Understanding Influenza

What is flu? “It’s a hit-and-run disease,” said Tom Chambers, PhD, who heads The OIE international influenza reference laboratory at the University of Kentucky’s Gluck Equine Research Center in Lexington. “When it hits, it causes big problems,

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What is flu? “It’s a hit-and-run disease,” said Tom Chambers, PhD, who heads The OIE international influenza reference laboratory at the University of Kentucky’s Gluck Equine Research Center in Lexington. “When it hits, it causes big problems, but then it disappears again.” Many horse owners are concerned about equine influenza following announcement of an outbreak in Japan.


The Japan Racing Association released information stating that a flu outbreak is occurring at two Thoroughbred training centers, and it has shut down racing throughout the country. Chambers has no further information about the outbreak.


Here are some flu facts:


  • Equine influenza is one of the most common causes of upper respiratory disease in horses;

  • Flu is a viral disease;

  • Flu is highly contagious;

  • Flu is spread by the aerosol route (i.e., sneezing, coughing);

  • Flu often causes a fever;

  • Infected horses can shed flu virus prior to having a fever or other clinical signs;

  • Flu can cause subclinical infections (no clinical signs);

  • Horses with no clinical signs can shed virus and infect other horses;

  • Flu does not produce chronic (long-lasting) infections;

  • Flu does not produce latent infections (the virus does not persist in the body and become reactivated at a later time due to stress);

  • Flu can spread down a barn aisle in a matter of days;

  • Flu changes over time;

  • A horse can get flu many times during his life;

  • Horses that get flu usually don’t get it again for 1-1½ years;

  • Flu is not a hardy virus, it can be easily killed through disinfection;

  • Vaccines might protect horses from getting sick (showing clinical signs), but might not prevent exposed horses from shedding the virus and infecting other horses;

  • Horses can shed flu virus for a week or more, so quarantine recommendations are for 10 days to two weeks for exposed horses;

  • Nasal swabs can be tested for the presence of flu virus in about an hour at a laboratory.

  • Because flu changes, vaccines need to be updated to reflect those changes.

Researchers know horses can spread flu virus from direct contact or coughing. “I strongly suspect flu can be spread by fomite transmission, meaning casual contact by inanimate objects such as hands, bits, or anything that comes into contact with a horse’s nose,” said Chambers.


He suspects there are many cases of equine influenza that are never diagnosed. “It wouldn’t surprise me if sooner or later that nine out of 10 horses are exposed,” said Chambers. “It very rarely kills a horse. It gives them a fever, cough, runny nose, and with no complications, a week later they are starting to feel better.” There can be secondary bacterial infections in horses that have the flu that can be dangerous if not treated. For that reason, antibiotic therapy is widely employed, he said

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Written by:

Kimberly S. Brown is the editor of EquiManagement/EquiManagement.com and the group publisher of the Equine Health Network at Equine Network LLC.

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