What Vaccines Does My Horse Really Need? The Basics of Equine Infectious Disease & Prevention

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Infectious diseases are important causes of equine suffering and mortality. You have a vital role in preventing these diseases in your horses. Vaccination is ONE important tool we have to reduce these diseases in our horses. But it’s not as simple as going to the feed store and buying a guarantee that your horse won’t get sick. To be the best equine caretaker, you need to understand a little about the diseases, their causes and prevention.


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Bacterial and viral organisms are the common causes of infectious disease in horses. These organisms, called “infectious agents”, are transmitted in many different ways (depending on the agent) from direct contact between horses, ingestion, inoculation into wounds,  transmission by insects, and many more. Whether or not a horse becomes infected in a given situation depends on a balance of three factors- Host (Horse), Agent (Bug) and Environment.   When I was in vet school, I found this concept, called the Infectious Disease Triad, really helped me to understand infectious disease, and it should help you understand it too.
  • Host (Horse) Factors – The horse’s susceptibility to the disease: A combination of genetics, health, general immunity and immunity to the specific disease in question (altered by vaccination). Good general health is critical to a functioning immune system and reduced susceptibility.
  • • Agent – The characteristics of the infectious agent: This includes how potent and contagious the infectious agent is, how high a dose of the agent contacts the horse, and the mode by which this dose is presented (swallowed, inhaled into the lungs, etc..)
  • Environment– Stresses on the horse, quality of feed, exposure to unvaccinated horses, feed quality, etc.
  • A useful way to think of infectious disease and whether a horse will become sick.

    A useful way to think of infectious disease and whether a horse will become sick

    A useful way to think of infectious disease and whether a horse will become sick. You can see from this triad that whether or not a horse gets sick depends on  more than just vaccination. Keeping horses disease-free requires excellent general health, maintenance of a favorable environment to maintain health and reduce exposure to disease agents, AND keeping up  immunity to key diseases through appropriate vaccination.


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Vaccines have historically been created by preparation of all or part of a particular infectious agent, either killed, or inactivated and alive, mixed with a delivery liquid called an adjuvant. The agent (or part of an agent) in a vaccine agent stimulates the immune system to produce antibodies against the disease.Vaccines on counter
In some cases, vaccination may only reduce the severity of disease, not prevent it entirely.
Antibodies are infection fighting proteins which bind to a specific agent and help destroy it. Some new vaccines take advantage of advances in our understanding of the immune system and genetic engineering. These products may contain only a small, key part of the organism or its genetic code, but still promote excellent immunity. They tend to be more effective and safer than older vaccines. New delivery approaches (like intra-nasal administration) tend to target key organ systems more directly and so may produce better immunity.
Vaccines are not all created equal and are constantly being improved. Your equine veterinarian should be current on new vaccine development and can help you select the best vaccine for your needs.


The American Association of Equine Practitioners (AAEP) regularly publishes updates and guidelines for horse owners and veterinarians regarding recommended vaccines.  Generally, I suggest following the AAEP’s recommendations, but every horse is different and your vet can help customize your vaccination program to your situation.
The AAEP lists 4 vaccines as Core Vaccines for horses. The American Veterinary Medical Association (AVMA) defines core vaccinations as those “that protect from 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. 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 the majority of patients.”
If you vaccinate for nothing else, vaccinate your horses for these diseases:


West Nile Virus has been a problem for the horse industry since the first cases in the U.S. on the East Coast in 1999. It spread rapidly westward over the next few years, killing large numbers of birds, horses and some people. Since then, West Nile infection has become less common, but it is now considered endemic (established permanently) throughout the United States, Canada and Mexico. This disease is transmitted by a mosquito that has fed on an infected bird, and is not transmitted directly from horse to horse.
Recommendation: West Nile is a core vaccine strongly recommended by the AAEP. All horses in the U.S. should be vaccinated for West Nile Virus once annually. This is generally done in the spring, before mosquito season. There are multiple vaccines available, which rely on different technology in their formulation. All are considered effective if used annually. Your veterinarian will have an opinion on which product is best for your situation.


Symptoms of Eastern and Western Encephalomyelitis (a/k/a sleeping sickness) are similar to those caused by West Nile Virus and the virus itself is similar. These important mosquito-transmitted diseases have been overshadowed by West Nile Virus in recent years, causing some horsemen not to vaccinate for them. Like West Nile, these diseases fluctuate on a cycle dependent on immunity in bird populations, and many other factors. In the summer of 2010, there was an epidemic of Eastern Equine Encephalitis in New York and Ohio that killed many horses. The severity and extent of the outbreak may partly have been caused by the poor economy. Financially stressed horse owners cut expenses by reducing vaccination, resulting in more susceptible horses.
Recommendation: Encephalitis is a core vaccine strongly recommended by the AAEP. The vaccine is safe, effective and inexpensive. It is usually combined with West Nile Virus and Tetanus. Recommendation is annual vaccination, timed before mosquito season in the spring.


