In 2004 researchers identified a highly contagious canine influenza virus strain (H3N8) that was closely related to an equine flu strain. Essentially that meant the virus had jumped from horses to dogs, and the scientists said it was “a very rare event of considerable scientific interest with regards to understanding influenza virus transmission across species barriers.” It also illustrated the horse is not a dead-end host for the virus as once thought.

Gabriele A. Landolt, DVM, MS, PhD, Dipl. ACVIM, an assistant professor at Colorado State University, reviewed virus mutation and interspecies transmission of equine influenza viruses at the 2006 AAEP Convention.

“In horses, influenza infection probably represents the most frequently diagnosed and economically important viral respiratory disease,” Landolt explained. “However, equine herpesvirus-1 and -4 are quickly catching up. While experimentally horses of all age groups are susceptible to infection, the highest incidence of disease seems to be in the younger population (between two and three years old).

“Vaccination of horses with inactivated or modified-live virus vaccines has been (and remains) the cornerstone of the control of equine influenza,” she said. “However, vaccine breaks (failures) do occur, and influenza virus infections still remain a serious health and economic problem in horses.”

Why do we see vaccine failure?

According to Landolt, insufficient vaccination and maternal antibody interference in young horses (less than six months of age) present problems for vaccine protection against influenza.

She added, “Another important reason for vaccine failures is due to the fact that virus continues to evolve genetically.”

Interspecies Transmission

There are two mechanisms by which influenza is transmitted from one species to another. One method is the exchange of genetic material (known as gene swapping or genetic reassortment) between flu viruses from different hosts that can result in the creation of a unique third virus that has the ability to infect and spread within a new host species. Examples of this are the viruses that caused the influenza pandemics (disease outbreaks impacting a large geographical area) of 1957 and 1968.

Another method is the transfer of a whole virus from one host species to another, and that virus has the ability to replicate in the new host. The best examples for this scenario are the pandemic of 1918, when an avian influenza strain was transmitted to humans and killed an estimated 675,000 people in the United States alone, as well as the recent infections of humans with the avian flu H5N1. However, in contrast to the 1918 strain, the H5N1 strain of influenza hasn’t shown the ability to transfer efficiently from human to human.

“The transmission of influenza from horses to dogs is especially interesting in light of the fact that dogs were not commonly regarded as hosts for influenza A viruses,” Landolt said. “Although circumstantial evidence from surveillance studies indicates that human-lineage influenza viruses occasionally cross the species barrier to dogs, none of these human viruses are known to have spread efficiently within the canine population. In contrast, the H3N8 equine-lineage virus that caused the recent canine influenza outbreak in the United States spread quickly among dogs.”

Scientists believe H3N8 used the whole-virus transfer method to move between horses and dogs, except canines are able to transmit the virus to other canines, which is only rarely seen with this type of viral transfer.

“Because equine H3N8 viruses apparently have the ability to infect dogs, it raises the question if such an event has happened before or if it could happen again in the future,” Landolt said. “Although it is unclear if the canine H3N8 viruses are still capable of infecting horses, they potentially represent a serious emerging disease threat to the American horse population.”

What might be even more troubling, she said, is current vaccines might not protect horses from future flu strains because of the ability of the virus to mutate.

“The sustained antigenic drift (small, irreparable changes in the virus’ makeup) of the equine H3N8 viruses continues to result in vaccine failures,” Landolt explained. “Thus, inclusion of viral antigens representative of contemporary circulating viruses (in vaccines) has to remain a priority.”

Get research and health news from the American Association of Equine Practitioners 2006 Convention in The Horse’s AAEP 2006 Wrap-Up sponsored by OCD Equine. Files are available as free PDF downloads