By Kevin Haussler, DVM, DC, PhD, DACVSMR

and Ann-Marie Hancock, DVM, veterinary osteopath

History of equine vaccination

Equine vaccination has a long and rich history. The practice of vaccination dates back to the late 18th century when Edward Jenner developed the first smallpox vaccine, utilizing cowpox to protect humans from smallpox.

The veterinary use of vaccines followed suit, with equine vaccination emerging as a means to prevent infectious diseases like tetanus, strangles, and equine influenza. In the 20th century, vaccines became more refined and targeted specific pathogens, leading to a reduction in equine mortality from infectious diseases. Today, vaccination plays a crucial role in maintaining the health and well-being of horses.

The purpose of vaccines

The primary purpose of vaccines is to stimulate the horse’s immune system to produce an immune response without causing the disease itself. Vaccines introduce a small, inactivated or altered form of the pathogen (such as a virus or bacteria), prompting the horse’s body to recognize and combat it. In this way, vaccines help the horse develop immunity to specific diseases, offering protection in case of future exposure.

Vaccines recommended by the American Association of Equine Practitioners (AAEP)

By understanding the various vaccines and their recommended frequency, horse owners can, in consultation with their veterinarian, tailor a vaccination plan that provides optimal protection based on their horse’s lifestyle and risk factors.

“Core” vaccines are those considered as essential for all horses, regardless of exposure risk, due to the severe consequences of these diseases. “Non-core” vaccines are those recommended based on specific risk factors such as geographic location, travel, competition, and breeding. “Frequency” refers to the recommended interval for vaccination, which may vary depending on the horse’s risk of exposure or specific regional guidelines.

Vaccine	Disease Targeted	Core/ Non-Core	Frequency	Indications
Tetanus (Clostridium tetani)	Tetanus	Core	Annually, or as recommended by vet	All horses due to the risk of contamination from wounds
Rabies (Rabies virus)	Rabies	Core	Annually	All horses, especially those with potential exposure to wildlife
Eastern Equine Encephalitis (EEE)	Eastern Equine Encephalitis	Core	Annually, during mosquito season	Horses in areas where mosquito-borne diseases are a risk
Western Equine Encephalitis (WEE)	Western Equine Encephalitis	Core	Annually, during mosquito season	Horses in areas where mosquito-borne diseases are a risk
West Nile Virus (WNV)	West Nile Virus	Core	Annually	Horses in areas where mosquitoes are prevalent, especially during summer/fall
Equine Influenza (EI)	Equine Influenza Virus	Non-Core	Annually or bi-annually (depends on exposure risk)	High-risk horses (competition, travel, etc.)
Equine Herpesvirus (EHV-1, EHV-4)	Equine Herpesvirus 1 & 4 (Rhinopneumonitis)	Non-Core	Annually, or every 6 months in high-risk settings	Horses in high-risk environments (breeding, racing, shows)
Strangles (Streptococcus equi)	Strangles	Non-Core	Annually or as recommended by vet	Horses at risk of exposure (shows, stables with outbreaks)
Potomac Horse Fever (Neorickettsia risticii)	Potomac Horse Fever	Non-Core	Annually	Horses in endemic areas (mainly in the U.S. mid-Atlantic and Midwest)
Botulism (Clostridium botulinum)	Botulism	Non-Core	Annually or per veterinarian advice	Horses at risk from consuming contaminated feed or in endemic regions
Equine Viral Arteritis (EVA)	Equine Viral Arteritis	Non-Core	Every 6 months (breeding stallions)	Breeding stallions and mares (as required by breeding centers)
Leptospirosis (Leptospira spp.)	Leptospirosis	Non-Core	Annually or as recommended by vet	Horses exposed to contaminated water, wet environments, or certain rodents
Influenza and Herpesvirus Combination	Equine Influenza and Equine Herpesvirus (EHV)	Non-Core	Bi-annually to annually in high-risk settings	Horses in high-risk environments (racing, shows)

Multivalent vaccines

There exist several different multivalent, i.e., combination vaccines. These combinations – referred to, for example, as a “3-way” or a “5-way” – can be completely different depending on the geographic region, and also differ from veterinary practice to practice. Although some veterinarians and owners consider this approach to vaccination to be more convenient and sometimes more cost-effective (i.e., fewer injections), these combined vaccines may increase the risk of reactions for certain horses.

The use of titers

Titer tests or “titers” measure the presence and concentration of antibodies in the blood. A titer can be used to determine whether a horse has enough immunity to a particular disease and whether a booster shot is necessary. This is particularly useful for diseases where immunity may be long-lasting or where reactions to vaccines are a concern.

See References at the end of this article for more information about titers.

