Every horse’s immune system is unique. Age, nutrition, stress, travel, and existing disease—such as pituitary pars intermedia dysfunction (PPID, equine Cushing’s disease)—all influence how the horse’s body responds to the threat of disease. This is why the American Association of Equine Practitioners states, “All vaccination programs should be developed in consultation with a licensed veterinarian.”
Regardless of the vaccination protocol you and your veterinarian ultimately land on, one thing is for certain: Those vaccines require routine boosters to offer optimal protection against infection. In this article, we’ll look at how vaccines work to prime the immune system, whether your horse is at risk, and why you should talk to your veterinarian about booster vaccines.
How Vaccines Work (Nutshell Version)
Vaccines contain either killed parts of a pathogen (disease-causing organism), a live but inactivated/weakened version of a pathogen, or gene-based vaccine technology. All three essentially trick the immune system into thinking a real pathogen has infiltrated the horse’s body and must be fought off.
The immune system’s response is complex and not completely understood. What we do know is infectious agents have proteins called antigens on their surfaces. Those antigens wave like flags in a storm, signaling to the horse’s body that an invader is present. One way the immune system responds to invading organisms (e.g., bacteria, viruses, fungi) is by producing antibodies. These specialized proteins bind to the pathogen’s antigens, orchestrating a precise and controlled destruction of the invading organism.
Once the infection is cleared, the immune cells that make those antibodies stand down but don’t disappear. If the horse again finds itself under siege by the same pathogen, then those immune cells spring into action, rapidly producing antibodies to destroy it. In other words, vaccination mimics natural infection and “preps” the horse’s immune system should the actual pathogen present itself.
Immune systems have relatively long memories. But as time goes on, memory—and therefore immunity—can fade. Routine boostering (revaccinating) jostles the immune system’s memory, keeping it fresh and at the ready.
“Once your horse receives its initial vaccine or vaccine series, its immune system needs to be reminded regularly how to battle that specific pathogen,” explained Liz Arbittier, VMD, CVA, Associate Professor in Equine Field Service at the University of Pennsylvania School of Veterinary Medicine’s (Penn Vet) New Bolton Center, in Kennett Square. “This will hopefully ensure adequate protection should (the horse) ever be exposed to the actual live virus or bacteria.”
Those exposures, for example, could occur when your horse is bitten by a mosquito carrying West Nile virus (WNV) or stabled near a horse sick with equine influenza (EI) on the winter show circuit.
When to Revaccinate
Not all vaccines protect a horse for the same amount of time. “The ability of a vaccine to provoke a good response is called immunogenicity, and some vaccines exhibit greater immunogenicity than others,” Arbittier said.
The rabies and botulism vaccines both have excellent immunogenicity, which is why they only need to be boosted annually.
“When administered correctly, the rabies and botulism vaccines provide extremely good protection in immune-competent animals when they are exposed to the diseases,” Arbittier said.
She added, “Other vaccines provide less immunogenicity and need to be given more often.”
Vaccines against equine influenza and equine herpes virus fall into this latter category. AAEP recommends boostering these twice annually, if supported by a risk-based assessment made by your veterinarian.
Not only do we need to consider when to revaccinate after the initial dose in a naïve (previously unvaccinated) horse, but we also need to consider when to boost vaccines relative to anticipated exposure.
Vectors are living organisms such as mosquitoes or other bloodsucking insects that can transmit infectious pathogens between animals, including horses and humans.
“For vector-borne diseases, it is really important to boost the vaccine a minimum of two weeks prior to the start of the (vector) season,” Arbittier said. “Making sure your horses have the highest level of antibody protection prior to exposure will give them the strongest protection against these diseases.”
In some states, Arbittier added, vaccinating for WNV in the spring and fall is recommended. “When mosquito season lasts for much of the year, revaccinating in the spring and fall will best maintain adequate immunity,” she said.
Potential exposure to infectious respiratory diseases via unfamiliar horses should also drive booster decisions. This could include horses who live at busy boarding barns, those heading into training, or mares visiting breeding farms.
For performance horses, certain risk-based vaccine booster requirements are clear-cut. The U.S. Equestrian Federation, as an example, requires proof of vaccination against EI and equine herpesvirus (EHV) within six months of entering show grounds for horses 7 months or older. These restrictions help protect populations of horses that frequently comingle and experience stress from exercise and travel, further compromising their immune systems.
Have a risk-based assessment discussion with your veterinarian to determine if your horse needs booster vaccinations—and which ones—to help optimize his immune system.
While vaccines offer the best line of defense against many diseases, no vaccine will protect 100% of horses from disease 100% of the time, even with a tailor-made vaccination schedule based off current vaccine guidelines. This doesn’t mean, however, you should skirt the syringe.
“Sometimes these vaccines don’t always prevent the horses from getting sick, but they can lessen the severity of the disease and/or reduce the amount of shedding that could infect another horse,” advised Arbittier.
Getting your horses’ annual wellness exams and spring vaccines is the first step to keeping them healthy. The next step is to have your veterinarian booster those vaccines at recommended intervals to keep your horses’ immune systems primed and ready to fight off foreign invaders. So, throw on your fall flannel, cozy up with your calendar, and schedule your fall farm call to ensure your horses are protected year-round.