Equine Flu Virus Infects Dogs; Can Horses Be Re-Infected?

Researchers recently identified a highly contagious canine influenza virus strain that is thought to be an adaptation of an equine flu strain, which was transferred from horses to dogs in 2004. It is unclear at this time whether the strain can re-infect horses.

Scientists from the Colleges of Veterinary Medicine at the University of Florida (UF) and Cornell, along with the Centers for Disease Control and Prevention (CDC), published a paper on the topic that appeared Sept. 29 in Science Express online.

The strain first surfaced in dogs at a Florida Greyhound racetrack in January 2004. Ruben Donis, PhD, chief of molecular genetics for the CDC's influenza branch, said, "Horses acquired the H3N8 influenza virus from an unknown species, perhaps an aquatic bird, in 1963. During the past 40 years, the virus circulated the horse population worldwide, adapting progressively to this species by a process of mutation and selection.

"The virus might have just recently reached the high level of adaptation to mammalian host that was necessary for infection of dogs," Donis explained. He called the illness' emergence "a very rare event of considerable scientific interest with regards to understanding influenza virus transmission across species barriers."

There are two ways influenza is transmitted between species. The first is through gene swapping between flu viruses from different hosts, which creates a unique third virus that can infect a totally different host. An example of this is the pandemic (impacting a large geographical area) outbreak of 1918, when a strain of avian flu was transmitted to humans and killed an estimated 675,000 people in the United States.

The other transmission method is the transfer of the whole virus from one host species to another, with the virus having the ability to replicate in the new host. Scientists believe the whole-virus transfer method is how the virus moved between horses and dogs, except dogs are able to transmit the virus efficiently to other dogs, which is not typically a characteristic of this type of viral transfer.

One gene of the canine virus has eight to 10 amino acid changes compared to the equine virus. Donis suspects these changes might affect the virus' interaction with canine cellular receptors, and other gene changes might affect aspects of the virus' interaction with the host.

UF's Cynda Crawford, DVM, PhD, said, "The risk (of re-infecting horses with a more virulent strain) is most likely insignificant, but we'll have a better handle on this risk when we complete studies on the susceptibility of horses to infection with canine influenza viruses." More information: www.TheHorse.com/emag.aspx?id=6235.

About the Author

Chad Mendell

Chad Mendell is the former Managing Editor for TheHorse.com .

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