Vaccinating Horses: What, When, and Why?

Regular, strategic vaccination is a safe, effective method to greatly minimize the chances of deadly diseases impacting your herd.

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Equine Vaccination: What, When, and Why?
Seek veterinary input when determining which vaccines to include in your herd’s program. | The Horse Staff

It’s spring and the sun is shining, the birds are chirping, and the foals are frolicking. Less visibly, but just as importantly, mosquitoes are hatching, Clostridium tetani spores are lying in wait in soil, manure, and on that old, rusty fence wire, and a confused-looking and possibly rabid raccoon just wandered through the paddock in the middle of the afternoon. While springtime brings warmth, new life, and equine activities, it also increases preventable disease risks. Regular, strategic vaccination is a safe, effective method to greatly minimize the chances of deadly diseases impacting your herd.

Core Vaccinations

The AAEP recommends all horses receive core vaccinations that “protect against diseases that are endemic to a region, are virulent/highly contagious, pose a risk of severe disease, those having potential public health significance, and/ or are required by law.” These include:


This is a frequently fatal neurologic disease caused by C. tetani bacteria. It is not contagious—a sick horse cannot infect another—but C. tetani is present in the intestines of horses and other animals. The spores are resistant to heat and can survive in the environment for many years, making exposure a constant risk.

Infection typically occurs through puncture wounds, lacerations, and surgical incisions. In addition to regular vaccination, ensure injured horses, horses undergoing surgery, or pregnant mares are current as recommended by your veterinarian.

Eastern/Western equine encephalomyelitis (EEE/WEE) and West Nile virus

These mosquito-transmitted encephalitic viruses cause severe neurologic signs and have high mortality rates. Disease frequency varies depending on location: EEE is more prevalent in the southeastern United States, while WEE is more frequently seen in the western U.S. Regions with large mosquito populations tend to have more reported cases.


While the number of rabid horses reported in the U.S. each year is small, the zoonotic disease is fatal. Because of the severity of this disease and public health concerns, most states require veterinarians to administer or oversee the administration of the vaccine.

Risk-Based Vaccinations

Risk-based vaccinations are those administered in certain populations after assessing risk vs. benefit. When determining which vaccines to include in a herd’s vaccination program, seek input from a veterinarian familiar with regional disease variation, horse lifestyle, and competition or transport requirements.

Risk-based vaccinations include:

Equine herpesvirus (EHV, rhinopneumonitis)

EHV-1 and EHV-4 can cause respiratory disease ranging from mild to severe. EHV-1 can also lead to outbreaks of abortion, as well as neurologic disease in a manifestation known as equine herpesvirus myeloencephalopathy (EHM). These viruses spread primarily via the respiratory route but also through contact with infected aborted fetuses and reproductive tissues. They can establish latent infection, leading to subclinical carrier status for some horses. Because EHV is endemic in many populations, vaccination is aimed at decreasing frequency of abortion in pregnant mares and reducing severity and duration of illness in young horses or those at high risk for exposure. Many competition facilities require record of EHV vaccination for entry. We don’t yet have a vaccine labeled to prevent EHM.

Equine influenza

This common infectious respiratory disease spreads through aerosolized droplets and fomite transmission. It is endemic throughout most of the world, with the exception of Australia, Iceland, and New Zealand. While influenza is rarely fatal, it can produce significant respiratory illness and spread rapidly. Many competition venues and facilities require record of influenza vaccination.


This highly contagious respiratory disease causes fever, lymphadenopathy, and mucopurulent nasal discharge. It can spread through direct contact or contaminated equipment or fomites. Vets recommend vaccination based on risk of exposure due to endemic status on a property or frequent transport or exposure to other horses. Vaccination in the face of an outbreak can help decrease disease but necessitates careful assessment due to increased risk of adverse reactions.

Other risk-based vaccines include those against anthrax, botulism, equine viral arteritis, leptospirosis, Potomac horse fever, rotavirus, snake bite, and Venezuelan equine encephalomyelitis. Appropriate vaccination schedules vary significantly; a trail horse living on a small farm in Florida, for instance, needs a different regimen than a show jumper in California who travels frequently or a broodmare on a large farm in Kentucky. Discuss with your vet how to keep your herd safe, weighing the risks of exposure with the benefits and protection vaccines can provide.


Written by:

Jennifer Madera, DVM, lives and practices in Ocala, Florida. She is a 2004 University of Missouri graduate. As an FEI endurance veterinarian, she is passionate about maintaining the health and welfare of equine athletes.

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