By Charlie Powell, College of Veterinary Medicine

PULLMAN, Wash. – An influenza virus, H3N8, is causing its typical seasonally-increasing number of cases in Western Washington.

H3N8 is among the most common causes of upper respiratory disease in horses.

So far, more than two dozen cases have been confirmed in Whatcom, Snohomish, Thurston and Skagit counties.

While such outbreaks are not unusual, horse owners should keep two things in mind during the cooler months.

“The single best protection against this virus is vaccination,” stressed Jennifer Gold, Clinical associate professor of equine medicine at Washington State University’s Veterinary Teaching Hospital. “Also, during the cooler months of the year, simply don’t congregate horses any more than necessary. Remember, the words, ‘vaccinate and isolate.’

“When it’s cold, everyone wants to park near the entrance to an event, and they pack their trailers and horses in tightly. Owners are best advised to park away from others and keep their horses isolated except during competition or showing.”

Gold explained the equine influenza virus is highly contagious and spreads rapidly when horses sneeze or cough. She says to avoid using another horse’s tack or brushes as these things can pass the virus, too. While the virus is not constantly circulating among horses, it is introduced by an infected horse. New horses being introduced into a herd should undergo 10 to 14 days quarantine to prevent the introduction of equine influenza as well as other diseases.

“We see this most often in stabled horses or those otherwise in close contact with other horses at events. Vaccination and keeping horses apart is the key to avoiding this illness.”

Like the human influenza virus, the equine influenza virus is constantly mutating and changing, although not as fast. A horse also loses some of its immunity to the virus over time as well. A horse that is immune this year may not be, or may only partially be, immune next year. For this reason horse owners should check with their veterinarians each year to ensure their horses are up‑to‑date on the latest vaccines available.

“In the case of equine influenza, I advise clients to seek vaccines that combine the isolated components from the most prevalent forms of the virus from the last five years. Even then, vaccinated horses can become infected and not develop the clinical disease. Nonetheless, they can shed the virus for up to a week infecting other horses. This is where isolation pays off.”

For exposed horses, the incubation period typically is 24–72 hours. Younger horses, ages 1–5 years, are the most vulnerable. The virus is easily destroyed by common disinfectants.

The disease is rarely fatal and causes the usual symptoms of a severe upper respiratory infection which are treated with supportive care. For more information, visit the American Association of Equine Practitioners website.