“There are a number of influenza vaccines available, and they are updated periodically,” said Dr. Katie Wilson of the Virginia-Maryland Regional College of Veterinary Medicine.

“The influenza virus strains mutate and change over time, just like in human strains,," said Wilson. "This enables the viruses to evade the immune system and the protection offered by vaccines. If the vaccine is not the current strain, it won’t be able to prime the immune system as well to fight off the virus that infects the horse. The strains don’t change as quickly in horses as they do in humans. For people, a new vaccine is created every year, based on what was the most common strain of influenza the previous year.

“This doesn’t happen that quickly in horses, but vaccine manufacturers should try to use the strains that have been involved in most of the outbreaks within the previous five years," noted Wilson. "The equine vaccines do get updated, but sometimes not that often.”

There are three different types of vaccines available. One is an inactivated, killed virus, given as an intramuscular vaccine, the second is a modified live virus given with an intranasal technique, and the third is a modified canary-pox virus, explained Wilson. Every equine vaccine company offers some type of influenza vaccine.

“The horse should have an initial three-dose series of injections (a month apart) if it has never been vaccinated before,” said Wilson.

The horse is not protected until about two weeks after the final injection. After that, the recommendation is for boosters twice a year for young horses that will be exposed and co-mingling with other horses.

The second class is the modified live virus (MLV) vaccine. “The virus has been modified so it is not infective and it won’t cause the disease, but it will stimulate the immune system to create antibodies," explained Wilson. "Generally the modified live vaccines stimulate the immune system better than killed virus vaccines because they appear more similar to the actual influenza virus. The modified live virus vaccine is an intranasal vaccine, squirted up the nose and into the pharynx." These tubes have recently been updated to be much shorter and easier to use and are included in the vaccination package.

Intranasal vaccines induce immunity locally—in the pharynx and nasal passages—rather than stimulating the immune system to develop antibodies that circulate through the body (bloodstream and lymph systems).

“Local immunity from the MLV vaccine is probably more effective because it attacks the virus where it enters the body," said Wilson. "For the intranasal MLV vaccine, the horse only needs one dose, even if it has never been vaccinated before. This vaccine does not need any initial boosters because it induces a different type of immunity and is very effective. The recommendation is for vaccination every 6 months after the initial dose.”

The third class of influenza vaccine is a modified canary-pox virus that serves as a vector to carry killed influenza virus parts, to trick the immune system into thinking the whole live virus is there. “It’s supposed to induce better immunity than the fragments of virus in a killed vaccine," said Wilson. "This vaccine is an intramuscular injection that should be boostered every six months.

“One thing that is critical for any horse that has never been vaccinated is to start with a two-dose series (with the injectable style vaccines),” said Mark Crisman, senior veterinarian with Equine Technical Services at Zoetis. “If you don’t give the follow-up booster shots, there will be no immunity. You need to give the priming dose, then a few weeks later the booster—which will stimulate the immune protection for whatever period of time it lasts.” It is important to follow label directions regarding administration, boosters and vaccination schedules.

“After the initial booster series, how often you vaccinate (once, twice, or more times annually) is totally dependent on the risk, and general environment of the horse," said Crisman. "Some boarding and training stables have strict rules about it and require a flu vaccination for horses in their facility.”

Most influenza vaccines come in a combination with rhino (herpes), and they can also be included in a 4- or 5-way vaccine with tetanus, Eastern and Western encephalomyelitis, or West Nile virus.

“There are some advantages (such as convenience) and disadvantages to combination vaccines,” said Wilson. “In general, the more vaccines a horse gets at one time, the less likely the immune system will be able to respond appropriately to each one of those diseases. We try not to over-vaccinate with too many diseases at one time. For flu and rhino it doesn’t matter whether they are in combination or not, but neither the MLV intranasal vaccine nor the canary pox vector vaccine have any combinations.”

Heather Smith Thomas