Immunizations are the cornerstone of disease prevention if given appropriately and in a timely manner.

One of the brightest spots of equine medical care for horses is the availability of many safe and effective vaccines to protect horses from infectious and noninfectious diseases. Sometimes it is confusing as to which ones your horse might need. Let's look at the vaccine options, the necessity of each, and how to plan for boosters throughout the year. Always keep in mind that your veterinarian is the best person to advise you on the vaccinations needed for your particular horses and your area.

Planning a Vaccination Strategy

Mary Scollay, DVM, previous chair of the Infectious Disease Committee of the American Association of Equine Practitioners (AAEP), urges, "I would like to reinforce that the development of an effective vaccination program requires a partnership between the veterinarian and the horse owner. A horse owner has an obligation to provide input when a vaccination program is being developed. It is not a passive process. Examples of information that should be shared include how the horse is used, whether it travels, and what other animals is it likely to come in contact with."

Vaccines are only one aspect of preventing disease; it is just as important to implement good horsekeeping and biosecurity strategies (see page 31 for more information) to minimize disease risk.

The AAEP developed a useful vaccination protocol that can be accessed at

Core Vaccines

Spring heralds more riding and transport, so schedule your horse's annual spring veterinary checkup. (For more information see article #11398 at Your veterinarian will administer "core vaccines," which are those considered important for every horse to have annually, regardless of geographical location or athletic use.

The AAEP and the American Veterinary Medical Association (AVMA) describe core vaccines as those "that protect from diseases that are endemic (prevalent with a high rate of occurrence) to a region, those with potential public health significance, required by law, virulent or highly infectious, and/or those posing a risk of severe disease." These include: tetanus, West Nile virus (WNV), Eastern and Western encephalomyelitis (EEE and WEE), and rabies.

TETANUS Horses spend a lot of time around dirt/manure, so they are at particular risk for contamination of even the smallest wound with Clostridium tetani spores. An annual booster of tetanus toxoid in the spring is recommended.

Debra Sellon, DVM, PhD, Dipl. ACVIM, a professor of equine medicine at Washington State University, suggests, "The tetanus toxoid is inexpensive and safe, and the disease is highly fatal. Therefore, I always recommend a booster tetanus toxoid injection in horses with wounds or with plans to undergo surgery if it has been more than six months since that horse received its last booster injection."

WEST NILE VIRUS This disease, which causes potentially fatal neurologic illness and is endemic in the lower 48 states, is carried by birds and transmitted by mosquitoes to horses. Luckily, there are three vaccines against WNV--all are safe and have demonstrated good efficacy. Horses should receive an annual booster following the initial vaccine series.

Keep in mind the timing of your annual boosters, making sure the horse is protected during mosquito season. In warmer climes where mosquitoes abound year-round, it might be necessary to administer boosters twice a year, depending on the vaccine product. Scollay says if your veterinarian recommends vaccinating twice yearly, "it might be prudent to consider vaccinating say, April 1 and Aug. 1, to enhance immunity during the period of high risk for exposure, instead of at a rigid six-month interval."

EASTERN AND WESTERN EQUINE ENCEPHALOMYELITIS Encephalomyelitis (or encephalitis) virus, which causes neurologic disease, also is carried by birds and transmitted by mosquitoes. The vaccination strategy for EEE and WEE is comparable to WNV--once or twice annual boosters, depending on length of mosquito season, following an initial priming series. Horses living in states directly bordering Mexico might also receive an annual booster for Venezuelan equine encephalomyelitis (VEE).

RABIES Scollay says, "Rabies is a fatal neurologic disease of warm-blooded animals; that means horses and humans." Wild animals such as bats, skunks, foxes, or raccoons can bite a horse and pass this virus without anyone being aware. Given that humans are constantly inserting their hands into horses' mouths when placing a bit, checking age, floating teeth, or administering dewormers and paste medications, Scollay asks, "Why would you risk contracting a fatal disease from routine contact with a horse, especially when the disease can be effectively prevented?"

Rabies vaccine is labeled to be given once a year, but Scollay says if you have concerns about a specific horse's immunity, it would be appropriate to consider a series of two vaccinations.

Risk-Based Vaccines

Vaccines against certain diseases are given based on anticipated degree of risk.

INFLUENZA VIRUS Horses that travel or encounter horses that have been traveling are at an increased risk of exposure to equine influenza virus. The 2007 Australian epidemic, in which thousands of unvaccinated horses were exposed to flu, displayed how readily the disease could spread. There are many effective equine flu vaccines, and a horse should receive two or more boosters a year (depending on which product is used), usually in the spring and fall, following the initial series of three injections and/or intranasal (IN) administration of certain products.

HERPESVIRUS OR RHINOPNEUMONITIS Equine herpesvirus (EHV-1 and EHV-4) can cause respiratory problems (this disease expression is known as rhinopneumonitis). Rhino is spread through respiratory secretions--on shared objects or airborne. Cough, runny nose, or fever can be readily apparent, but EHV can be latent (hidden) in the horse, meaning it sits in the lymphatic tissue without producing any proteins and, therefore, the horse does not "respond" to it. Stressors such as transport, weaning, castration, mixing of horses, or foaling can reactivate the virus, which the asymptomatic horse sheds in respiratory secretions. EHV-4 causes mostly respiratory disease, whereas EHV-1 can cause respiratory disease, abortion, or neurologic disease.

Scollay says research has shown EHV vaccination programs help reduce clinical disease and the period of viral shedding in adult horses. "It is reasonable to assume that many of the horses that experience these benefits were initially infected as foals," she notes. "This is an important 'herd health' concept--that by minimizing clinical disease and viral shedding in horses that respond well to vaccination, you are also providing increased protection to horses in the same population that did not, for whatever reason, develop a good immune response to a vaccination."

