Saturday, 15 June 2024

First equine WNV case of year found in Sonoma County

LAKE COUNTY – With the state's first case of equine West Nile Virus diagnosed on the North Coast this month, state officials are reminding horse owners that the best way to protect their animals is through vaccination.

California Department of Food and Agriculture (CDFA) officials reported on Wednesday that the first case of equine WNV so far this year was confirmed in a Sonoma County horse on Feb. 8.

“Outbreaks of West Nile virus are expected to continue this year,” said California State Veterinarian Dr. Richard Breitmeyer. “Horse owners should contact their veterinarians as soon as possible to ensure current vaccination status, so that horses will have maximum protection against the disease.”

CDFA reports that horses contract the disease from carrier mosquitoes. Affected horses, the agency added, are not contagious to other horses or people. Not every horse exposed to the virus will die.

Signs of West Nile virus include stumbling, staggering, wobbling, weakness, muscle twitching and inability to stand.

Dr. Jeff Smith, a Middletown veterinarian who has dealt with several local WNV cases over the last few years, explained that WNV is a form of encephalitis that results in neurological symptoms, including brain swelling.

The horses he's treated, he said, have a “lazy gait,” and drag their feet or cross their legs, and play with their lips. If horses get the point where they can no longer stand, they have to be euthanized, he said.

Smith said because the disease has no treatment, it has to be waited out. He's used hyperimmune serums to treat his WNV patients, although that treatment hasn't yet been proved to cure the disease. Smith said he also works to reduce brain swelling in the animals.

Some animals, he said, spontaneously recover from WNV.

Equine WNV was first diagnosed in a horse in San Diego County in 2003, according to the state's WNV information site,

That was the only case for 2003. But by 2004, the disease had reached 32 of California's 58 counties, infecting 540 horses and causing 228 to die or be euthanized.

In 2005, 456 horses in 43 counties were diagnosed, and 200 of them died.

Steve Lyle, CDFA's director of communications, explained that as state officials watched the disease move westward over the last several years, they noticed that it had a pattern of explosive growth and a two-year peak cycle.

In California's case, the peak years appear to have been 2004 and 2005, he said.

That pattern held true in Lake County. CDFA statistics showed that Lake County had four cases of equine WNV in 2004, with one animal death. In 2005, 10 Lake County horses were diagnosed with WNV, and eight of them died or were euthanized.

The numbers of horses diagnosed have since dropped off. Last year, 58 horses in the state were known to have contracted the disease, with 24 deaths. In Lake County there were only two equine cases, but both were fatal, according to CDFA.

In the great majority of those cases the horses were either not vaccinated or vaccinated improperly, CDFA reported.

Smith said the animals he's treated for WNV hadn't been vaccinated. Most were older animals; like people, the immune system of horses weaken as they age, said Smith.

Lyle said scientists believe that animals exposed to the disease either develop immunity or get sick, and overall infections decline.

“What we're seeing is a natural cycle of decline that has been witnessed elsewhere,” said Lyle.

This recent diagnosis in Sonoma County, said Lyle, is a reminder that WNV is “still out there and populations are still vulnerable, just in smaller numbers that before.”

As a result, officials continue to encourage horse owners to vaccinate their animals, which Lyle said is the only way to protect animals from infection.

Vaccinations don't, however, guarantee horses won't be infected, Lyle added. But in cases where vaccinated horses did contract the disease, they are more likely to survive, he said.

University of California at Davis' vaccine regimens have changed several times in the last few years, said Smith. The current recommendation is that horses be vaccinated every four months or in the face of an outbreak, he said.

Animals that survive WNV usually recover fully, said Smith. “It's not the average horse that would be left debilitated by it.”

The West Nile Virus Information Line can be reached at 800-268-7378.

For updated statistics on West Nile Virus statewide, as well as information on how to report cases, visit

E-mail Elizabeth Larson at This email address is being protected from spambots. You need JavaScript enabled to view it..


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