Vaccination Basics For Horses

Vaccines are "cheap insurance" to protect your horses from more costly--and potentially deadly--diseases.

Photo: Erica Larson, News Editor

Vaccines are “cheap insurance” to protect your horses from more costly— and potentially deadly—diseases.

Vaccination used to be simple. You bought a couple of vaccines at the feed store and gave them once a year to your horse. Today there are so many vaccination options that if you still administer vaccines yourself without at least talking to your veterinarian, you might be selling yourself, and your horse, short.

"Vaccination is a complicated business," says Paul Lunn, BVSc, MS, PhD, MRCVS, Dipl. ACVIM, who served on the committee that updated the vaccination guidelines released in 2008 by the American Association of Equine Practitioners (AAEP). "There are a lot of considerations in devising vaccination strategies. You need a formula for your horse or your farm, and you will have a heck of a time coming up with that unless you work with your veterinarian."

Mary C. Scollay, DVM, who chaired the vaccination committee and is the equine medical director of the Kentucky Horse Racing Authority, agrees: "Even within the same barn, each horse is unique and has unique needs."

Experts recommend vaccinations be tailored to the horse, his situation, the time of year, what he is going to do, where he is going to do it, and who he is going to meet.

"Your vaccination program will be more effective if you do a critical analysis of how you use your horse, how your horse travels, how your horse is managed, and the environment in which the horse lives," explains Scollay. "The owner of the horse has to be a partner with the veterinarian to determine what the best vaccinations are specific to the horse's needs."

Because of this need to work together, the AAEP vaccine guidelines are available to owners and veterinarians at The guidelines are extensive and provide information about diseases, available vaccines, their safety and effectiveness, and why they are recommended, as well as how often they should be given.

The AAEP guidelines divide the vaccines into two categories: core vaccines and risk-based vaccines.

Core Vaccines

Every horse, even the backyard pet that rarely leaves the farm, should receive the core vaccines, which are tetanus, Eastern/Western equine encephalomyelitis, West Nile virus, and rabies.

Tetanus, a disease that is caused by a spore-forming bacterium called Clostridium tetani, was a serious equine problem until vaccines became available, according to Lunn, who is professor of clinical sciences at the Colorado State University College of Veterinary Medicine. The C. tetani organisms can invade the body through a puncture wound, open cut, surgical incision, or umbilical cord, or through the reproductive tract of postpartum mares. After the initial vaccination the horse should receive an annual booster, but if he sustains a wound or undergoes surgery six months or more after his last tetanus shot, he should be revaccinated immediately.

Many owners want to know why people need a tetanus shot every 10 years, while horses need one yearly. According to Lunn, the studies in horses needed to determine how long tetanus immunity lasts would be expensive. Because an effective and safe vaccine already exists, there is little chance a company would pay to conduct these studies. In Europe there is a licensed vaccine that can be given every two years.

"Tetanus vaccination is extremely important because the organism that causes tetanus is in horse manure, so horses cannot avoid it," says Julia H. Wilson, DVM, Dipl. ACVIM, associate professor of large animal medicine at the University of Minnesota College of Veterinary Medicine, who also served on the vaccination committee. "If the horse has any type of wound, it can get into the wound and can be fatal unless treated."

There are several vaccines to protect against the common equine encephalitides, including Eastern equine encephalomyelitis (EEE), Western equine encephalomyelitis (WEE), Venezuelan equine encephalomyelitis (VEE), and West Nile virus (WNV). In the United States, EEE, WEE, and WNV are core vaccines. VEE is not because the disease has not been diagnosed here in almost 40 years. The viruses are transmitted by mosquitoes, and the vaccines are normally given in the spring. In areas where mosquitoes thrive all year the vaccines are given more often.

"If you live in a high mosquito area, for instance the southeastern United States, even an annual vaccination isn't enough to keep up protection," says Lunn. "In some parts of the country people vaccinate more than once a year if they have year-round mosquitoes."

Travel is an important consideration when developing a vaccination plan for your animals because you have to consider the disease threats at the destination as well as in your hometown.

"There are diseases that are of no concern in New Jersey in the winter because there is a killing frost, but when you go to Florida for the winter you might encounter insect-borne disease because there has not been a killing frost," says Scollay. "So, the geographic location of the horse and the season that the horse is going to be in that location can affect your vaccination protocols."

EEE/WEE vaccines are given intramuscularly to at-risk horses. "The risk of exposure is pretty extensive across the country," says Scollay. "The vaccine is cheap and effective in preventing the disease. If the horse is unvaccinated, the effects of the disease can be dire if not fatal for the horse."

Since 1999, when West Nile virus was first reported in this country, there have been more than 24,000 reported cases of equine WNV encephalitis. Although cases have declined overall, in part due to vaccination, the disease is still a threat to horses, veterinarians say, and about one-third of horses that contract WNV die.

