Your horse’s immune system does extraordinary and complex work, but even this intricate defense mechanism sometimes needs assistance. Vaccination is an extremely effective and safe way to provide a targeted boost to the immune system to help safeguard horses against deadly infectious diseases.

Unfortunately, an abundance of misinformation and myths continues to circulate about vaccination. Some people wrongly believe that vaccination is unnecessary if a disease is rarely seen in a region or on a particular farm. Another myth holds that “natural” immunity is better than the protective response stimulated by a vaccine. Finally, some people are concerned that the immune system will be overloaded by vaccination. These untruths are often perpetuated by the “Dr. Google” phenomenon.

A better understanding of the equine immune system, along with how vaccines enhance immune responses can be helpful in dispelling these myths and improving the overall health and welfare of our equine companions. The relationship you have with your veterinarian, who is your partner and your horse’s best line of defense against infectious disease, is a vital link to making the best health care decisions for your horse.

The basics of immunology

Immunity is the body’s ability to recognize and dispose of potentially harmful foreign substances. There are many ways to classify immunity, but the two major components of the immune system are natural and acquired immunity.

1. Natural immunity (also known as native or innate immunity): Fast-acting, non-specific immune response that occurs at the site of infection and is always present and readily activated. This is the horse’s built in, first line of defense that is present from birth. Non-specific chemicals and cells are the major components of the natural immune response. Examples of mechanisms responsible for natural immunity include certain types of white blood cells, proteins, antimicrobial compounds, and even natural physical barriers such as the skin, mucous membranes and stomach acid. Animals and people have some degree of natural immunity to almost all infectious agents.

2. Acquired immunity (also known as adaptive immunity): Antigen-specific, acquired immunity that is induced and remembered after exposure to specific antigens. It occurs after initial exposure to a specific pathogen and requires several days to weeks to develop. Acquired immunity consists of two parts:

     A. Passive immunity, such as achieved through the feeding of colostrum (first milk-containing antibodies) to a newborn foal and/or through administration of plasma containing antibodies.

     B. Active immunity, which occurs following exposure to an infectious agent or to a vaccine. There are two types of active immune response.

          i. Humoral immunity (production of antibodies). Examples include the antibody response to tetanus, rabies and other vaccinations.

          ii. Cell-mediated immunity (CMI). An example of CMI would be a response that does not involve antibodies. What occurs, instead, is the release of cytokines (proteins that activate numerous components of the immune system) that help direct cell function in response to an antigen or vaccination. This type of response is generally thought to be a more robust immune response.

Both types of immune responses work in tandem to protect the horse from invading pathogens. For example, acquired immunity relies on the natural (or innate) immune system to communicate information about the type of pathogens potentially invading the body. This, in turn, helps the united immune system mount the appropriate and most profound response.

Vaccines and the immune system

Killed versus modified-live vaccines

Different types of vaccines stimulate the immune system differently. “Killed” (inactivated) vaccine antigens are processed and recognized by the immune system as exogenous antigens – those having originated externally. Killed vaccines primarily induce an antibody response, but a less aggressive CMI response also occurs. “Live” (modified-live) vaccine antigens have been altered (attenuated) so they can no longer cause clinical disease. Modified-live vaccines are recognized and processed by the immune system as endogenous antigens – having originated within – which mimic how a natural viral or bacterial infection is processed. These antigens elicit both antibody and strong CMI responses.

Route of administration can also have a substantial impact on the immune response to vaccination. For example, intranasal modified-live vaccines have an enhanced ability to stimulate the natural (innate) immune response in conjunction with stimulation of an adaptive (acquired) immune response.

A study conducted at Colorado State University demonstrated how the modified-live intranasal influenza vaccine (marketed as FLU AVERT® I.N.) stimulates innate, protective immune responses in the equine upper respiratory tract (URT) similar to wild-type influenza.[1]

Monovalent versus combination vaccines

The advent of combination vaccines for horses has changed the way veterinarians approach vaccination. Combination vaccines, which include multiple disease antigens in one injection, are conveniently used to help eliminate the need for multiple injections. Most cover the majority of core vaccine recommendations, as well as important risk-based diseases, such as equine influenza and herpesvirus. Monovalent (containing one antigen) vaccines are still used and work well, but do not necessarily improve protection over combination vaccine products.

