Public health authorities are looking into how equine influenza spread among 5,000 horses during an outbreak in Australia in 2007. Although the team identified a few possible methods for spreading the virus, including spread by birds and other animals, they reported in a recent review that they were unable to prove whether these potential transmission scenarios occurred.

"The virus is thought to be transmitted through direct contact with infected horses and contaminated equipment or indirectly from contact with droplets generated when the animals coughed or sneezed over a short distance," said Paula J. Spokes of the New South Wales Department of Public Health Officer Training Program. But in some instances during this outbreak, horses became infected without a clear mode of transmission.

"The speculation that birds might have served as carriers strikes me as plausible," said Tom Chambers, PhD, head of the OIE Reference Laboratory for Equine Influenza at the University of Kentucky's Gluck Equine Research Center, who commented on the report. "However, none of the Australian horses were vaccinated. In the USA, effective vaccines are available. If a horse is vaccinated using vaccines containing appropriately protective virus antigens, then we can hope and expect that transmission by birds would be unimportant, even if biologically possible."

The team also investigated the possibility of windborne spread of the virus