Equine Immune System Basics Part III

How the Body Systems of the Horse React to Stress
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Immune System Reaction of the Muscular System


When a performance horse has completed a difficult competition or a hard-fought race, nearly the entire muscular system can get overworked due to damaged cells and depletion of cell nutrients. The stress of performing a competitive task often triggers stress hormones, most notable of which is adrenaline. This adrenalin rush makes the horse feel powerful by increasing blood flow to prepare the muscles for exertion, as well as increasing the rate of respiration and carbohydrate metabolism. The contractions of the muscles during hard exercise also causes the release of immune system responders that trigger inflammation, and some that stimulate production of satellite cells which help repair and develop new muscle cells.

But as this rush declines, it is followed by a "crash" that can make the horse feel nervous and depressed due to the presence of other stress hormones, like cortisol, which have anti-inflammatory effects. Moderate exercise is recommended following an adrenalin crash to relieve some of its negative effects and to help bring the immune system back into balance by triggering the release of endorphins, which have a calming effect. Chronic and extreme muscle inflammation, however, can disrupt the balance of controls that the immune system has for the muscular system, and the net effect can be muscle loss.

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In the performance horse, too much stress can manifest itself in various ways. The horse can be moody and less willing to train, and may have a decrease in appetite. An over-stressed horse can also develop stiffness due to overuse of muscles, ligaments, tendons, cartilage and joints.

Immune System Reaction of the Digestive System

The digestive system of the horse is known to be the body’s "food processor," breaking down food into nutrients small enough to be absorbed into the body. It also gets rid of waste that is no longer needed or is toxic to the body. In addition, the digestive system is a very important part of the immune system.

As food enters the mouth, the immune system starts "scanning" it as it is chewed for pathogens, allergens or toxins, and the immune messengers and defenders in saliva start to work to communicate to other defense mechanisms further down the system. Chemical acids and enzymes in the stomach not only start breaking down the food but also kill a lot of pathogens even before they enter the intestine. Once the food enters the small intestine, the gut barrier of the mucosa surfaces inside the intestine are actively scanning microbes, killing the invaders and letting the good allies move on to the large intestine and cecum, where they aid in digestion and also populate this area to prevent the invaders from taking over.

If invading pathogens succeed in breaking through the barriers, they can enter the body. This alerts and activates the whole army of the immune system (systemic immune activation). The metabolism rate increases to support the immune response; blood flow increases to the intestinal system, transporting even more defenders and repairers; inflammation occurs; fever develops to help kill the invaders; motility of the intestine increases; and more fluid enters the intestine to help flood the pathogens out, causing diarrhea. The horse receives signals to slow down its eating, resulting in loss of appetite, so that the intestine can clear out the invaders, repair damage and get back to its normal function.

Immune System Reaction of the Respiratory System

The respiratory system (nose, trachea and lungs) also plays a very important role in the immune system. The mucosa surfaces (mucus membranes) lining the nasal area, trachea and lungs serve as a barrier to pathogens in the air that we breathe, performing essentially the same functions of scanning and defending as those of the mucus barrier in the intestinal system.

Although allergens in the air are not pathogens, the immune system responds to them as it would to pathogens. This stimulates inflammation, increases mucus production to help flush them out, causes coughing and sneezing to blow them out, and can generate fever in an attempt to kill them. In the case of people or animals with severe allergies to certain allergens, the respiratory system can be overly reactive to the point of causing excessive inflammation, which can restrict air passages. Allergens in food can cause similar allergic reactions in the digestive system. Extreme allergic reactions require interventions with anti-inflammatory substances. Adrenalin is often used as a potent, fast-acting anti-inflammatory to open airway passages.

An Oral Form of Serum Proteins Can Positively Affect the Immune System

Research demonstrates that orally-dosed serum based bioactive proteins such as LIFELINE Equine Performance Supplements have multiple effects on the immune system reaction in different body systems. Studies in model species have shown that orally-dosed bioactive proteins reduce respiratory inflammation when animals are challenged with a bacterial or viral infection.

Serum-based bioactive proteins administered to animals helped support and maintain balance of the immune system response to various inflammatory induced stressors. Use of serum-based bioactive proteins in supplements for horses, particularly during stress-related life events, may help maintain a balanced immune response and reduce inflammation.

Author Pam Maley is an educator who, with her husband, co-owned and -managed a Thoroughbred broodmare farm in Lexington, Kentucky, where she was closely involved with all aspects of equine life. An avid former foxhunter, her journey with horses continues as a writer and editor with EquestriSol. This is the third part of her three-part series detailing Immune System Basics. Part I examined how the immune system works. Part II examined common stressors for horses and the immune system’s reaction to that stress.