Horses can respond to vaccines differently, and it is believed that the response to vaccines can be related to the health of your horse’s intestinal tract. Most vaccines require boosters because an animal’s immunity decreases over time, and a booster stimulates immunity.

There are conditions that decrease the horse’s response to vaccines. Any disease process can do this, especially equine metabolic syndrome, or EMS, an insulin-resistant condition. Horses commonly have equine metabolic syndrome, and those horses are usually overweight and have a different number and type of normal organisms in the gastrointestinal tract.

The number and type of organisms in the GI tract are called the microbiota, and disruption of these organisms can affect the immune system. Dr. Kathleen Crandall, Equine Nutritionist, Kentucky Equine Research, says in the publication The Horse that 70% of a horse’s immune system is located in the intestinal tract, where both beneficial and non-beneficial bacteria reside in the intestinal lumen and usually stay in balance. If an imbalance occurs, the immune response can trigger inflammation.

Dr. Amanda Adams, Ph.D., Assistant Professor at the University of Kentucky Gluck Equine Research Center, says that one study showed feeding prebiotics changes the immune response to influenza vaccination in horses. However, it is not known if these horses were better protected against the disease.

There is limited research to clearly identify which supplements may optimize the microflora to support a horse’s immune system best. Although there seem to be hundreds of supplements available claiming to support the immune system, there is very little research to indicate that these supplements work.

Ask your veterinarian about immune system supplements you may be considering for your horse.