Canine influenza virus (CIV) is an emerging threat to dogs in the community.

Virtually 100% of newly exposed dogs become infected with CIV, and up to 20% may develop a severe systemic course.²

The Jump from Horses to Dogs

There are three major types of influenza viruses: A, B, and C. Influenza A infects the widest host species range, is the most mutable, and is responsible for human influenza pandemics. Normally, however, even this virus maintains species specificity.⁴

In the case of CIV, the H3N8 equine virus strain appears to have mutated and gained the ability to cause active disease in dogs. The virologists at Cornell determined that the strain of influenza A virus isolated from a Greyhound affected by the initial outbreak in Florida was genetically similar to the H3N8 equine influenza virus.¹¹ The finding was so unexpected that it was confirmed by investigators at the Centers for Disease Control and Prevention (CDC), who performed a nucleotide sequence of the hemagglutinin gene of a laboratory strain of equine influenza virus and the new canine isolate. Comparisons of these sequences with those in databases "clearly showed that the Cornell isolate was different and not a lab contaminant."¹⁰ It is unknown how the virus made the leap from horses to dogs, but it is hypothesized that it may have occurred following consumption of raw or undercooked horsemeat.⁴ This interspecies jump has been said to have the potential to be as serious as the rapid worldwide spread of canine parvovirus in the late 1970s, which is believed to have evolved from the closely related feline panleukopenia virus.²¹

In light of the worldwide avian influenza epidemic, this recent interspecies jump raises the question of the risk of this virus infecting humans. To date, there is no evidence that CIV causes disease in humans, and there has not been a single reported case.²² However, it does appear that the dog-adapted virus can still cause infection in horses.⁴

Because of the increasing spread of CIV infection among the canine community, it is recommended that pet care professionals have a high index of suspicion for this condition in dogs that present with respiratory problems. They should also be aware of current reports about outbreaks, particularly those affecting their local area. Importantly, they should develop a clear plan for quickly recognizing and handling outbreaks.^4,8,11