Following the vaccination guidelines recommended by the American Association of Equine Practitioners lays a solid foundation for protecting horses against infectious disease. However, not all horses respond alike to vaccines. Certain factors might help improve or maximize horses’ vaccination response, providing optimal protection.

Reasons for Waning Immunity

Horses typically respond well to most vaccines and only require periodic (e.g., annually for rabies and tetanus) boosters. You might need to booster other vaccines, such as the equine influenza or flu vaccine, more frequently. Boosters are necessary for several reasons, including:

1. Annual or booster vaccines are critical because immunity from the original vaccine fades over time, and the booster can help the immune system maintain protection.
2. Horses might have underlying disease processes that preclude an adequate immune response. Researchers demonstrated that horses with equine metabolic syndrome (EMS) appear to have different microbiota numbers and diversity than healthy horses and, therefore, might have a reduced response to vaccination compared to metabolically normal horses (Elzinga et al., 2018).
3. Immunosenescence, defined as a gradual decline of the immune system with age, could result in inadequate immune responses and leave older horses at increased risk.
4. Dysbiosis, or disruption of the equine intestinal microbiome, might negatively affect a horse’s immune system and response to vaccination.

A Closer Look at Diet and the Microbiome

“The intestinal microbiome, which in essence can be considered a distinct organ, plays an integral role in the horse’s immune system,” explains Kathleen Crandell, PhD, a nutritionist at Kentucky Equine Research, in Versailles. “About 70% of the immune system is located in the digestive tract.

“The lymphatic system, hugely responsible for the body’s defense, has a prominent presence in the mucosal (inner) layer of the digestive tract,” she continues. “These defenses include the Peyer’s patches in the mucosa, lymphocytes and plasma cells that reside in the basement membrane of the intestine, and lymphocytes found between the mucosal cells.”

Both beneficial and nonbeneficial bacteria make up the gut microbiome in the intestinal lumen, and they typically remain in balance in the healthy horse. Some of the many beneficial bacteria produce butyrate. This short-chain fatty acid helps maintain that healthy mucosal layer.

“When an imbalance between the beneficial and pathogenic bacteria in the gut occurs, the immune response can trigger inflammation,” says Crandell.

Amanda Adams, PhD, associate professor and specialist in equine immunology at the University of Kentucky, in Lexington, says her research has produced “data showing treatment (with prebiotics) changed immune responses to influenza vaccination in horses.” In her study, however, “all groups of horses responded to vaccination, so with (prebiotic) treatment having some effects, it doesn’t necessarily mean that the horse is ‘better protected.’”

“Unfortunately, limited research has yet to clearly identify which nutritional supplements of specific ‘-otics’ are capable of optimizing the microbiome to best support a horse’s immune system,” says Crandell.

Adams agrees, saying “more research is needed in this area because the gut-immune interaction is so complex, and we have a lot to learn there before we can really say probiotics or modulating gut responses help boost immune responses.”

In the meantime she says owners can start by offering their horses a balanced diet. “Make sure the horse is receiving all required nutrients daily and is in good body condition and healthy—that will be the foundation for a horse to respond to vaccination,” she says. “In my opinion, other factors that may be of greater importance in influencing the immune responses to vaccination are factors of age, obesity, and stress.”

Take-Home Message

Diet, in addition to vaccination, might affect a horse’s microbiome and immune response. Evaluating your horse’s diet to be sure he’s getting sufficient nutrients can help him get the most out of vaccination, but more research is needed to confirm the effect gastric health might have on a horse’s vaccine response.

Elzinga, S., Reedy, S., Barker, V. D., Chambers, T. M., & Adams, A. A. (2018). Humoral and cell-mediated immune responses to influenza vaccination in equine metabolic syndrome (EMS) horses. Veterinary immunology and immunopathology, 199, 32–38.