Influenza is caused by a virus and is one of the most common infectious respiratory diseases of horses. The equine influenza virus causes upper respiratory disease and the horse has a fever—which can be 106 degrees or higher. A horse with a high fever doesn’t feel good, and it is lethargic and is off feed.
The sick horse generally has a clear nasal discharge, and a harsh, dry cough. When the horse coughs, fluid from the airways is being aerosolized. The virus can be spread many meters away on those tiny droplets, according to Mark Crisman, DVM, senior veterinarian, Equine Technical Services at Zoetis.
He said the influenza virus is effectively spread by nose-to-nose contact and by coughing. Even if another horse is downwind quite a ways from the coughing horse, it might inhale some of those virus-bearing droplets and become exposed to the disease. “The viral infection usually runs its course quickly,” said Crisman. It attacks the epithelium in the upper airways and damages this lining (and the cilia), but this damage will heal within a week to three weeks if there are no secondary complications.
“The virus destroys the lining of the trachea and its cilia—the tiny hair-like appendages that continually move mucus and debris up and out of the airway,” said Dr. Katie Wilson of the Large Animal Medicine department at the Virginia-Maryland Regional College of Veterinary Medicine.
The upper airway is the first line of defense to keep pathogens from getting down into the lungs. Bacteria and other pathogens commonly enter the upper airway as they are inhaled by the horse. The tiny cilia that line the epithelium move in a wavelike motion to push dust or any other inhaled debris—including pathogens—up and out of the airway to be coughed out or swallowed, keeping them out of the lungs.
The influenza virus removes that defense by destroying the cilia. Wilson said you therefore don’t want the horse exercising, breathing deeply, and pulling dust/debris into an airway that cannot prevent these particles from getting down into the lungs.
“Destruction of the cilia and epithelium leaves a horse vulnerable to secondary bacterial infection along with the influenza infection," said Wilson. "This complication is fairly common, and the horse may develop a thicker white/yellow nasal discharge and may be sicker and for a longer time. The respiratory infection may progress into the lungs (pneumonia), which can sometimes be life-threatening if it is not recognized and treated early.”
Without this complication, usually the horse with influenza will get rid of the virus without help within about a week. “Because it’s a virus, antibiotics are not effective," said Wilson. "Treatment consists of supportive care. The sick horse can be given anti-inflammatory medications to keep fever down and keep the horse comfortable so it will continue to eat and drink.
“If the horse seems to be getting a secondary infection, or is at high risk for secondary infection, we may put him on antibiotics," continued Wilson. "Complications delay recovery, but the virus itself usually isn’t fatal. Horses occasionally develop serious complications from influenza that are immune-mediated. The immune system, while trying to kill the virus, starts to recognize parts of its own body as foreign, and attacks them. Very rarely, horses may develop immune-mediated myositis (a muscle disease) or sometimes myocarditis (affecting the heart muscle). These are some of the serious complications that may occur.”
Crisman added, “Influenza is a pesky little virus. It’s not the virus itself that’s a big problem; it’s mainly the secondary complications that can occur that can be tragic. This sequel can happen if people don’t follow biosecurity guidelines, or rest the horse long enough before going back to work. If you take care of the horse, the virus will run its course and the horse will recover.”