Tetanus is a severe paralytic disease caused by a toxin produced by the bacteria Clostridium tetani. This organism is very common in the environment, is present on many surfaces, and becomes a potentially fatal problem when it is introduced into wounds. It is not infectious from horse to horse.
Recommendation: Tetanus is a core vaccine strongly recommended by the AAEP. This vaccine is highly effective, safe, and inexpensive, and should be given to every horse each year. It is often combined with encephalitis and West Nile vaccines and so is often given at the same time, in the spring. While immunity to tetanus probably persists longer than one year after vaccination, I still recommend vaccinating annually. Until there is more convincing scientific evidence that equine immunity lasts longer than a year, it makes more sense to be safe, rather than sorry.


Rabies is a rare but fatal disease in horses that is transmitted by the bite of an infected animal (bats and skunks) in the southwestern US, (but foxes, raccoons and other wildlife in other regions).
Recommendation:  Because of the public health risk associated with rabies, it is another of the CORE vaccines listed by the AAEP,  and the current recommendation is to administer the vaccine annually. The vaccine is relatively safe, effective and inexpensive but should ALWAYS be administered by a veterinarian.
I wrote a separate article about when to use the “optional” vaccines- Deciding When to Use “Risk-Based” Vaccines – but here are the basics.


Equine Influenza (flu) virus is fairly common in horses. It causes high fever, cough and nasal discharge. It is highly contagious, spreading rapidly through a group of horses. Virus is expelled by coughing horses and can infect horses up to 30 feet away. As with all flu viruses, the equine influenza virus is constantly changing, making it difficult to make a vaccine which contains the most current version of the virus.
Recommendation: If you are going to use flu vaccine, you need to use it at least twice annually for at-risk horses. The vaccine gives protection for up to 6 months, probably not more. Horses in contact with others (racehorses, show and performance horses, horses boarded at barns) are at risk for the disease and should be vaccinated frequently. Vaccination may not be needed for isolated horses. Vaccines containing more current strains have been shown to produce better immunity to more currently relevant versions of the virus.


Equine Herpes Virus (a/k/a rhino or rhinopneumonitis) are several related viruses that can cause respiratory signs, neurologic signs, or abortion in horses. In the past 15 years, there have been significant outbreaks of EHM, (Equine Herpes Myeloencephalopathy), the neurologic form of disease, mostly at facilities with large concentrations of horses. Many horses have died or been euthanized in these outbreaks.  One unique characteristic of most equine Herpesviruses is that they can lay dormant in infected horses and then can causes signs of disease when immunity decreases because of stress or other factors. Unfortunately, it is unlikely that vaccination with our current vaccines reduces the likelihood of the neurologic form of the disease (EHM). What it may do is generally decrease the spread of EHV in the population and so reduce exposure.
Recommendation: We recommend frequent (at least twice a year) vaccination using specific, newer generation vaccines, based on the specific situation and risk. Pregnant mares should be vaccinated as directed with a vaccine licensed to protect against the abortive form of this disease. The benefit of giving rhino vaccine may be less for older and more isolated horses.


“Strangles” (a/k/a Distemper) is a respiratory disease caused by an abscess forming bacteria, Streptococcus equi. The classic sign of strangles is severe swelling and abscess formation around the throat and under the jaw. The name “Strangles” comes from these often large swellings, which in their most severe form can cause difficulty in swallowing and breathing.
The abscesses typically break open and drain yellow pus. Horses with Strangles also often have thick Nasal discharge sketchyellow nasal discharge, fever and cough.  The disease is common and is highly contagious to at-risk horses. It is easily transmitted by contact with secretions and drainage containing the organism. It is also carried on the hands of handlers and on shared tack and equipment and through shared water.
Vaccination has proven difficult for this disease.  The problem with all of the vaccines has tended to be relatively high adverse reaction rate and limited effectiveness. There is a modified-live intranasal strangles vaccine considered the most effective.  This vaccine can cause a form of strangles in a very small percentage of vaccinated horses., but when it occurs, it is a serious problem!  This vaccine is also dangerous because it can be inoculated into and cause abscesses in the tissues at other vaccination sites.
Recommendation: We currently recommend the use of the intranasal vaccine 1-2 times per year, only for horses in which the risk of strangles infection outweighs the risk of side effects from the vaccine. It’s pretty well accepted that the vaccine does help reduce likelihood of infection and severity of disease in a population of horses.
A critical part of reducing strangles incidence is management of horses to reduce strangles exposure (quarantine before introduction of new horses into a herd) and immediate quarantine of any affected horses. Your veterinarian can help you decide the need for this vaccine and to administer it for you if needed. I believe that only a vet should administer the intra-nasal vaccine.


