Vaccination Schedules for Adult Horses

As desirable as it would be to have a national (or even regional) one-size-fits-all protocol for vaccinating adult horses, vaccination recommendations are best tailored to individual circumstances. These primarily include the areas of the country the horse lives in or travels to (the specific disease risks that abide in said area) and whether a horse is exposed to transient populations.

“Some vaccines are given based on specific geographical factors, a good example of which is the widespread advocacy for vaccination against West Nile encephalitis for horses in the USA during the past few years,” explains Philip J. Johnson, BVSc (Hons), MS, Dipl. ACVIM, MRCVS, professor of internal medicine in the Equine Medicine and Surgery department at the University of Missouri, Columbia.

When West Nile virus (WNV) first hit the United States, it was considered to be a regional problem, thus after the development of the WNV vaccine, recommendations were to vaccinate horses only in endemic areas. Since then, WNV has spread throughout North America, as have recommendations for protective measures against the disease.

Sometimes unique local conditions dictate which vaccinations are added to a protocol. “Horses in Kentucky tend to be vaccinated against botulism to a far greater extent than horses in many other parts of the country,” Johnson continues, “because botulism has been recognized as a common and important condition in parts of Kentucky. Moreover, there exists a vaccine for type B botulism—the type that tends to occur in that part of the country.”

Botulism strikes quickly, mortality is high in untreated adults, foals can become extremely ill, and treatment is expensive. This means if you are shipping a mare to Kentucky to be bred, you should have your veterinarian vaccinate her prior to shipping. The same is true for endemic diseases in other parts of the country.

Local climate influences not only what you vaccinate against, but when. “In those parts of the USA for which there exists a ‘vector’ (mosquito) season, vaccines against the mosquito-borne encephalitis viruses tend to be given a month or so before the insects are anticipated,” states Johnson. “This seasonal approach to ‘strategic’ vaccination is important for Eastern equine encephalitis (EEE), Western equine enceph-alitis (WEE), and West Nile encephalitis. For horses living in parts of the country (such as Florida) in which the mosquito-borne encephalitis viruses are a risk year-round, the vaccines against these diseases are given more frequently.”

These encephalitic diseases are difficult to treat, can leave neurologic deficits in survivors, and, in the case of EEE, carry a high mortality rate.

Besides geographic recommendations based on risk, protocols should consider individual situations. Pregnant mares, for example, are routinely vaccinated against equine herpesvirus type 1 (EHV-1) to reduce the risk of abortion.

Another example is that stay-at-home horses isolated from transient populations are at less risk for strangles, influenza, and rhinopneumonitis, so there would not be as much indication for protection against those diseases. In contrast, horses that show or stay-at-home horses exposed to other horses that come and go are more likely candidates for rhino, flu, and strangles inoculations.


Your veterinarian is the best resource for helping determine which diseases you should vaccinate against, how often, and at what time of the year.
Nevertheless, there are some diseases for which vaccination is routinely recommend-ed, regardless of geographic location, Johnson states. “Perhaps the most important one of these is tetanus—a globally important disease for horses which, as a species, are particularly prone!”

Tetanus carries a high mortality rate, and treatment is costly and complicated.

“The tetanus toxoid vaccine is safe, afford-able, and very effective; tetanus is rarely seen these days because horses tend to be well-protected by this vaccine,” Johnson adds.

Rebecca S. McConnico, DVM, PhD, Dipl. ACVIM, associate professor of equine medicine in the Equine Health Studies Program at Louisiana State University, adds, “Tetanus toxoid should be given at the same time as the other vaccinations and is usually provided commercially in combination with the flu or encephalitis vaccine.”

Vaccinations against the encephalitis viruses—EEE, WEE, and WNV—are also widely recommended. But for these mosquito-vectored diseases, timing varies.

“In the Southeast states, horses should be vaccinated at least twice a year for encephalitis viruses; timing should be one or two months prior to peak periods of exposure (early spring and mid- to late summer),” says McConnico. “In states with four distinct seasons (several weeks or months of below-freezing temperatures), vaccination against encephalitis viruses should occur three to four weeks prior to mosquito season.”

Rabies is a relatively uncommon disease, but it is always fatal, thus it often makes the must-have list, decidedly so in endemic areas. Timing doesn’t matter; it’s usually administered annually along with other vaccinations.
McConnico says that equine protozoal myeloencephalitis (EPM), a potentially debilitating, difficult-to-treat, and sometimes fatal disease, is another infection that some people consider worth protecting against.

Individual circumstances tend to dictate whether other vaccines should be considered. These include:
Travel—“Horses that travel or are exposed to any new horses should be vaccinated for respiratory tract diseases, especially equine influenza and equine herpesvirus 1 and 4 (rhino),” McCon-nico says. While flu has a high morbidity (illness) rate and can keep a horse out of competition and training for several weeks, it has a low mortality rate. Equine herpesvirus type 1 can cause respiratory disease, abortion, or fetal damage (and rarely neurologic disease); type 4 can cause respiratory disease (and rarely weak foals and abortion), says Johnson.

New data suggests that the intranasal flu vaccine might give a better immune stimulus than most of the intramuscular preparations, Johnson says.

“Vaccination against respiratory diseases should be given at least three to four weeks prior to travel/show/race activities,” McCon-nico advises. “Horses traveling to other states, regions, or internationally may need to be vaccinated for additional diseases. Own-ers should contact their local or state veterinarians for specific requirements or recommendations (for ex-ample, horses living in or traveling to states bordering Mexico should be vaccinated for Venezuelan equine encephalitis).”

