird patterns," says Dr. Tim Boone, staff veterinarian with California's Animal Health and Safety Services equine programs. "This was our year to encounter enough birds that acted as reservoirs for mosquitoes to spread the virus to horses and humans."

More than half of the country's confirmed cases reside in California, according to Centers for Disease Control; at presstime, 559 veterinary cases had been reported nationwide; 547 of those were reported in horses. And the numbers in California are expected to grow due to the short, mild winters enjoyed by much of the state.

"Based on past experience and the information I get from our state's mosquito experts, we expect the activity to extend until at least December, and we likely will see some problems in Southern California well into the winter months, as well as next year when the mosquito populations build back up," Boone says.

Two vaccines have been approved by the U.S. Department of Agriculture for the virus, but Boone says antibodies will take some time to permeate the equine population in California.

"As we've observed in other states, there appears to be a two-year period for the mosquitoes to build up a rate of infection in which the horses are either exposed naturally through mosquito bites or have been vaccinated," Boone says. "So, we are expecting more West Nile next year; we obviously don't know the extent, but it is a serious concern."

Boone says the wide variety of California mosquitoes have proven to be effective transporters of the virus, especially Culex tarsalis. While controlling the insects is possible in closed-quartered areas by using insecticides and fans, the most effective treatment is a vaccine, Boone says.

"It's not a death sentence; most horses bitten by infected mosquitoes do not develop clinical infections at all; but 30 percent of the horses that develop clinical signs either die or must be euthanized," he says. "Other states also have figured that 17 percent of cases develop permanent debilitating neurological signs, which means that one out of every two horses that develops clinical West Nile either dies or cannot perform the duties that they were purchased for."

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