However, there are many other influenza viruses in the world. The ‘H’ and ‘N’ varieties (called “subtypes”) of influenza A viruses now go up to H18 and N11 with the recent discovery of new subtypes in bats. Most of these subtypes are rare in mammals but common in wild waterfowl. The only influenza subtypes that have been confirmed to infect horses naturally are H3N8 and H7N7, and the horse-adapted H7N7 viruses appear to have disappeared from horses nearly 40 years ago.
Does this mean that horses cannot be infected by influenza viruses from other species of animals? The answer is no, possibly they can be. Transmission of influenza viruses between different species definitely occurs. Humans, swine, dogs, cats, whales, seals, and sometimes other mammals such as mink have occasionally been infected by influenza viruses from birds. This was long thought to happen only rarely, but since 1997 in southeast Asia there have been annual occurrences of humans contracting bird flu subtypes such as H5N1 or H7N9 and these cases are often lethal. Almost all of these cases have been dead-end transmissions, meaning that each case appears to be a separate event with very little sign that they are capable of spreading from human to human.
Can bird flu viruses infect horses? The answer is most likely yes. One piece of evidence is that the H3N8 subtype was not always circulating in horses; it first appeared in 1963, and its genetic ancestors seem to have been bird flu viruses. In 1989 in northern China a strain of bird flu was positively confirmed to cause a large-scale disease outbreak in horses. Its subtype was also H3N8. Was that coincidence or is there some unique characteristic of the H3N8 subtype that makes it more apt to infect horses? Those questions remain unanswered.
It is known that the molecular receptors present on the surface of horse tracheal respiratory cells are a little different from other species, which could be a contributing factor to the specificity of the influenza viruses that infect horses. Work in the author’s laboratory suggests that this not the entire explanation and other factors must be involved.
A related question is whether influenza viruses can be transmitted from horses to other mammals. That answer is definitely yes. About 15-20 years ago, H3N8 horse flu infected dogs in the United States and has persisted in dogs ever since. Can it jump from dogs back into horses? Maybe, although once the virus adapted to dogs, it became much less adapted to horses.
Are humans at risk of infection from equine influenza viruses? Experimental infection of human volunteers conducted 50 years ago suggests that infection can happen but the result is mild or even sub-clinical. Humans with horse exposure do sometimes develop antibodies against equine influenza virus. The only report of a suspected naturally occurring clinical disease from equine influenza virus in a human never actually demonstrated the presence of equine influenza virus in that patient.
The message for readers is: Transmission of influenza viruses from one species to another can happen and sometimes does happen. If your horse is sick with the flu, take elementary biosafety precautions such as washing hands and clothes or equipment that came in contact with that horse, as soap will kill influenza viruses. And if you are sick with the flu, it is probably best not to cuddle your pets.
CONTACT—Thomas M. Chambers, PhD—firstname.lastname@example.org—859/218-1126—University of Kentucky Maxwell H. Gluck Equine Research Center, Lexington, Kentucky
This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd’s, London.