Equine influenza (Horse flu) refers to varieties of Influenzavirus A that are endemic in horses. Horse flu viruses were first isolated in 1956.

There are two main types of virus called equine-1 (H7N7) which commonly affects horse heart muscle and equine-2 (H3N8) which is usually more severe. Horse flu is endemic throughout the world.

The disease has a nearly 100% infection rate in an unvaccinated horse population that has not been previously exposed to the virus with an  incubation time of  one to five days.

Horses with horse flu can run a fever, have a dry hacking cough, have a runny nose, and become depressed and reluctant to eat or drink for several days but usually recover in 2 to 3 weeks.

“Vaccination schedules generally require a primary course of 2 doses, 3-6 weeks apart, followed by boosters at 6-12 month intervals. It is generally recognised that in many cases such schedules may not maintain protective levels of antibody and more frequent administration is advised in high-risk situations.”

In countries such as New Zealand which hasn’t  had the disease, there is  no resistance in the local equine population. Since the Australian outbreak in August 2007, New Zealand is the only country left with a major equine population that remains free of the disease in its general population.

While most horses get over the disease, veterinarians report more post-infection problems with equine flu than with most other respiratory infections. Heart problems and pneumonia, for example, are not uncommon.