Fighting Equine Influenza: Winning the Battle but Losing the War?

Over the course of a few short months, the Australian horse flu outbreak cost the country approximately $100 million AU ($92.6 million US) and caused racing, breeding, and other performance horse-related activities to effectively grind to a halt.

Australia, which was influenza-free until August 2007, was hard hit by this virus. The combination of a naïve population (one with no immunity or prior exposure) and a virus that is easily spread is a recipe for disaster, which was clearly demonstrated in the Australian outbreak.

The horse flu is caused by influenza A virus strain, subtype H3N8. Vaccines are available against this virus subtype; however, the vaccine is not able to completely protect horses from becoming infected with the horse flu.

"No vaccine is 100% effective," explained Scott Weese, DVM, DVSc, Dipl. ACVIM, from the Ontario Veterinary College at the University of Guelph in Canada. "Some horses may not mount a protective immune response after vaccination, assuming the horse is actually exposed to the strain of influenza in the vaccine."

Unlike Australia, in North America the horse flu has been endemic for decades. Nonetheless, this disease continues to pose both economic and health-related concerns.

According to Weese, influenza viruses are particularly problematic due to their propensity to change. Minor changes in their structure can make vaccines less effective, and major changes can render current vaccines completely incapable of protecting the horse.

"While mutations do not occur as frequently in horse strains of influenza compared to human strains, the possibility always exists," said Weese. "Our main concern at this time is that a new equine influenza strain that is moderately different from the vaccine strains will emerge."

This would render current horse flu vaccines useless and, essentially, the horse population would be naïve to this new strain, akin to the pre-flu status in Australia.

Since influenza is easily spread and horse transport is common, a widespread North American outbreak like the one in Australia could rapidly develop should one or more mutant strains develop.

In North America, a second important concern regarding the horse flu virus is its demonstrated ability to cross the species barrier. This was first identified in 2005 following the canine flu outbreak that began in racing greyhounds in Florida when influenza effectively "jumped" from horses into the canine population, infecting racing greyhounds in Florida.

For more information on the equine influenza in Australia, its occurrence in dogs in in North America, and current horse vaccination guidelines, see:

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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