New Research on the Equine Immune System

In horses, as in humans and many other animals, the immune system protects against disease in several ways. The first line of defense is a physical one in which the skin and structures in the nasal passages block harmful bacteria and other pathogenic organisms from entering the body. Those organisms that do manage to enter the horse’s body are met by defensive cells that trigger the immune system to produce antibodies against the foreign material. If the immune system is not functioning strongly or is overwhelmed by harmful organisms, disease develops. Antibodies can also be produced by vaccinating a horse against a particular disease, so that the horse is already prepared to fight microbes that can cause that disease before they actually enter its body.

Several recent research projects have led to a better understanding of the equine immune system. Researchers at Colorado State University have developed an artificial membrane that has the characteristics of the horse’s respiratory tract lining. Using the membrane, it is possible to study actions of the viruses that cause equine influenza and herpes much more closely than was possible by observing horses suffering from these diseases.

Another Colorado State University breakthrough is a new model of the circulatory system that serves the horse’s brain and spinal cord. Both developments allow researchers to study drugs, vaccines, and treatment protocols in ways that would not be possible using live horses.

A foal depends chiefly on antibodies derived from its dam’s colostrum for protection from disease in the first weeks of life. Mares can have antibodies to diseases they have had and also ones they have developed after being vaccinated against specific diseases. Researchers are now looking at other roles that may be played by the dam in the development of her foal’s immune system. Some evidence has suggested that the pregnant mare’s nutrition, housing, exercise, and stress level may impact the development of her foal’s immune system well into the foal’s lifetime.

As a horse ages, its immune system tends to weaken in its response to a disease challenge. Trials have shown that older horses may develop a stronger immune response when they are vaccinated with a vaccine made from live influenza virus rather than a killed form of the same virus. If this holds true for other vaccines, it could lead to a recommendation as to the type of vaccine that is most effective in preventing diseases in older horses.

Another area of research is examining the role of the immune system in regulating inflammation. The body’s fight against infection includes inflammation, which is considered necessary for healing. However, inflammation is also a response to training and muscle stress. Muscles tend to become inflamed after hard exercise, but this response becomes less severe as training continues, possibly because the immune system becomes conditioned as the muscles increase in strength. When medications such as non-steroidal anti-inflammatory drugs (NSAIDs) are prescribed to treat stiffness and soreness resulting from exercise, they may impair the ability of the immune system to adapt to training. Some evidence is emerging that masking pain and limiting immune system adaptation may leave the horse at an increased risk for more serious injuries further along in his training and performance.