Vaccination is one of the best and most effective ways to prevent infectious disease in horses. Vaccines are typically made from an inactivated (dead) or a weakened form of a disease-causing microorganism, which stimulates the horse’s immune response. The immune system creates a defensive response in the form of antibodies which recognize that same microorganism in the future. For some vaccines there is also a cell mediated response that can further assist in protection against infection in exposed horses. Once vaccinated, the horse’s immune system “remembers” the real microorganism as foreign and acts to destroy it.
Core vaccines have a clearly demonstrated high degree of safety and efficacy and thus are considered to provide a high enough level of patient benefit and low enough level of risk to justify their use in the majority of horses regardless of where they reside in the USA or what they do for a living. The 5 core vaccines for horses are tetanus, Eastern EEE and Western Equine Encephalomyelitis WEE), West Nile Virus (WNV) and rabies. Criteria to identify a core vaccine include the following
- Protect horses from diseases that are a risk for horses in a region
- Have potential public health significance (e.g. rabies)
- May be required by law
- Protect against virulent (deadly) and/or highly infectious organisms
- There is a perceived risk of exposure for most horses
At-risk vaccines are recommended by a veterinarian after they have considered the risks and benefits for your horse based on the horse’s history, the group of horses it lives among, its occupation and the amount of associated commingling with other horses, and/or the region the horse lives in. Owners, trainers, barn managers, and event organizers are encouraged to consult with their veterinarian when assessing disease-risk and the appropriate immunization protocol for their horses and / or facility. Risk-based vaccines include those that protect horses against equine influenza virus (EIV), equine herpesvirus 1&4 (EHV1&4), Strangles, Potomac Horse Fever, anthrax, botulism, leptospirosis and equine viral arteritis.
The following factors should be taken into consideration when determining a vaccination program:
- Risk of the disease to the individual and/or herd. Some diseases such as equine influenza virus (EIV) or equine herpesvirus (EHV1&4) are highly contagious; it is beneficial to ensure that all or a high percentage of your herd is properly vaccinated to limit spread of these diseases within a barn, at a racetrack, or at an event such as a horse show. Biosecurity measures are also critical in the management of highly contagious diseases. Other diseases such as Eastern Equine Encephalitis (EEE), West Nile Virus (WNV), and tetanus, which are potentially fatal or cause high morbidity, are not spread from horse-to-horse, but are acquired from insects or environmental sources. Vaccination against these diseases is recommended for all horses.
- The consequences of the disease itself. Some diseases such as EEE have a high case fatality rate often exceeding 90%. Rabies is not only uniformly fatal for the unvaccinated horse but also poses a dangerous public health hazard.
- The efficacy of selected vaccines. Not all vaccines are 100% effective; however, proper vaccination against tetanus or rabies offers a high degree of protection against those diseases. In some cases, vaccines will decrease the severity of a disease making vaccination a valuable procedure.
- The impact of the disease versus the cost, efficacy and risk of vaccination. While Equine Influenza is rarely fatal, there may be a considerable loss of income associated with missed race or horse show days due to sick horses.
- The potential risk of adverse events associated with vaccination. Vaccination always carries the potential risk of an adverse reaction. Vaccine reactions may be local and characterized by focal swelling and pain at the injection site. Other reactions may be systemic and can vary from low grade fever, malaise, loss of appetite, hives, infection at the injection site and, rarely, fatal anaphylaxis. All adverse events should be reported to the veterinary practitioner, the vaccine manufacturer, and to USDA APHIS VS Center for Veterinary Biologics.
- Onset of immunity. Onset of protection following vaccination is not immediate and often requires a repeat vaccination (booster vaccination) and sufficient time for the horse to mount an optimal immune response. Immunity provided by tetanus vaccination, for example, is optimal after a booster vaccination followed by annual boosters.
- Individual horses will vary in their response to vaccination. The immune response to vaccination can be negatively impacted by
- Age. In young foals, high levels of maternally-derived colostral antibodies may diminish their immune response to initial vaccination; the guidelines provide specifics related to when to first vaccinate foals based on the vaccination history of their dams. Senior horses may not exhibit a robust response to vaccination due to the effect of aging on the immune system (known as immunosenescence).
- Poor health will adversely affect immune response. Only healthy, afebrile horses should be vaccinated unless a veterinarian advises differently.
- Concurrent or recent administration of immunosuppressive medications such as corticosteroids may blunt the immune response.
- Stress, such as long-distance transportation or strenuous exercise, can also suppress the immune system.
- Poor nutrition can reduce the likelihood of an optimal immune response.
- Improper storage, handling, or administration of the vaccine can affect the efficacy of the product and may increase the risk of an adverse event.
All vaccination practices should be undertaken with realistic expectations. No vaccine is 100% effective in preventing disease in all horses. Vaccination without good management practices and biosecurity protocols will not prevent infectious disease. Onset of protection following vaccination is not immediate and may require more than one dose of vaccine and / or sufficient time for the horse to mount an optimal immune response. See the AAEP Vaccination Guidelines for more information