Vaccines for All Ages

Horses at different life stages have a variety of immunization requirements, but they’re not as complicated as they might seem.

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Vaccines for all Ages
Vaccination remains the most effective way to help protect horses against infectious disease, mosquito-borne diseases, and more. | Photo: Kevin Thompson/The Horse

Horses at different life stages have a variety of immunization requirements, but they’re not as complicated as they might seem.

In a herd of various-aged horses, vaccinating is rarely a one-size-fits-all endeavor. Imagine: Adult horse Patches is recovering from an illness and has a busy show season ahead; broodmare Sunny and her young filly are heading off to the breeding shed for a date with mom’s next suitor; and ol’ Dobbin is enjoying his golden years at home, despite an equine Cushing’s disease diagnosis.

So, who needs what vaccines? Are immunization requirements the same across our herd’s vast age spectrum? And do these horses need to receive all of the commercially available vaccines, or just certain ones?

Don’t panic. Dale Brown, DVM, an ambulatory practitioner at Rood & Riddle Equine Hospital, in Lexington, Kentucky, and Elizabeth Davis, DVM, PhD, Dipl. ACVIM, a professor and section head of equine medicine and surgery at Kansas State University’s College of Veterinary Medicine, in Manhattan, are here to help, making clear the cloudy issue of vaccinating decisions. 

Vaccination Guidelines

Both Brown and Davis strongly recommend horse owners consult the American Association of Equine Practitioners’ (AAEP) vaccination guidelines (available at when planning vaccination protocols with their veterinarians. These guidelines include both “core” and “risk-based” vaccines.

“Core vaccinations protect against diseases that are endemic to a region, those with potential public health significance, required by law, virulent/highly infectious, and/or those posing a risk of severe disease,” the guidelines’ authors state. “Core vaccines have clearly demonstrated efficacy and safety, and thus exhibit a high enough level of patient benefit and low enough level of risk to justify their use in all equids.” 

The AAEP’s core vaccines include Eastern/Western equine encephalomyelitis (EEE/WEE), rabies, tetanus, and West Nile virus (WNV), and our sources says all horses should have these on board.

Meanwhile, “risk-based vaccinations are those having applications which may vary between individuals, populations, and geographic regions,” the guideline authors write. “Risk assessment should be performed by, or in consultation with, a licensed veterinarian to identify which vaccines are appropriate for a given horse or population of horses.”

The AAEP classifies anthrax, botulism, equine herpesvirus-1 and -4 (EHV-1), equine viral arteritis (EVA), equine influenza, Potomac horse fever, rotavirus, and strangles vaccines as risk-based options.

Regardless of Age

There are a few vaccination pointers owners should remember, regardless of whether their horse is a few months old or getting up there in age:

Vaccines are important While it might seem like an easy way to save money on horse care, skipping vaccination might mean the difference between life and death for a horse. “The majority of vaccines on the market are highly efficacious, and although no vaccine can assure 100% prevention of a particular disease, they can significantly reduce the risk of disease or clinical signs, many of which can be fatal in the unvaccinated horse,” Brown says. In other words, it’s typically significantly less expensive to vaccinate a horse regularly than it is to treat him for a disease he contracts because he wasn’t immunized.

Veterinarians know best which vaccines a horse should receive Include your veterinarian in any discussion of vaccines your horse should get and when. “Veterinarians are continually educated on any new or emerging diseases and will have the best idea on which particular vaccines are and aren’t necessary for your horse based on region, environmental factors, and risk of exposure to diseases,” Brown explains.

Adverse vaccine reactions can happen Many owners are concerned about their horses developing reactions, such as mild swelling and soreness at the injection site, fever, diarrhea, poor appetite, and potentially deadly anaphylactic shock. Fortunately, “although adverse vaccine reactions are possible, they are quite rare when the vaccines are given appropriately following the labeled dosage, route of administration, and using aseptic (sterile) technique,” Brown says. And remember that just because a horse has an adverse reaction doesn’t mean he shouldn’t be vaccinated. “You should consult your veterinarian to decide which vaccines have the highest benefit and lowest risk for your horse,” he adds.

Don’t fear overdoing it “Overvaccinating horses is always a concern, and that is the reason there is no one set of guidelines that should be followed for every horse or herd,” Brown says. “A vaccination program should be tailored for each individual (or herd) based on the theory of disease risk versus vaccine benefit.”

Additionally, Davis says, “we really don’t have evidence that we have disease that’s occurring because of overvaccination. (The AAEP guidelines) are based on safety and efficacy: We’re trying to protect our horses from potentially life-threatening diseases they could get sick from, but we also don’t want to do it more frequently than that because of the potential for reactions.”

Vaccinating Foals

Foal owners have the responsibility of starting a horse out right for the rest of his life, and that includes vaccination.

