Vaccinate Against Equine Flu to Minimize Spread of Disease

Equine influenza is a common respiratory infection in horses caused by the equine influenza A type-2 virus. While influenza is not particularly harmful to affected horses (i.e., it has a very low mortality rate), it is associated with explosive outbreaks that have a large economic impact on the industry. Vaccination of at-risk horses and continually updating the flu vaccines play a crucial role in limiting the damage caused by this virus.

"Influenza is a moving target, often changing its coat to confuse the immune system."
–Dr. Tom Chambers
"While not considered a core vaccination by the American Association of Equine Practitioners (AAEP), it is recommended that all horses should be vaccinated against the flu unless they are part of a closed herd," explained Tom Chambers, PhD, who heads the OIE Reference Laboratory for Equine Influenza at the University of Kentucky's Gluck Equine Research Center in Lexington.

All of the current influenza vaccines are believed to provide protection for at least six months duration. Thus, bi-annual vaccination against flu for at-risk horses (e.g., mature performance or show horses) is recommended. Recommendations for other horses vary and are available at  

Despite vaccinating horses appropriately, outbreaks of flu can still occur.

"This is because the equine influenza virus undergoes antigenic shift and drift which means that the proteins on the surface of the virus are altered in such a way that the virus appears 'new' to the horse's immune system," relayed Chambers. "Influenza is a moving target, often changing its coat to confuse the immune system."

To overcome the virus' ability to evade the horse's immune response, vaccines are constantly updated to ensure the most current forms of the virus are included in the vaccine.

According to Chambers, "The only way to keep the vaccines up to date and effective against the current 'version' of the influenza virus is to have samples of the viruses submitted to our laboratory. This requires nasal swabs."

Nasal swabs also serve to "facilitate the timely and accurate diagnosis of respiratory diseases to help prevent an introduced disease from exploding into a major outbreak."

Chambers added, "A rapid diagnosis also enhances the effectiveness of quarantine and vaccination, which are the two most successful means to prevent the spread of disease. In the case of equine influenza, vaccination in the face of an outbreak can work--if the horse has already been primed by previous vaccination, and if there is at very least a seven-day interval between vaccination and exposure to the disease."

Chambers explained that this is because the horse's immune system needs this seven day window start mass-producing the necessary antibodies to protect against disease. The sooner a disease diagnosis is made, the better the chance that unaffected horses can be protected.

Additional details regarding nasal swabbing and submitting samples to the reference laboratory are available at the Gluck Research Center Web Site.

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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