Veterinarian emphasizes need to vaccinate horses against West Nile
--First California case has been reported already for 2007
by Elizabeth Larson, Capital Press

March 02, 2007

While the worst of California's West Nile virus epidemic in horses appears to have passed, a respected veterinarian in the state's premier veterinary school explains that continuing to vaccinate horses is crucial.

During the last few years California has seen large outbreaks of West Nile virus, with hundreds of horses dying from the disease.

This past year equine West Nile virus cases were markedly down in California, with 58 infected horses reported in 2006, according to the California Department of Food and Agriculture, as opposed to 436 the previous year.

However, the disease is still very much with us, said Dr. W. David Wilson, a professor of large animal medicine and director of Large Animal Clinical Services in the University of California-Davis Veterinary Medical Teaching Hospital.

California has already had its first equine West Nile virus case of the season this year. The California Department of Food and Agriculture confirmed that a Sonoma County horse was diagnosed with the disease Feb. 8.

Horses contract the disease from carrier mosquitoes that feed on infected birds, Wilson explained.

CDFA reported that West Nile virus symptoms include stumbling, staggering, wobbling, weakness, muscle twitching and inability to stand.

Wilson said it remains critical that horses be vaccinated against the disease, which he called "cheap insurance" for a horse's health.

The vast majority of equine West Nile virus cases that have occurred in California resulted because the animals hadn't been vaccinated, he said.

Although there's been no cure developed for the disease, there are several vaccines now available to prevent it, said Wilson, which began appearing on the market in 2001.

"We have more new technology available through vaccines for the prevention of West Nile than for any other equine disease," Wilson said.

There are four vaccines, three of them licensed and available on the market, he said. The vaccines use both traditional and sophisticated new technologies, Wilson said. An example of the high-tech aspects of the vaccines is how some of them use related viruses to deliver protection.

All of the vaccines, Wilson added, are effective.

Wilson has been an advisor on state and national vaccination protocols; as a member of the American Association of Equine Practitioners' vaccination task force, he's written vaccination guidelines for vets around the country.

Horses should be vaccinated in the spring, by early April at the latest, he suggests. Until recently, he said, horses owners have been advised to vaccinate horses twice annually to ensure strong immunity throughout the year.

However, the different vaccines have different vaccination schedules. Some require only one annual booster, Wilson said.

"What we're recommending now is talk to your local vet because local conditions vary and risk varies," he said.

"I think under most circumstances in most areas of California, a single annual booster would be sufficient for horses that have been vaccinated previously," Wilson added.

West Nile virus has been a deadly disease for the state's horses since the disease got a foothold in California in 2003.

One case of equine West Nile virus was reported in the state that year. That horse was in San Diego County, according to California's West Nile virus website, westnile.ca.gov.

However, by 2004, the disease had reached 32 counties and infected 540 horses, 228 of which either died or were euthanized.

In 2005, the disease again struck large numbers of horses across the state - 456 in 43 counties, with 200 horses dying.

Steve Lyle, CDFA's director of communications, said 2004 and 2005 were the disease's peak years in California.

"We watched this disease move from east to west for a number of years," Lyle said. "By the time it arrived here we knew to expect it would be a two-year cycle of a high volume of disease outbreaks, and after that a decline."

There are several reasons for that peak and decline cycle, Wilson said.

The first is that within a few years of the disease's arrival, most susceptible horses become infected, he said. Subclinical infection, along with vaccinations, helps induce immunity, said Wilson, resulting in fewer animals susceptible to the disease.

Around 5 percent of horses that become infected with West Nile virus will actually develop clinical disease symptoms that include neurological signs, Wilson said.

"The problem with the disease is that by the time the horse shows clinical signs, the virus has already infected the tissues in the neural system," Wilson said, which can result in residual damage, even for horses that survive.

Wilson estimated that more than 23,000 horses have been clinically affected by West Nile virus since it appeared in the U.S. in 1999; of those, he said, more than one-third died.

The disease's impact on an area also is determined by climate conditions that help mosquitoes survive, said Wilson.

Wilson warns that while California's peak infection years appear to have passed, peaks sometimes come again several years later. He pointed to spikes in the numbers of infected horses in Pennsylvania and New Jersey that appeared several years after the initial peaks.

That's why vaccination remains crucial, he said.

"The virus is going to be with us," Wilson said. "It's going to continue to cause disease in horses that are not adequately vaccinated."

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Elizabeth Larson is based in Lucerne. E-mail her at elarson@capitalpress.com.

This article is reprinted with the permission of the Capital Press. Content © 2007 Capital Press