When Horses Get the Flu (Can You?)

With all the concern over "bird flu" being transmitted to people, influenza is very much in the news these days. And although horses can get influenza, too, the good news is, you're not likely to "catch it" from your horse. Even so, there are things you'll want to know about equine influenza.

As a rule, influenza viruses have strict requirements for survival and transmission. Horses, people, birds, etc., are generally only infected by the specific strains that affect that particular species. However, on occasion, viruses do develop the ability to "jump" species, like the bird flu did in Asia.

Interestingly enough, an equine outbreak of severe influenza in China in 1989 came from a strain more commonly found in birds. Twenty percent of the infected horses died. Equine influenza virus has also been found to jump species and infect dogs, causing very severe and potentially fatal pneumonia.

When a species jump happens, the disease tends to be particularly severe. Unfortunately, because the immune system of the new host species has never been exposed to it before, it has no antibodies or specific cellular defenses to recognize and destroy the invader. The virus, in effect, gets a head start of 10 days to two weeks on the immune system.

Until the sophisticated arms of the immune system can get up to speed, the work of fighting the virus falls to the primitive immune system. That means the new host responds with exaggerated inflammatory responses. The body's extreme reaction makes the animal or person feel very sick and can damage more tissue than a more sophisticated immune reaction would.

Fortunately, it's a relatively rare occurrence for influenza to jump species. However, influenza viruses within their own species can cause enough problems as it is.

Catching the Flu
A horse catches the flu by inhaling virus particles that become airborne when an infected horse coughs or sneezes. Therefore, to contract influenza, a horse has to be relatively close to an infected horse.

The need for proximity explains why horses that are not exposed to other horses have little risk of coming down with influenza. Outbreaks tend to occur when horses are brought together in large groups and are in close contact, such as at shows, racetracks and inside boarding stables. The more horses you bring together, the higher the odds that one of them will have influenza and can spread it to the others.

There is no "flu season" per se, and outbreaks can and do occur year-round. However, just like with people, when horses are together in an enclosed area with poor circulation, the amount of virus particles in the air can rise sharply. Barns that are closed up tightly in the winter, horse trailers and vans, and indoor show facilities are particularly high risk environments.

Foals with no immunity to influenza viruses, and older horses or sick horses with poor immune responses, are the most vulnerable to infection. Otherwise, the combination of vaccination and repeated low-level natural exposures keeps most horses free of the disease unless they are in close quarters with a horse producing very large amounts of virus.

There hasn't been a large-scale influenza outbreak in the United States since the early 1980s, but it could happen at any time. Epidemics tend to occur when the virus mutates in a way that it avoids triggering a "memory" immune response. When this happens, even vaccinated horses are unprotected. Epidemics also occur when imported horses bring influenza virus into a part of the world that had previously been free of the disease. The virus is well established in the United States and Europe, but many other countries are largely free of flu.