Four licensed vaccines are available. As with WEE/EEE vaccines, the WNV vaccine is generally given once a year, right before the mosquito season. However, in areas that are warm all year, you can vaccinate against these diseases more often.

Rabies is the final core vaccine. Although not common in horses, rabies does occur and can be fatal. In addition, the vaccination committee thought rabies could be a threat to people; think of all the people that come in contact with a horse at a county fair. "One rabid horse could have tremendous consequences for human health," Scollay says.

"Horse owners have never quite understood why it could be considered a core vaccine for their dogs, but not for their horses," says Wilson. "I think it is a very good move (to consider it a core vaccine) because the same vectors that might bring rabies to your dog or cat can certainly give it to your horse."

Risk-Based Vaccines

Anthrax, botulism, equine herpesvirus, equine viral arteritis, equine influenza, Potomac horse fever, rotavirus, and strangles are in the risk-based vaccine category. The need for these vaccines depends on the horse's situation and lifestyle.

Lunn explains, "We've moved to a risk-based decision-making process because there is no single way to vaccinate a horse. They live different lives; they need individual health care plans."

The recommendation for most of the risk-based vaccines is readily apparent: Most would be limited to a geographic region, right? Not necessarily. Potomac horse fever tends to occur in the mid-Atlantic region, but it also can be found in the Midwest and in Northern California; and anthrax tends to occur in the Southwest, but horses and other livestock might leave the Southwest. Finally, equine herpesvirus (EHV) and equine influenza are ubiquitous--so why aren't they core vaccines?

"If you have a horse that travels, mingles with other horses, if you go to competitions or trail ride with people whose horses are stabled in other barns, you should consider equine herpes or influenza vaccination," says Scollay. "The two retired show horses that live on my farm don't travel and don't come into contact with any other horses. It's a 'closed' herd, so we don't currently vaccinate for EHV-1 and influenza.

"But I think the majority of people who own horses should consider these vaccines because if you are in a boarding stable or your horses mingle with horses outside the barn population, there is a risk of exposure and a risk they could bring the viruses back to the barn," she adds.

Botulism and rotavirus are more of a concern in foals than in older horses, and usually these vaccines are given to the pregnant mare to protect her young through passive transfer of antibodies. In the case of rotavirus, the decision to vaccinate might be related to previous outbreaks of rotaviral diarrhea on a property.

"This is definitely one that you need to discuss with your veterinarian," says Scollay. "If you send one mare out to be bred at a farm that has had rotaviral diarrhea, it may affect how you treat your own farm, even if you have not had diarrhea on your farm. It requires vigilance and constant reassessment because your needs change as your situation changes."

While often a subclinical problem, equine viral arteritis (EVA) can cause abortion in pregnant mares and death in neonates. In addition, breeding stallions can become carriers, which would be a concern to any breeding establishment.

"Outbreaks are believed to be uncommon in this country," says Lunn. "The most serious implication is abortion. EVA is one of those diseases that may not cause major problems for lengthy periods, but when we do have an outbreak, it can have serious economic and health implications. We don't routinely suggest you vaccinate except in stallions, because intact stallions can become carriers."

Strangles is a very contagious disease, and as with EVA a horse can be a carrier. However, the reaction rate of vaccination is a little higher for this vaccine than for some of the others; therefore, it tends to be used in response to an outbreak or on endemic farms.

"My horses have had annual strangles vaccination for some times and not others, depending on whether they were going anywhere or being boarded," says Wilson.

"Plenty of horses are vaccinated as a result of an outbreak in the area," says Scollay.

Strangles bacterial transmission requires direct contact with the horse's nasal secretions. Besides horse-to-horse transmission, bacteria can be spread by people who don't disinfect their hands or change their clothing after handling horses that have strangles, on equipment such as buckets, and via tack used on multiple horses.

"In evaluating whether or not to vaccinate for strangles, not only do you have to evaluate the likelihood of horse-to-horse contact, but you have to evaluate the personnel who come in contact with the horse," Scollay says. "What are their practices and protocols? What are the risks of disease transmission by those individuals?"

Vaccination Risks

Vaccines have decreased the incidence of infectious diseases among horses, but "there are always risks, which is why you should discuss vaccinations with your veterinarian," Lunn says. "Most are local, which is mild soreness and swelling at the site of the injection. They can be systemic, and that can be anything from a low-grade fever and being off feed for a bit to severe reaction that could lead to anaphylactic reaction, which happens very infrequently."

Take-Home Message

Experts call vaccination "cheap insurance" because prevention is cheaper than treating disease. And luckily for horses, many new and innovative products have come on the market that have helped decrease the incidence of many infectious diseases. Vaccines are one tool that, along with a good biosecurity plan for your farm, will protect your animals.

About the Author

Marie Rosenthal, MS

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