Equine Influenza Virus (EIV) is often cited as the most commonly diagnosed and economically important causes of viral respiratory disease in horses. Therefore, the AAEP states that “all horses should be vaccinated against EIV unless they live in a closed and isolated facility.” Like many other respiratory viruses, it produces fever, nasal discharge, and coughing. So what sets this virus apart from other equine respiratory viruses?
Influenza is considered endemic in the horse population of North America and most parts of the world, with the notable exceptions of New Zealand and Iceland. It is highly contagious, spread rapidly by aerosolized droplets created by the hacking, propulsive cough that usually marks infection with the virus. Clinical signs typically develop in as little as 24-48h, and the distance that the infectious agents can travel mean that once influenza is present within a barn, it is usually difficult to contain its spread before all animals have been exposed. Also, it is important to note that horses that are partly immune to the virus can be a common occurrence, and these horses can be subclinically infected with the disease, showing no significant outward signs and yet still spread the virus. They are often the source of virus touching off an outbreak when no sick horses were known to be brought onto the premises.
Unfortunately, immunity to the virus (either following vaccination or natural exposure) is often short-lived. Even after a horse is naturally infected by influenza and recovers, its immunity to that same strain of the virus lasts only for about one year. Similar to human influenza viruses, equine influenza virus undergoes frequent “antigenic drift,” meaning the virus changes just enough over time that the immune system is no longer able to recognize a strain of influenza to which it had previously been exposed. This also has important implications for vaccine production. It is important to choose vaccines that contain current strains of the virus, and also select a vaccine appropriate to your area, as the strains circulating within North America are different from those found elsewhere in the world. Horses travelling internationally, or mixing with international horses at shows or other sporting events, should be vaccinated with an agent that contains both the North American and the Eurasian strains.
Because the immunity formed after natural or vaccine exposure is so short, frequent vaccination is required to maintain adequate protection. In high risk horses, vaccination every six months is recommended. In other adult horses, once yearly vaccination is probably sufficient. Those horses at the highest risk are young horses (aged 1-5 years, although older horses can have their immunity overwhelmed if they are frequently exposed at competitions or other places where horses mingle), those that are in frequently in contact with a large number of horses, and those who are not adequately vaccinated.
Explosive outbreaks, therefore, tend to occur at intervals of several years, when the immunity of the horse population wanes and there is enough antigenic drift that the virus can evade the natural or vaccine induced immunity present. A presumptive diagnosis of influenza is often based on the rapid spread of acute fevers and nasal discharge (usually starting clear, but turning more yellowish after a few days), especially if a significant, dry hacking cough is present. The number of involved horses on a premise can often reach 100%, although the mortality rate is typically low. The disease can, however, cause more serious disease in donkeys and mules. Although the nasal discharge and fever typically resolve in 1-2 weeks, the cough can persist for several weeks, and even moderate exercise can exacerbate the cough and cause it to persist well beyond this time frame. Significant weight loss is often very commonly associated with influenza infection.
However, clinical influenza does not circulate continuously, even in large groups, and instead is occasionally introduced by a new horse. The virus is also quickly cleared by the immune response mounted by the horse after exposure or infection. Together, this allows for fairly effective prevention of spread onto a new premise if all horses are appropriately vaccinated and new arrivals are effectively quarantined for as little as 14 days.
Contact the clinic for more information about Equine Influenza, and for assistance in determining if vaccination, and at what interval, is appropriate for your horse.