The H3N8 canine influenza virus was established as the causative agent for respiratory disease in Florida greyhounds from racetracks in 2004; it was assumed that this virus originated in horses. A 2003 outbreak of equine influenza in the United Kingdom yielded H3N8 infection in canine contacts of infected horses, although the infection does not appear to be maintained in the canine population as of this writing. This study described a 2007 Australian equine influenza outbreak and the presence of respiratory disease in canine contacts of affected horses. Nucleotide sequencing and phylogenetic analysis of nasal swabs and serum samples confirmed the presence of an identical H3N8 virus infecting both horses and dogs. This study suggested that equine influenza virus may directly spread to dogs in close contact with horses, although there was no support for dog-to-dog transmission or persistence in this population.

Commentary: This report emphasized the volatile nature of the influenza viruses and the potential for zoonotic transmission between species. It also urges the veterinarian to be aware of the potential transmission between humans and companion animals.

Influenza virus transmission from horses to dogs, Australia. Kirkland PD, Finlaison DS, Crispe E, Hurt AC. EMERG INFECT DIS 16:699-702, 2010.