Pregnant mares—Pregnant mares should be vaccinated during the fifth, seventh, and ninth months of pregnancy with equine herpesvirus type 1 vaccine to protect against abortion, McConnico says. “Broodmares should have booster vaccinations for tetanus, the encephalitis viruses, and equine influenza about one month prior to foaling to ensure adequate antibody levels in the mare’s colostrum.”

Johnson states that influenza vaccines should not include type 1 influenza. “The current recommendations by the Influenza

International Surveillance Panel from April 2005 recommend the current vaccines contain updated strains (A/eq/South Africa/4/03 or A/eq/Ohio/03, and A/eq/Newmarket/2/93, A/eq/Suffolk/89, or A/eq/Borlange/91),” says Johnson.
For more information on vaccinating broodmares, see “Crossing Boundaries” in the October issue,

Stallions—Vaccinating breeding stallions against equine viral artertis is a good idea, Johnson says. Vaccinating open mares before breeding to positive stallions might be recommended; discuss options with your veterinarian.

Maybe, Maybe Not

Both the intranasal and intramuscular strangles vaccines (for protection against Streptococcus equi) can cause concern among veterinarians. “Although effective in stimulating a protective immune response, this vaccine carries the possibility of complications including fever, anorexia, muscle pain, and abscess formation,” McConnico reports. The vaccine does not offer long-lasting protection, she says. While it might reduce severity of clinical signs, its efficacy isn’t stellar. She says some veterinarians prefer to let the disease strike, run its course (under veterinary supervision), and produce natural immunities.

“The strangles vaccination is not usually necessary for horses older than three who are not exposed to young horses,” says McConnico. “The intranasal strangles vaccine must be administered by a veterinarian after all other injections have been administered (it’s recommended to give it on a separate day).”
Adds Johnson, “There’s not much argu-ment that both the intranasal and injectable vaccines have some risks—the intramuscular vaccine can cause muscle abscesses if it is either inappropriately or inadvertently injected (usually by owners), or the intranasal can cause problems if the contents of the vaccine accidentally contaminate the hands of the person giving subsequent intramuscular injections.”

The AAEP Guidelines for Vaccination of Horses lists strangles as “optional,” suggest-ing semi-annual vaccinations for adult hors-es when endemic conditions exist or the risk is high.

There are vaccines against Potomac horse fever (PHF) and various endotoxins, but recommendations for their use varies, in part because there aren’t yet a lot of cases to study.

Do It Right

When it’s time for your horse to get its vaccinations, it’s best to let your veterinarian administer the vaccines. Explains Johnson, “There is the very rare risk of an al-lergic reaction; if a horse does develop a reaction to the vaccine given by a horse owner, the horse might die from the reaction.” A veterinarian at the scene is better equipped to deal with any adverse reactions.

Be sure to vaccinate the whole herd, not just select horses. “If you just immunize one horse in a group,” says Johnson, “the other horses that are not immunized could generate the virus as they get sick to the extent that the virus will overcome the effects of the vaccine in the protected horse. Vaccines represent a part of an overall disease prevention program; broad use of vaccines is intended to decrease disease incidence in a population as a whole (owners are often under the impression that the vaccine is mainly for the individual).”

Take-Home Message

Vaccines are easily obtained today by horse owners, but they shouldn’t be taken lightly. They’re still pharmaceuticals that can cause unwanted reactions if handled unwisely. Also, the need for protection against various diseases dif-fers among locales, and even between farms in the same location. Discuss your vaccination needs with your veterinarian.

Questions Remain

Despite a long history of vaccination use, questions and concerns still remain regarding the efficacy and safety of these products.

Says Philip J. Johnson, BVSc (Hons), MS, Dipl. ACVIM, MRCVS, professor of internal medicine in the department of Equine Medicine and Surgery at the Univesity of Missouri, Columbia, “Many horse owners may be surprised to know that vaccine manufacturers do not have to show that the vaccine product is safe, contains substances that are supposed to be in a vaccine, and has a logical basis (that the vaccine can stimulate antibody production, for example, which does not mean it will stop the disease).”

Probably even less known, although vaccine manufacturers are supposed to be tailoring influenza vaccines based on the current/circulating influenza viral strains, is that many influenza vaccine products for horses continue to incorporate the type-1 strain of the virus that hasn’t been reported worldwide since the early 1980s, Johnson reports., “I am curious as to why they persist with this policy,” he says.

There is also a lot of current interest, Johnson says, in whether or not vaccination should be recommended for protection against equine herpesvirus type 1 (EHV-1) encephalomyelopathy (the neurological form of herpesvirus infection).

Johnson states that there’s a “fair bit of discussion about the fact that veterinarians might be over-vaccinating animals. I am uncertain that this is the case for the important horse diseases such as tetanus. However, older, solitary horses that do not travel so often or mix with younger horses probably do not need to be vaccinated as often against influenza, etc.”

In contrast, under-vaccinating could be an issue in some situations. “Many veterinarians administer annual shots as a routine,” Johnson says. “Probably for younger horses, there might be value in administering influenza vaccine more often (every three to four months); I would make a similar comment for EHV-1 when it’s being used to prevent the respiratory form of the disease (rhinopneumonitis). However, I do not think that once-a-year shots against EHV-1 are likely very effective.”—Marcia King

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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