Equine Influenza Virus Transmitted to Canines

Scientists Unsure if Horses Could 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, in conjunction with the Centers for Disease Control (CDC) and Prevention, published a paper on the topic that appeared Sept. 29 in Science Express online.

The flu strain first surfaced in dogs at a Florida greyhound racetrack in January 2004. Clinical signs of the canine flu virus are similar to the clinical signs caused by the bacterium Bordetella bronchiseptica, which causes kennel cough. Those signs can include coughing that persists for 10-21 days, nasal discharge, and a low-grade fever.

Ruben Donis, PhD, chief of molecular genetics for the influenza branch of the CDC, said at a press conference, "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 mechanisms by which influenza is transmitted from one species to another, according to Science Express" One method is the exchange of genetic material (known as gene swapping) between flu viruses from different hosts that creates a unique third virus that can infect a totally different host," said UF's Cynda Crawford, DVM, PhD. An example of this is the pandemic (a disease outbreak impacting a large geographical area) outbreak of 1918, when a strain of avian influenza was transmitted to humans, and killed an estimated 675,000 people in the United States, according to the CDC.

"Another method is the transferring of the whole virus from one host species to another, and that virus has the ability to replicate in the new host," Crawford said. "This is similar to the avian flu H5N1 that is infecting people in Asia now." However, the H5N1 strain of influenza has not shown the ability to transfer from human to human.

Scientists believe the whole-virus transfer method is how the virus moved between horses and dogs, except canines are able to transmit the virus to other canines, which is not typically a characteristic of this type of viral transfer. "The data indicate that the virus is being transmitted efficiently from dog to dog, and this indicates that the equine virus was transmitted to dogs and is now well established in the dog population," Donis said.

The study described the strain as "an unprecedented interspecies transfer of a complete equine influenza virus to a dog, and the emergence of a new canine-specific influenza virus associated with acute respiratory disease."

Researchers found the hemagglutinin (H, a surface protein that allows the virus to attach to and enter host cells) gene of the canine virus had eight to 10 amino acid changes compared with the equine virus.

"We speculate at this point--because we haven't finished the research--that these changes may affect the interaction of the virus with cellular receptors (in canines)," Donis said. "In addition to that, there are changes in other genes that might affect other aspects of the interaction of the virus with the host, and this is the most important part of the study from the perspective of understanding interspecies transmission."

"The risk (of re-infecting horses with a more virulent strain) is most likely insignificant, but we will have a better handle on this risk when we complete some studies on the susceptibility of horses to infection with canine influenza viruses," Crawford explained. "We hope to complete these studies in the upcoming months."

About the Author

Chad Mendell

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

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