West Nile Virus Appearing in Washington Horses

In 2002, Washington confirmed its first two equine cases of West Nile virus (WNV) in Pierce and Thurston counties. The disease was not seen again until 2005, when one positive horse was found in Yakima County. In 2006, six cases were confirmed, the first during August in a horse from Yakima County. By the year's end, four others were diagnosed from Yakima County and one from King County.

Nationwide, there were 1,035 equine cases of WNV reported in 2006, with 338 occurring in Idaho, 35 in Oregon, and 24 in Montana, according to data from the USDA. The disease kills approximately 1/3 of infected horses, although most survive without complications. Most cases occurred from August through October.

"Since 2000, the disease has gradually moved westward across the United States," said Debra Sellon, DVM, PhD, a professor and board-certified specialist of equine medicine at the Washington State University College of Veterinary Medicine. "Each year the number of equine cases has been highest in the states at the westernmost edge of the virus spread. Last year was Idaho's year. Next year it may be Washington's turn."

Three people in Washington developed disease due to WNV infection, as well as 69 in Oregon, and 34 in Montana, but no deaths occurred. In contrast, 984 people in Idaho contracted the disease, 14 of whom died.

Fortunately, about 80% of people infected with WNV develop no symptoms, and about 20% experience mild flulike symptoms, according to the Centers for Disease Control and Prevention (CDC). Less than 1% of people bitten by an infected mosquito will become severely ill with inflammation of the brain or surrounding tissues called encephalitis, or meningitis, respectively.

Currently, there is no vaccine available for human use, although researchers are working to develop one.

Several Equine Vaccines Available

For horses, there are presently three vaccines approved as aids in the prevention of WNV, one of which was released last year. Because the fatality rate is so much higher in horses than humans, it is recommended that horse owners vaccinate for the disease and incorporate yearly booster shots as part of a routine vaccination program.

"The three vaccines for West Nile virus are a little different from each other, so horse owners should talk with their veterinarian about the best one to use for their particular horse," Sellon said. "The first time a horse is vaccinated for the disease, the vaccines available through Fort Dodge (West Nile-Innovator) and Merial (Recombitek) are applied in a series of two or three shots over several weeks," she said. "The new vaccine produced by Intervet (PreveNile) requires only one injection the first year, so immunity is established faster. But each of the three vaccines requires an annual booster shot for a horse to remain immune. The annual booster is generally best given just before the start of mosquito season."

Signs of the Disease

West Nile virus is a mosquito-borne disease that can cause inflammation of the brain and spinal cord in humans, horses, and birds. Some other animals are affected to a much lesser degree, including bats, cats, chipmunks, skunks, squirrels, and domestic rabbits.

The disease cycle starts when mosquitoes bite infected birds capable of migrating long distances. The mosquitoes become infected and pass the virus on when they bite other birds, horses, humans, or other mammals. Once exposed, horses may show signs of West Nile virus in less than a week.

The horse is a dead-end host, meaning if mosquitoes bite an infected horse, they cannot pass it on to other horses, birds, or humans.

"One of the reasons that the Yakima area is affected is because it follows a major migratory route for birds traveling from Oregon," Sellon said.

Lethargy or lack of energy, low-grade fever, hindquarter weakness, involuntary muscle twitching, loss of coordination, convulsions, paralysis, and coma are all signs that a horse may be infected with WNV. Horse owners should contact their veterinarian if they notice any clinical signs of the disease.

Vaccinations can help prevent or decrease the severity of the disease, but owners can also help prevent infection through mosquito control. It is especially important to eliminate stagnant water around a property and enclose horses in a barn at dawn and dusk, when mosquitoes most actively feed. There are also a variety of measures to control mosquitoes inside barns, such as repellents.

Unfortunately, it only takes one mosquito bite to infect a horse. Veterinary attention can be worthwhile for horses that contract the disease. There are no specific drugs to treat or kill the virus, but good supportive care and the use of hyperimmune plasma with high levels of antibody to the virus can help sick horses.

More information about WNV can be found on the WSU College of Veterinary Medicine's Web site at www.vetmed.wsu.edu, the CDC's Web site at www.cdc.gov/ncidod/dvbid/westnile/index.htm, or at the USDA's Web site at www.aphis.usda.gov/vs/nahss/equine/wnv/index.htm.  

Area horse owners who have questions about WNV can also call the WSU Veterinary Teaching Hospital at 509/335-0711.

Reprinted from the WSU College of Veterinary Medicine Equine News Spring 2007 issue.

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