We often get asked if each fraction (antigen) within these combination vaccines is proven to be efficacious. In other words, is each fraction (disease antigen) in a combination vaccine proven to provide protection against that disease antigen in the combination vaccine? The answer is yes! Efficacy requirements for licensing of multiple antigen vaccines (combination vaccines) are the same as those for single antigen (monovalent) vaccines.[2] So we know that combination equine vaccine products are efficacious in preventing and lowering the risk of the diseases addressed by each vaccine.

The immune system cannot be overloaded

The remarkable work of the immune system is showcased by its ability to respond to literally thousands of antigens every day. Equine immunologist David Horohov, Ph.D., chair of the Department of Veterinary Science at the University of Kentucky College of Agriculture, Food and Environment and director of the Gluck Equine Research Center in Lexington, Ky., says it is unlikely that a combination of antigens in a vaccine product would exhaust or compromise the immune response.

“Every day, the horse, like all animals, encounters a multitude of foreign antigens in its environment. The body has evolved a number of protective mechanisms to deal with this exposure. While vaccination does provide a concentrated exposure to specific antigens, and even if multiple vaccines are administered, one would not expect any untoward effect on the immune system because of these mechanisms.”

Adjuvants and preservatives are safe

An adjuvant is a substance included in many inactivated vaccines to enhance the immune response to specific antigens. Killed vaccines generally require an adjuvant to enhance the vaccine antigen presentation to the horse and elicit an efficacious immune response. Many adjuvants can help elicit both humoral (antibody) and cellular response. Most adjuvants are derived from safe chemicals, microbial components and/or mammalian proteins.

“The increased efficacy of most equine vaccines over the past 20 years has been almost entirely due to the development of more effective adjuvants,” says Dr. Horohov. “Adjuvants are used to enhance the immunogenicity of the vaccine by both prolonging the exposure of the immune system to the antigen and by providing necessary co-signals through the interaction of the adjuvant with antigen presenting cells.”

Adjuvants are generally considered safe – non-allergenic, non-toxic and non-antigenic. However, adjuvants can create local inflammation in the tissue of some horses. This most commonly results in injection-site reactions (bumps, swelling), fever, local soreness, lethargy and reduced appetite. As described, these reactions are generally mild and self-limiting.

Vaccine manufacturers place a great deal of importance on research and development efforts, and safety testing to determine the appropriate balance of adjuvant to antigen to elicit an optimal immune response with the least potential reactivity.

Vaccines may include a preservative to increase shelf-life, for safe storage, and to prevent bacterial and fungal contamination that could compromise the vaccine.

“The concentration of these preservative materials is quite small,” says Dr. Horohov. “Once injected into the horse, their effect is diluted nearly to the point of extinction. While a great deal of concern has been expressed concerning exposure to these agents via vaccinations, there is no evidence that such fears are warranted.”

Importance of veterinarian-directed vaccination

Still uncertain about vaccination? Talk with your veterinarian. He or she is a great resource and is critical in designing an appropriate vaccination program for your horses and your farm. He or she understands the individual needs of your horse(s), as well as endemic and regional disease considerations and need for vaccination.

Equine infectious disease can strike suddenly and be devastating to your horse—and others. Vaccination is proven to be one of the safest and most reliable life-saving measures you can take for your horse. Get the facts on vaccination to help ensure a lifetime of health and happiness for your equine companion.

[1] Landolt GA. Comparison of innate immune responses in equine respiratory epithelial cells to modified-live equine influenza vaccine and related wild-type influenza virus. Abstract ACVIM 2014.

[2] United States Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) Code of Federal Regulations (CFR) for licensing of animal biological products: 9 CFR 113.7,