Page 32 - NMHBA Summer 2017

Equine influenza is caused by a virus and is one of the most common infectious diseases of the respiratory tract of horses. It is endemic in the equine population of the United
States and throughout much of the world, where young horses aged one to five are generally the most susceptible. Older horses may have some immunity if they have previously encountered the virus. Equine Influenza is one of the diseases a horse might be exposed to when leaving the farm and going to the racetrack.
Mark Crisman DVM, who is the Senior Veterinarian at Equine Technical Services, Zoetis in Blacksburg, Virginia, works for Zoetis and also teaches at the University. He says equine influenza has been around for centuries—with outbreaks recorded throughout North America and Europe. “One incident that really got our attention was the outbreak in Australia in 2007. Australia had been influenza-free, so horse owners there did not vaccinate for this disease. It came in to that continent with an imported horse and the native equine population was completely susceptible,”
he says.
“More than 10,000 premises (and
all the horses on those premises) were affected. Australia spent more than a billion dollars getting that outbreak resolved,” says Crisman.
“The incubation period for this respiratory disease is very short—just a couple of days. We’ve seen outbreaks that literally swept through a stable or barn, and in a teaching hospital, where within 48 hours every horse in the barn was coughing,” he says.
The clinical signs include fever, cough and nasal discharge. “I’ve seen fevers up
to 104 degrees and 105 and higher with influenza. The horse also has a harsh, dry cough. During that phase, the virus is being aerosolized and can be spread quite
a distance just from the coughing and sneezing.” If another horse is downwind from the horse that is coughing, it may inhale some of those virus-bearing droplets.
“The viral infection usually runs its course quickly,” says Crisman. It attacks the epithelium in the upper airways and damages this lining (and the cilia), but this damage will heal within one to three weeks if there are no secondary complications.
by Heather Smith Thomas
“The length/duration of viremia (virus in the blood), which is the period of time the virus is being shed and can be spread to another horse, depends on existing immunity of that particular horse. For instance, in Australia, where there were many naïve horses, the disease basically ran amok. Here in the U.S., it’s a bit different because many horses have had some exposure and have some immunity because they are generally vaccinated, which gives some protection,” he says.
“The severity of clinical signs can vary
a great deal. Horses with some level of immunity, such as recent vaccinations, won’t be as sick and won’t shed the virus as long. We tell horse owners that they should rest the horse one week for every day of fever.” In other words, if the horse had a fever for only one day, a week of rest might be adequate, whereas if the horse had a fever for three days, you should rest him for at least three weeks before working him again, even if he feels good before that time is up.
“The virus attacks the epithelium
in the upper airway. The upper airway
is designed to be a major defense component of the horse’s immune system,” he says. This is the first line of defense to ensure that pathogens don’t get down into the lungs.
Bacteria and other pathogens exist in the upper airway, inhaled by the horse. The tiny cilia (hair-like “fingers” that line the epithelium) constantly move in wavelike motion to move dust or any other inhaled debris—including pathogens—up out of the airway so they can be coughed out or swallowed, keeping them out of the lungs.
30 New Mexico Horse Breeder
Exercising a horse too soon after infection can cause secondary issues. The deep breathing in of dust and debris draws it into the airways and can create a respiratory infection.
Ty Wyant
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