Vaccination can prevent the return of disease, suppress virus so it remains latent, and stop shedding in nasal secretions, limiting transmission to naïve horses.

STRANGLES Available vaccines do not protect entirely against Streptococcus equi-caused disease, and there are controversies surrounding its use in some animals. Sellon weighs in: "Strangles IN vaccine is used in horses at risk of exposure to strangles. The exception to this is horses with very high previous, especially if recent, exposure to strangles. Most experts agree that vaccination of horses with either IM (intramuscular) or IN vaccines, if they have a pre-existing high titer to the bacterial organism, is associated with an increased risk of adverse effects. I recommend IM strangles vaccine for broodmares in the last 30 to 60 days of gestation if they or their foals are at risk of exposure.

All strangles vaccines have been associated with immune-mediated reactions, such as vasculitis (inflammation of blood vessels) and myositis (inflammation of muscles). The IN vaccine, which is (made with) modified-live bacteria, may cause abscesses, rarely. The IM vaccine often causes soreness, swelling, or potential abscesses at the vaccination site. Base your decision to vaccinate for strangles on assessment of the potential risks (farm history, lots of horse traffic on and off farm) and benefits.

Consider diagnostic testing to determine if the horse is harboring S. equi before vaccinating. (For more information see article #10688 at Following a primary series, veterinarians administer strangles vaccines once or twice annually in high-risk areas.

EQUINE VIRAL ARTERITIS This disease is encountered most commonly in the semen of an infected carrier stallion, yet it can be passed from horse to horse in respiratory secretions. Most times this vaccine is used to protect breeding stallions, mares with planned breeding to a known infected stallion, and nonbreeding horses in the event of an outbreak. Pregnant mares should not receive the EVA vaccine. Before vaccinating, you can screen a horse for previous exposure to EVA with a blood test. Vaccinating for EVA might also preclude a horse's entry into some countries, as it is difficult to determine natural versus vaccine titers. (For more information see article #10215 at

POTOMAC HORSE FEVER (PHF) This is a diarrheal disease (and occasional cause of abortion) caused by the organism Neorickettsia risticii. Its effect on horses follows a seasonal pattern, usually between late spring and the fall during hot weather (vaccinate prior to insect hatching and warm weather). Horses are infected by ingesting infected insects derived from aquatic environments. Current vaccines do not have challenge information based on this natural route of infection, but, instead, are based on a transmission method that was suspected and now has been disproven (ticks, so tests to determine vaccine efficacy were done with blood challenge).

ANTHRAX This is a fatal disease caused by Bacillus anthracis, occurring in specific geographical locations where the spores remain in the soil for decades. This vaccine is usually only administered to pastured horses in high-risk areas. A primary series is followed by an annual booster.

BOTULISM This fatal neurotoxic disease disease is caused by Clostridium botulinum. A vaccine is available for C. botulinum type B, which is particularly useful to protect foals against shaker foal syndrome that have acquired botulism through ingestion of the spores.

Sellon comments, "Botulism should be included in broodmare vaccinations if the horses reside or will travel to areas where type B botulism is known to occur. This generally means Kentucky and the mid- Atlantic region of the eastern United States."

Pregnant mares in high-risk areas should be receive a primary series at least four to six weeks prior to foaling to ensure transfer in colostral antibodies for the foal. Foals should also receive this vaccine series in high-risk areas. Vaccinate adult horses in these areas based on a veterinarian's recommendation.

ROTAVIRUS In the case of the diarrheal disease rotavirus, vaccinate the mare to protect the foal, especially if there have been previous problems with this disease on the farm or in the area. Sellon counsels, "All breeding operations, large and small, should have in place reasonable biosecurity plans to decrease the chance of accidental introduction of the disease on the premises. Vaccination for rotavirus should never be considered as a replacement for this type of husbandry."

Administer a three-vaccine series to a pregnant mare by the last month prior to foaling. This way a foal receives colostral antibodies that provide resistance to rotavirus for the first 30 to 60 days. This vaccine is not necessary for other adult horses.

Foal Vaccine Program

A foal's first-year immunizations begin as a series of two to three injections (depending on the product), followed by boosters once or twice a year. Most foals are born in the spring and will not receive EEE, WEE, WNV, and tetanus immunizations until 4 or 5 months of age or later. Don't start flu and rhino until 6 to 9 months of age, depending on the mare's vaccination history.

Scollay explains that foal vaccination timing is based on maternal antibody interference. "While the antibodies in the mare's colostrum provide a foal with early protection against infectious diseases," she says, "those same antibodies can also inhibit the foal's own immune system from 'learning' from a vaccine and developing its own immunity to disease. If the mare was vaccinated late in pregnancy, the foal's vaccinations should begin later than if the mare was not vaccinated late in pregnancy. If you don't know the mare's vaccination status, you must assume she was both vaccinated and unvaccinated."

How Many Vaccines At Once?

Many horses can receive multiple vaccines at one time and have no adverse reactions, particularly if using separate injections rather than multivalent products, but not all horses fare well in this scenario.

Scollay gives the vaccinations in two sets, 10 to 14 days apart. She says, "I don't know if this benefits in terms of developing better immunity, but I do think the horses are more comfortable with less localized muscle soreness and general 'punkishness.' "

Determine how your horse fares with individual vaccine products, then try to minimize future adverse reactions.

Take-Home Message

Ensure that your horse receives his core vaccines annually, along with any other vaccines against diseases for which there is a high risk in your area, and make sure he gets his boosters. Collaborate with your veterinarian to tailor the best strategy for your horse, based on exposure and risk.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her recent book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care (available at or by calling 800/582-5604). She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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