Vaccination is one of the most effective ways to prevent infectious diseases in horses. Vaccines typically use an inactivated (dead) or weakened form of a microorganism to stimulate the horse’s immune system. This process generates antibodies that can recognize and combat the microorganism if encountered in the future. Some vaccines also trigger a cell-mediated response, enhancing protection. Once vaccinated, the horse’s immune system “remembers” the microorganism and can quickly respond to future exposures. For more details, refer to the AAEP Vaccination Guidelines.

Core Vaccines

Core vaccines are highly recommended due to their safety and effectiveness. They provide significant benefits and low risks, making them suitable for most horses in the USA, regardless of location or activity. The five core vaccines for horses are:

1. Tetanus
2. Eastern Equine Encephalomyelitis (EEE)
3. Western Equine Encephalomyelitis (WEE)
4. West Nile Virus (WNV)
5. Rabies

Criteria for identifying core vaccines include:

• Protection against diseases prevalent in the region
• Public health significance (e.g., rabies)
• Legal requirements
• Protection against virulent and/or highly infectious organisms
• Perceived risk of exposure for most horses
  

At-Risk Vaccines

At-risk vaccines are recommended by a veterinarian based on individual horse factors such as history, living conditions, occupation, and regional disease prevalence. Owners, trainers, barn managers, and event organizers should consult their veterinarian to determine the appropriate vaccination protocol. At-risk vaccines include:

• Equine influenza virus (EIV)
• Equine herpesvirus 1&4 (EHV1&4)
• Strangles
• Potomac Horse Fever
• Anthrax
• Botulism
• Leptospirosis
• Equine viral arteritis
  

Factors in Determining a Vaccination Program

Several factors should be considered when developing a vaccination program:

• Risk of Disease: Some diseases, like EIV and EHV1&4, are highly contagious, making it crucial to vaccinate most of the herd to prevent spread. Diseases like EEE, WNV, and tetanus, which are often fatal or cause severe illness, are not spread horse-to-horse but through insects or environmental sources. Vaccination against these is recommended for all horses.
• Disease Consequences: Diseases like EEE have high fatality rates, often exceeding 90%. Rabies is uniformly fatal and poses a significant public health hazard.
• Vaccine Efficacy: Not all vaccines are 100% effective, but vaccines like those for tetanus and rabies offer high protection levels. Some vaccines may reduce disease severity, making them valuable.
• Impact vs. Cost: While diseases like equine influenza are rarely fatal, they can cause significant economic losses due to missed events.
• Adverse Events: Vaccination carries potential risks, such as localized swelling or systemic reactions like fever and malaise. Severe reactions are rare but possible. Report any adverse events to your veterinarian, vaccine manufacturer, and USDA APHIS VS Center for Veterinary Biologics.
• Onset of Immunity: Immunity is not immediate and often requires booster shots. For instance, tetanus protection is optimal after a booster and annual follow-ups.
  

Factors Affecting Immune Response

The effectiveness of vaccination can be influenced by:

• Age: Young foals with high maternally-derived antibodies may have a diminished response to initial vaccinations. Senior horses may also have a weaker response due to immunosenescence.
• Health: Only healthy horses should be vaccinated unless advised otherwise by a veterinarian.
• Medications: Immunosuppressive medications, like corticosteroids, can blunt the immune response.
• Stress: Factors like long-distance transport or strenuous exercise can suppress the immune system.
• Nutrition: Poor nutrition can hinder an optimal immune response.
• Vaccine Handling: Improper storage, handling, or administration of the vaccine can reduce efficacy and increase the risk of adverse events.
  

Realistic Expectations

No vaccine is 100% effective in all horses. Good management practices and biosecurity protocols are essential alongside vaccination to prevent infectious diseases. Remember, protection following vaccination is not immediate and may require multiple doses or time to develop an optimal immune response.