“In that first year of life, from an immunologic standpoint, the foal’s immune system is naive to different antigens (any foreign substance that stimulates an antibody response, whether a virus, bacterium, or parasite), and some of those may be pathogenic (disease-causing),” Davis explains. “By providing a low-dose of antigen exposure (via vaccines), specifically pathogens, the foal’s immune system will be prepared for future exposure. Low-level, low-dose antigen exposure, in the form of a vaccination, administered multiple times in that first year of life should let that individual be prepared in case they encounter that pathogen to have an effective immune response and prevent the individual from getting sick.”

The AAEP guidelines recommend foals receive all the core vaccines in a three-dose series, except for rabies, which is recommended in a two-dose series:

  • EEE/WEE, tetanus, and WNV Administer the first dose at 4 to 6 months, followed by the second dose four to six weeks later, and the third dose at 10 to 12 months of age, prior to the onset of the next EEE/WEE/WNV vector season.
  • Rabies Administer the first dose at 6 months of age, followed by the second dose four to six weeks later.

“In addition, certain risk-based vaccines such as flu, rhino, botulism, and/or others may be necessary, depending on the region, environmental factors, and risk of exposure to particular diseases,” Brown says.

The dam’s vaccination status and time of year the foal is born also affect vaccine decisions. It’s important to begin vaccinating foals born to unvaccinated mares earlier than foals born to vaccinated mares (which get maternal antibodies from colostrum). Additionally, foals born later in the foaling season (say, May or June) might require early vaccination to prime their immune systems for seasonal antigen exposure.

“If we waited to start vaccinating them until 4, 5, or 6 months of life, they would go through the summer not getting vaccinated for (and, thus, having protection against) things like EEE, WEE, and West Nile virus until relatively late in the season,” Davis says. She suggests vaccinating those foals closer to 3-4 months of age.

This intentional approach to disease prevention can help jump-start a lifetime of health for the horse, says Davis: “What we would want to do, if we have the opportunity, is to start vaccinating in the first year of life, and then keep vaccinating for the rest of their lives.”

Vaccinating Adult Horses

Simply put, “everyone should always be vaccinated,” Davis says. For previously vaccinated adult horses this means, at the very least, annual tetanus and rabies boosters, along with EEE, WEE, and WNV boosters each spring before mosquito season. Veterinarians in some areas with heavy, prolonged mosquito activity might also recommend a second EEE, WEE, and WNV booster in the fall.

Depending on the horse’s geographic location, job, and travel frequency, he will likely benefit from receiving one or more risk-based vaccines. Some horses that travel or mingle with other equids frequently might need semiannual influenza or EHV-1 vaccines, for example, while others, perhaps those residing in pastures in certain areas of the country with natural water sources, might benefit from annual Potomac horse fever inoculations. Both Brown and Davis agree owners should consult a veterinarian to determine which risk-based vaccines are appropriate for each horse.

But not all horses come with a vaccination history, and in such cases Davis recommends starting from scratch to ensure the animal has appropriate disease protection: “If someone’s getting a new horse and they don’t know the vaccine history or if they suspect that maybe they haven’t been vaccinated, then they should get started on an initial series (similar to that given to foals in the first year of life) followed by appropriate boosters,” she says.

Vaccinating Seniors

Brown and Davis agree that senior horses (those older than 20) should be vaccinated in the same manner as mature horses as far as core and risk-based vaccines. But there are some seniors owners and veterinarians will need to manage differently.

“Just like with people when they get a little older, horses’ lymphocyte (a type of white blood cell responsible for mounting immune responses) numbers decline with age,” Davis explains. This means senior horses’ bodies can’t mount as strong of an immune response when faced with a challenge—like a mosquito-borne disease—as younger animals.

Davis says researchers are looking into whether older horses should be vaccinated more frequently than adult horses to compensate for their waning immunity, but “currently we don’t have any recommendations for that sort of protocol.”

Additionally, a senior horse’s disease status could impact your vaccine strategy.

“If the senior horse has other conditions—maybe if the horse has Cushing’s disease, for instance—he may not respond to a certain vaccine in an optimal way,” Davis explains. Thus, she recommends owners work with their veterinarians to diagnose and treat any potential underlying medical conditions that could negatively affect a horse’s vaccine response.

Take-Home Message

It’s important to note that some owners might need to make special vaccine considerations for their horses—regardless of age—based on the animals’ occupations and health status.

Vaccinating horses across a span of ages doesn’t have to be a complicated process. All it takes is some communication between you and your veterinarian.

“Have the horses as healthy as possible at the time of vaccination so they have the opportunity to have the best immune response possible to that vaccine,” Davis adds, and be ready to work and compete. And remember, “It takes a week or two after vaccination to have an optimal immune response to what that vaccine contains.”


Written by:

Erica Larson, former news editor for The Horse, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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