Influenza is a risk-based vaccine, according to Dr. Mark Crisman, senior veterinarian, Equine Technical Services at Zoetis. The horses most at risk should be vaccinated three or four times a year. For a backyard horse that never goes anywhere, and might only be exposed unintentionally by an unforeseen encounter with other horses, annual vaccination might or might not be adequate.

Credit: Thinkstock Foals might be protected for a few months by maternal antibodies if their mothers were properly vaccinated.

Even if you live at the end of a back road and don’t think your horse would ever be at risk, horses from a farm down the road might get loose and come visiting, which means there would be nose-to-nose with your horse.

“Decisions regarding influenza vaccination should be made by the owner in consultation with his/her veterinarian,” said Crisman. “Influenza is a risk-based vaccine (rather than one of the core vaccines recommended by the AAEP for every horse).” Risk-based means you need to understand the level of risk that your horse might face for exposure to equine influenza.

Some horses are more vulnerable to the influenza virus than others. Foals might be protected for a few months by maternal antibodies if their mothers were vaccinated or had some natural immunity from exposure. Older horses have generally encountered the virus at some point and might have some natural immunity, according to Dr. Katie Wilson, of the Large Animal Medicine department at the Virginia-Maryland Regional College of Veterinary Medicine.

“It seems like natural immunity lasts about a year or so,” said Wilson. “There may be some longer-term effects on the immune system that enable older horses to not be as affected by this virus. The horses that tend to get equine influenza most readily are young horses that are co-mingling, such as at horse shows and race tracks.”

Sending youngsters to the track is like sending young children off to school; they pick up every “bug” that’s going around.

“Most horses won’t die from influenza (just as most people aren’t going to die from a “cold” virus), but because it destroys cilia in the air passages, it will be awhile before full recovery,” she said. Any horse that will be leaving the farm and coming into contact with other horses—and especially any horses in show careers that can’t afford any “sick time”— should be vaccinated. Recovery time will be prolonged if the horse isn’t given enough time off from work once it is affected by the influenza virus.

“It takes about three weeks for the mucociliary mechanism to regenerate,” said Wilson. “In the meantime, the horse should be laid off from work. If he continues to work and exercise, with more air turbulence in trachea, there will be no protection from bacteria that may enter the respiratory system. The layoff should be at least one week for every day that the horse had a fever during the disease—with a minimum of three weeks, since that’s how long it takes for the mucociliary apparatus to heal.” 

Thus a case of equine influenza in a performance horse can be a fairly costly disease because of time lost in training, showing or competing.

Since foals and young horses are also a high-risk group, there has been discussion regarding the best age to vaccinate foals for influenza. “Young foals tend to keep the maternal antibodies (acquired from their dam’s colostrum) for influenza for a longer time than the antibodies against other diseases,” Wilson explained. “If a foal still has circulating antibodies (from the dam), and you vaccinate that foal with any vaccine that contains parts of the virus, the circulating antibodies will neutralize those virus parts, and they won’t work to stimulate the immune system.

“Therefore the recommendation is to not vaccinate foals for influenza until they are at least 6 months old,” Wilson continued. “It won’t hurt them if you vaccinate them too young, but it just won’t stimulate immunity; it won’t give them the protection you are hoping for.”

If the dam has never been vaccinated or has not been exposed to the disease and has no antibodies in her colostrum, the foal will not have maternal antibodies that would interfere with vaccination immunity. In that situation, a person could vaccinate the foal at an earlier age. A person needs to know the vaccination history of the mare.

“The recommendation is to vaccinate broodmares to protect the foal during the last 30 days of pregnancy and to produce antibodies for the colostrum,” said Wilson. Those antibodies give the foal a lot of protection during the first months of life.