In Part III of this series, we examined the immune system and its layers of defense.  In Part IV we looked at the times of stress in a horse’s life when the immune system most often comes into play.  Here we consider how the specific body systems react to stress, and the immune responses that follow. 


The Muscular System

After a demanding competition or a hard-fought race, the entire muscular system of the performance horse can get overworked due to damaged cells and depletion of cell nutrients.  The stress of performing a difficult competitive task can trigger the release of stress hormones, most notably adrenaline, that make the horse feel powerful.  The blood flow is intensified, and the rates of respiration and carbohydrate metabolism increase.  In addition, as the muscles work during hard exercise, immune system responders that trigger inflammation are released.

But in the ‘crash’ that follows the burst of adrenaline, anti-inflammatory stress hormones are released in an attempt to bring the immune system back into balance, causing the horse to feel nervous, depleted, and depressed.  This up-and-down cycle of hormones, if it becomes chronic, can disrupt the balance of controls that the immune system has for the muscular system, leading to muscle loss, pain, and stiffness.


Digestive System

The digestive system of the horse is the ‘food processor’ that breaks down food into nutrients small enough to be absorbed into the body, and it 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.

The immune system begins scanning the food as it is being chewed, searching for pathogens, allergens, or toxins, and the messengers and defenders in the saliva swing into action, communicating to other defenders further down the system.  When the food reaches the stomach, chemical acids and enzymes go to work to break the food down, as well as to kill a lot of pathogens before they enter the intestine.  In the intestine, the gut barrier of the mucosa surfaces inside the intestine scans for pathogens, kills the invaders, and allows the beneficial microorganisms to move on.  In the large intestine and the cecum, these good allies aid digestion and populate the area to prevent the invaders from taking over. 

If, however, the invading pathogens overpower the defenders and break through the barriers and enter the body, the whole army of the immune system swings into action (systemic immune activation).  The rate of metabolism increases to support the immune response, blood flow increases to the intestine to transport more defenders, inflammation occurs, fever develops to help kill the pathogens, motility of the intestine increases, and more fluid rushes into the intestine to flood the pathogens out, causing diarrhea.  In addition, to give the intestine time to clear out the invaders, the immune system sends signals to the brain to tell the horse to slow down its eating until the intestine can return to its normal function. 

From The Merck Manual for Pet Health, online version. Susan E. Aiello, ed. Copyright © 2015 by Merck & Co., Inc., Whitehouse Station, NJ. All rights reserved. Used with permission. Available at: Accessed November 11, 2015.


The Respiratory System

The nose, trachea, and lungs make up the respiratory system, which also plays an important role in the immune system.  The mucus membranes that line these organs scan and defend in much the same way as those of the mucus barrier in the intestine.  

The respiratory system deals mostly with allergens.  While not pathogens, the immune system deals with them as if they were, stimulating inflammation, increasing mucus production to flush them out, causing coughing and sneezing to blow them out, and sometimes generating fever to try to kill them.  A severe reaction can cause excessive inflammation, restricting air passages, requiring emergency treatment.


Stress Relief

Research has demonstrated that orally consumed serum based bioactive proteins, marketed as LIFELINE®, have beneficial effects on the body systems.  When given to experimental animals, they helped support and maintain the normal balance of the immune system response to stressors present in the different body systems.  Use of these performance supplements may help the horse to maintain a balanced immune response, particularly during stressful life events.

Next week, Part Six will discuss a study done in Denmark on mares and foals.