Equine Viral Arteritis (EVA) is a viral respiratory disease that is transmitted either through venereal means (through breeding) or through transmission from coughing horses. It is carried by infected stallions, which can infect mares through their semen. this is an important disease for breeders to be aware of.
Recommendation: Commercial breeders should familiarize themselves with this disease and their state regulations related to it. This may include annual vaccination of breeding stallions.


There are common diseases for which we do not have a working vaccine. An example of this is Dryland Distemper (a/k/a “Pigeon Breast” or “Pigeon Fever”). This disease is common in the Southwest USA, and most commonly presents as large abscesses on the chest, belly or sheath in male horses.  The bacteria is spread by flies. Since there is no commercial vaccine, control measures for this disease are limited to management, and this centers mostly on good fly control.
There are other, less common infectious diseases that affect horses in the United States for which there may be vaccines available, but the cost/ benefit of their use may be questionable. An example of this is Potomac Horse Fever, a significant disease in the northern USA. This disease can cause life-threatening diarrhea in horses. It is a difficult disease to diagnose  definitively. The vaccine is of questionable effectiveness. For these reasons we do not recommend routine vaccination for this disease in our area. However, in the Northeast where the disease is more common, the cost/benefit is different and vaccination is probably a good idea.


Specific protocols should be followed for pregnant mares and foals. They have special vaccination requirements that I am not going to discuss here. Talk to your vet about the protocol that makes the most sense for your situation.


In the world of infectious diseases, the only constant is change.  Diseases wax, wane, and evolve over time in response to poorly understood and complex biologic pressures. The effort to prevent disease in our animals also adapts as we learn more about the diseases and develop new vaccines and control methods. Vaccine types and availability change frequently as well. For that reason,  I update this article every few years to keep it current. See also Deciding When to Use “Risk-Based” Vaccines that helps you decide what vaccines you need to use, when.


There is currently debate in the horse community (and beyond) over how much vaccination is necessary. There is some suggestion that “holistic means” are sufficient to prevent infectious disease and that vaccination is not necessary. But current research says you should still vaccinate as part of basic horse health care.
Ask your veterinarian before you rely exclusively on alternative approaches. While we cannot rule out that some holistic or natural agents may be helpful, there is very little known at this time about their true effects. The best policy for now is not to take a chance.
We are lucky to have highly effective and safe vaccines which have been proven to safely reduce your horse’s chances of succumbing to serious disease. Let your veterinarian help meld your own philosophies on equine health care with the latest research.


Anyone can give a vaccine if they have learned the proper technique involved. While many vaccines are now available over-the-counter, there are several key reasons to continue to involve your equine veterinarian in vaccinating and examining your horses;
  • • Your veterinarian stays educated on new vaccine development and so can provide the most effective, current and safest vaccines.
  • • They administer these vaccines correctly and on a proper schedule, and they maintains complete vaccination records.
  • • As of the time of this writing, many vaccine manufacturers will stand behind their vaccines. If after vaccination a horse becomes infected by a disease that the vaccine was intended to prevent, some manufacturers will pay for part or all of the veterinary treatment required. However, in order to receive such compensation, many of these companies require that their product be administered by a licensed veterinarian.
  • • Keep in mind that your veterinarian may actually save you money. They will discourage you from over-vaccinating your horses or using inappropriate or unneeded vaccines.
Most importantly, I believe vaccination of horses should be combined with veterinary consultation and examination of the horse. For me, this is an opportunity to talk to my client and examine the horse, and to get to know horses in health rather than in crisis. I enjoy discussing goals and management of my clients’ horses as it relates to their health care, and I feel that the time spent is of real value to horses and their owners.



By Doug Thal, DVM DABVP. Certified in Equine Vet Practice, Creator of Horse Side Vet Guide
Dr. Thal is an equine veterinarian who has been in practice for 24 years. Over that time, he has worked to help horse people better understand their horses physically and mentally, and better understand how to get the most from their equine vet. To do this, he created a Smartphone app for horse people called Horse Side Vet Guide. It’s the most highly rated equine health app in the world and a GREAT tool to help you care for your horses. You should have it on your phone! Learn more here!
Horse Side Vet Guide is the most highly rated equine health app - get it today!

Horse Side Vet Guide is the most highly rated equine health app – get it today! Available on iTunes & Google Play

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