California confirms fifth West Nile case in a horse; other states report infections

Electron microscopy of West Nile virus. Photo: PhD Dre CC BY-SA 3.0 via Wikimedia Commons
Electron microscopy of West Nile virus. Photo: PhD Dre CC BY-SA 3.0 via Wikimedia Commons

California has confirmed two more cases of West Nile Virus infection in horses this year, with other states also reporting recent infections from the mosquito-borne virus.

West Nile virus is spread by mosquitoes that have fed on infected birds. The disease can sicken people, horses, birds and other animals, but it does not spread directly from horses to people or other animals.

The disease is fatal to horses in about a third of the cases in which clinical signs are apparent, although most horses do not become ill and show no symptoms.

Horses that do become ill can appear to be displaying loss of coordination, loss of appetite, confusion, fever, stiffness, and muscle weakness, particularly in their hindquarters.

The California Department of Food and Agriculture confirmed the fourth and fifth cases in the state this year.

On Thursday, an unvaccinated eight-month-old Quarter Horse filly in Kern County and an unvaccinated three-year-old Quarter Horse filly in Contra Costa County displaying neurologic signs were confirmed positive for the virus.

The Kern County filly was euthanized due to the severity of the disease and the Contra Costa County filly is recovering.

The positive horses were located in Contra Costa, Fresno, Kern and Riverside counties. Two of the cases were in Kern Country.

It said four horses were unvaccinated and one horse had unknown vaccination status. Four horses died or were euthanized and one horse is recovering.

The agency said it was continually monitoring and investigating equine neurologic cases for the presence of West Nile Virus in California. It urged horse owners to consult their veterinarian about a vaccination program to ensure maximum protection for their horses.


Meanwhile, on Friday the Utah Department of Agriculture and Food said a two-year-old Quarter Horse mare from Carbon County was confirmed with the disease on August 3.

On presentation, the horse was unsteady on its feet and showed hind-limb weakness. As of Tuesday, she had responded well to treatment and was expected to recover. It was the first West Nile case for Utah in 2017.


Washington state confirmed its first equine West Nile case this week.

The state’s Department of Agriculture said it was notified that a 10-year-old quarter horse in the town of Four Lakes, Spokane County, was infected.

The horse, which was unvaccinated for the disease, was reported to have coordination problems with its rear legs and muscle tremors, but was now improving.

The department had 27 confirmed equine West Nile virus cases last year. Seven of those horses died or were euthanized. In all, 10 counties reported West Nile virus cases involving horses last year, including Benton, Franklin, Grant, Kittitas, Lincoln, Okanogan, Pend Oreille, Spokane, Stevens and Yakima counties. Spokane County led with eight cases.

“Horses can still benefit from first-time vaccinations or an annual booster vaccination,” state veterinarian Dr Brian Joseph said. “It’s never too late to vaccinate your horse, but it’s more effective to do it earlier than now.”


Cases have also been reported in recent weeks in Kentucky, Michigan and Wisconsin, the latter also reporting two cases of Eastern Equine Encephalitis in yearling fillies. One of the fillies died and one was euthanized.


South Carolina and Florida have also reported recent cases of Eastern Equine Encephalitis.

States authorities have urged owners to discuss vaccination with their vets.

Besides vaccination, horse owners can reduce mosquito populations and their possible breeding areas.
Recommendations include removing stagnant water sources, keeping horses inside during insects’ feeding times, typically early in the morning and evening. Consider using equine-approved mosquito repellants, place fans inside barns or stalls to maintain air movement, and avoid using incandescent bulbs inside stables at night.

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