How to prophylactically manage vaccination

Proactive management involves ensuring that your horse is in optimal health before vaccination. Here are some tips to minimize the risk of adverse reactions:

• Ensure the horse is not under stress – stress can compromise the immune system and increase the likelihood of a reaction.
• Vaccinate during the off-season. If possible, vaccinate during times when your horse is less likely to be exposed to other pathogens (e.g., outside peak show or transport seasons).
• Maintain regular health checks. Ensure that your horse is well-nourished and free of underlying infections before vaccinating.
• Consider holistic therapies, e.g., bodywork, nutritional support, herbals, liver and kidney support, detoxification or homeopathics when considering vaccination or for treatment of vaccination after-effects.

Vaccine reactions

Vaccines can sometimes cause reactions in horses, which may range from mild to severe. These reactions are typically a result of the horse’s immune system responding to the vaccine as it would to an infection. While most reactions are mild and transient, such as swelling at the injection site or a slight fever, others may be more significant. Reactions can be influenced by factors such as the horse’s health status, age, the vaccine used, and the horse’s previous exposure to pathogens.

Short-term effects usually resolve within 24-48 hours as the immune system finishes processing the vaccine. These include:

• Localized reactions – swelling, heat, or tenderness at the injection site
• Systemic reactions – mild fever, lethargy, or a decrease in appetite

Long-term effects are rare but can include chronic issues such as allergic reactions or autoimmune conditions. However, these effects are uncommon and often occur in horses with preexisting health concerns or a predisposition to hypersensitivity.  Vaccines can be part of an accumulation of inflammatory events that can predispose to other disease processes over time.

When to call the veterinarian

While most side effects are mild and self-limiting, severe reactions warrant immediate veterinary intervention. If your horse experiences any of the following after a vaccination, contact your veterinarian:

• Severe allergic reactions: Swelling around the face or neck, difficulty breathing, hives, or collapse
• Persistent fever or significant lethargy lasting longer than 48 hours
• Signs of infection: Discharge, increased swelling, or abscess formation at the injection site

How to treat vaccine reactions

Always follow the advice of your veterinarian when managing reactions. Most vaccine companies will compensate for any costs incurred when treating acute reactions, and also want your veterinarian to report all reactions to them.

Treatment depends on the severity of the reaction:

• Mild reactions typically require no treatment other than observation and possibly the use of anti-inflammatory medications such as phenylbutazone to reduce swelling or discomfort.
• Severe reactions may require antihistamines, corticosteroids, or other medications to manage inflammation or anaphylaxis. If an abscess forms at the injection site, your veterinarian may need to drain it and administer antibiotics.

Taking the best decision for your horse(s)

Vaccine schedules

Vaccines carry certain risks, but these must be weighed against the risks of exposure to diseases.

Pre-vaccination health assessments are essential to identify any potential issues that may make a horse more susceptible to reactions. Stress, poor health, or a compromised immune system can increase the likelihood of reactions to vaccines.

Age is a key factor. Foals require a series of vaccinations starting at 3 or 4 months of age, followed by booster shots until they are about 6 months old. Older horses may require less frequent boosters but should be assessed for risks based on exposure.

Exposure risks should also guide vaccine choices. Horses that have higher exposure to other horses, travel frequently, or compete in events may be at higher risk of contracting certain diseases and thus require more comprehensive vaccination.

Therefore, vaccination schedules can vary greatly depending on multiple factors:

• Minimal schedule: Young or inactive horses with limited exposure to other horses or pathogens may only need core vaccines (e.g., tetanus, rabies) every 1-3 years, or even less frequently, depending on local risks and regulations.
• Moderate schedule: Horses with moderate risk, such as those who compete locally, may need annual boosters (or titers) for certain vaccines. They may also need additional vaccines, such as those for equine influenza, equine herpesvirus, or strangles.
• Extensive schedule: Horses involved in high-risk activities such as racing, breeding, or international travel will require a broader vaccination schedule, including both core and non-core vaccines. And they may need vaccines every 6-12 months, depending on the disease and the vaccine.

Vaccination decision tree

A vaccination decision tree can help owners determine the best course of action:

1. Health status: Is the horse healthy and free from disease?
2. Risk of exposure: Will the horse be exposed to other horses or travel to high-risk areas?
3. Age and breed: Are there age-specific vaccination needs?
4. Previous reactions: Has the horse had any adverse reactions to previous vaccinations?
5. Veterinary advice: What do the latest veterinary guidelines and research suggest?
6. Owner’s comfort level: What do you as the owner consider to be important? What are your concerns? What is your comfort level with vaccinations?

In closing…

Vaccination is one of the most effective ways to protect horses from a variety of infectious diseases. By understanding the purpose of vaccines, how to avoid or manage reactions, and the factors influencing vaccination schedules, horse owners can make informed decisions to safeguard the health of their horses.

Always consult with your veterinarian to develop a vaccination plan tailored to your horse’s individual needs and risks. Remember that you are your horse’s caretaker and you have the final say – become educated and empowered to make the wisest decision for your horses and the one that “sits best” with you.