The human “influenza season” in North America is now about to begin again, while in South America spring is approaching and their influenza season is almost over. The influenza season happens every year during autumn, winter, and early spring months, and the influenza viruses that circulate each season tend to be the usual suspects: influenza A/H1N1, A/H3N2, and influenza B viruses.

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