As you are going about your morning barn chores, you hear coughing from inside one of the stalls. Peering over the stall door, you see that your horse appears listless and has been just picking at his hay as he intermittently expels a deep, dry cough. You check his rectal temperature to find that he has a fever. That coupled with an obvious nasal discharge has you calling your vet. A thorough veterinary exam and diagnostic testing from secretions your vet gathered using a nasopharyngeal swab confirms that your horse is stricken with equine influenza virus (EIV).
Equine influenza virus is one of the most common infectious respiratory diseases of horses. It has a significant impact on not just you as a casual or competitive rider, but also on the equine industry. While human influenza cases have an option of using products such as Tami-Flu to slow progression of an influenza infection, such respiratory anti-viral medications are not available to horses. Instead, we depend entirely on vaccination and excellent biosecurity practices to prevent infection in the first place.
What vaccination does is stimulate production of antibodies that block attachment of hemagglutinin (HA) protein that would otherwise bind to cell receptors to create infection; high enough levels of specific antibodies conferred through vaccination are able to neutralize the influenza virus against cell invasion. This sounds simple in theory, but the challenges come because many viruses, and especially influenza, are able to mutate or experience antigenic drift that adversely impacts vaccine efficacy. Such modifications in HA make virus less recognizable by those specific antibodies.
Influenza A, and specifically H3N8, is the culprit in equine infections. The reservoir for infection is birds, and in particular waterfowl such as ducks. The birds spread it via the oral-fecal route, then bird migration brings it to other geographic areas, and especially to local poultry. However, this virus can jump species, namely to man, swine and horses. Historically, two influenza viral lineages have been of concern—H7N7 and H3N8. Fortunately, H7N7 might be extinct, thus is no longer a problem. Therefore, the only strain that needs to be included in equine vaccines is H3N8.
Originally identified in Florida in 1963, H3N8 is now responsible for worldwide outbreaks with the exception of Iceland and New Zealand. The issue is that this virus is highly contagious and can cause explosive outbreaks in naive, susceptible populations, which is extremely disruptive to the equine industry. Shows get cancelled, horses are laid up, with the result that there is an economic impact in addition to the emotional one of having a sick horse. One such major outbreak in susceptible horses occurred in Australia in 2007, affecting up to 100,000 horses. (That country had a naive population when it came to influenza; the virus wasn't normally circulating in the horses.)
The mobility of horses and the tendency of the influenza virus to mutate negatively affects efficacy of vaccines.
An important point is that even vaccinated horses that show no clinical signs are able to shed virus!
Another critical point to know as a horse owner is that congregation of horses at equestrian events increases the potential risk for exposure. Then those exposed horses disperse to various geographic areas, potentially taking virus with them to susceptible horses on the farm.
That is why it is so important to practice the concept of herd immunity that calls for immunization of all horses on a property regardless of their activity levels or travel experiences.