small mutations new strain which can be combined in many ways. The numbers in the virus designation refer to the specific type of these antigens that are present. “Any combination of these makes each individual virus in each host species different,” Wilson says. For example, the infamous Spanish flu of 1918 was caused by H1N1. The “bird flu” outbreak of 2003 was caused by H5N1. With a few exceptions, most individual flu viruses can infect only one species of animal---a healthy person cannot catch influenza from a horse, at least not with the current varieties of equine influenza virus.
EIV, like its relatives in the influenza family, has two characteristics that make it difficult to control. For starters, it spreads quickly from host to host via airborne droplets. “The incubation period is very short---just a couple of days,” says Mark Crisman, DVM, MS, DACVIM, senior veterinarian for Zoetis, who also teaches at Virginia–Maryland College of Veterinary Medicine. “We’ve seen outbreaks sweep through a stable or barn, and in a teaching hospital, where within 48 hours every horse in the barn was coughing.”
The second notable characteristic of influenza viruses is their ability to change constantly. Over time, as the virus spreads from host to host, it undergoes small genetic changes---a process known as antigenic drift. At
Both antigenic shift and antigenic drift can produce viruses that “jump species” into new populations with no immunity. Equine influenza virus jumped to dogs in 2004, when an outbreak occurred in racing Greyhounds in Florida.