Note to the reader: Deborah Heubsch, who suggested this new feature in EN, asked, "What is the connection between colic during or after endurance rides and the particular stresses that are endemic to the sport of endurance such as sustained exertion, fatigue, dehydration, etc.?" and several other colic related questions. Her questions led to this first column.

Over centuries man has been witnessing painful and devastating colic in horses. This has led many to consider the bowels of the horse to be the weakest link.

There is a significant and undeniable relationship between the design of the sport of endurance and colic in our horses. This close relationship is the result of many common management practices combined with the effects of prolonged exercise on the normal physiological parameters of the horse. This complexity is the reason that we see horses die from colic before, during and after competitions on a fairly regular basis.

Over the years we have recognized the significant stress we place on the horses with long-distance hauling. We have also recognized the danger of dehydration whether it comes from travel, competition or plain hot weather. Now it is time to consider even more complicating factors and attempt to explain their relationship to two common forms of colic seen in the endurance horse.

Impaction colic is most commonly seen in the large intestine. It is described as a mass of ingesta slowed or stopped within the bowel lumen, causing unpredictable levels of pain. The significance of pain and the danger that this type of colic presents is related to the extent of the impaction, certainly, but is also related to cause, time to recognition, and treatment.

The object of treating this type of colic is to provide a fluid environment, control pain, and reestablish motility to the area. This will allow breaking up and evacuation of the impaction without harming the bowel itself.

Ileus, a more and more commonly seen type of colic, describes the slowing or stopping of motility of the small intestine and results in distension of the stomach. Again, the level of pain as well as the seriousness of this type of colic is related to the cause, timely recognition, and appropriate and timely treatment. Treating an ileus consists of keeping their stomach empty to relieve pain and prevent rupture until motility of the small intestine is reestablished. This sounds simple enough but, trust me, it is not at all simple.

As for the cause of colic in relation to endurance exercise, we must first address the age-old enemy, dehydration. Dehydration results in poor perfusion and loss of motility to the gastrointestinal tract. Increased contact of feces with the surface of the gut results in continual absorption of fluids. This is the normal response to improve hydration, but unfortunately under these circumstances it results in dry hard feces and abnormal motility. During the typical endurance ride, a level of dehydration becomes apparent, if measured within minutes of loading the horse in the trailer, and continues until the horse is returned home.

The severity of this lurking dehydration can be affected by certain management techniques for better or worse. Eating hay in the trailer, availability of water, distance traveled, and environmental factors can significantly effect the level of dehydration.

When the horse arrives for the ride in an already dehydrated state his ability to recover prior to the ride is unlikely. It is also questionable whether the horse has the ability to improve his level of hydration once the ride has started. Speed, distance, weight carried, level of conditioning, environmental temperature, availability of water, as well as level of anxiety will continue to effect his hydration status. As most of us recognize, the majority of endurance horses finish considerably more dehydrated than when they start. And again, they stand, eat dry hay, and haul yet more hours home. We can manage around some of these factors, but the sad truth is dehydration is a significant concern for the endurance horse, from the time he leaves home until he returns. This factor alone contributes to the large majority of impaction colics seen before, during and post ride.

Another significant and complex set of factors can affect the metabolics of the horse and result in colic. All of the above factors, speed, distance, etc., as well as hair coat, the amount of fat he carries, conformation, diet, and any pain or discomfort he may been in, will all contribute to his ability to regulate his core temperature during exercise. Again, management can affect these parameters to some extent.

As exercise continues, energy is made and used, and heat is produced within the muscles. Circulation shifts away from the bowel, both to supply oxygen to the muscles for continued work and to relieve the build up of heat in the muscles. Consequently, the gut slows, as digestion is not a priority while the horse is running. The horse is now shifting his blood to his muscles and skin to allow for moving and cooling. Sweating, also leading to increased dehydration, continues as long as the level of hydration allows.

Moving before cooling is a priority to the horse. It allows him to outrun his enemies when necessary. At the point of severe dehydration and serious overheating, the horse continues to move as the muscles takes priority. The sweat takes on a different appearance and eventually stops. The skin becomes clammy and the core temperature continues to rise. The circulation continues to shift away from the bowel and the skin, allowing for increased core temperature, increased dehydration, and increased likelihood for impacation or ileus.

An example to remember: Speedo may continue moving well past his ability to cool, and he will not be sweating, so he may not appear hot. If this point of overheating is obtained 10 minutes prior to reaching a stop, Speedo has the potential to raise his core temperature .22 degrees C per minute. If Speedo was actively cooling at 102 degrees F but stopped being able to control it at 104 degrees, by the time he arrives at the stop he could reach 108 to 109 degrees F, doing considerable metabolic damage. There is certainly enough heat to disrupt the motility of the gut, resulting in severe metabolic disease as well as ileus and impaction. At this point, these may well be life-threatening.

I have tried to give a very brief overview of some of the factors that put the horse at risk for colic when we are competing. I have tried to make you aware of some of the environmental and management factors and their ability to contribute to the disruption of normal physiology ending in colic and, too frequently, death. I would be happy to continue this explanation in more detail or expand to cover possible treatments etc., as the readers choose.

Jeannie Waldron, DVM, is a member of the AERC Veterinary Committee and practices at Rectortown Equine Center in Rectortown, Virginia.