Written By Dr. Kris Hiney

Earlier we posted an article on the typical causes of laminitis and some feeding strategies that may help in preventing laminitis (Feeding Horses for the Prevention and Management of Laminitis).  We also discussed how we might approach feeding a horse which has already experienced  laminitis.  This month we will begin to delve deeper into the causative factors of laminitis and how to prevent its development.

Remember that laminitis is actually a systemic disease, with just the symptoms being visualized within the hoof.  Some insult to the horse’s system creates an alteration in circulation, ultimately leading to tissue damage of the sensitive laminae.   This period of time, referred to as the development phase or prodromal phase, includes the actual insult prior to the development of symptoms.    It is thought to be due to some change within the vasculature, that leads to ischemia, or lack of blood flow to the digit.  Whether it is a lack of nutrients or oxygen due to the decreased blood flow, tissue damage or death results.  This initial insult is then followed by the acute phase of laminitis.

In the acute phase, blood flow returns to the foot, typically at an increased rate, as this is the body’s normal response to tissue injury.  Even this reperfusion response can cause tissue damage.  This is when the owner now recognizes the development of symptoms. The horse will appear very stiff and reluctant to move, and may even lie down.  A particular stance may be assumed by the horse as he appears to rock back on the hindquarters and place the forefeet forwards in order to limit weight bearing.  Due to the pain and anxiety the horse is experiencing, his heart rate and respiration rate may both be elevated.  Finally, the owner may even be able to detect an increase in temperature of the hoof wall and a bounding or throbbing digital pulse.

If permanent changes to the architecture of the foot occur, whether in PIII displacement, permanent changes to the normal lamellar architecture of hyperkeritization of the hoof wall, the horse has entered into the chronic phase. These horses are typically more susceptible to recurrent episodes of laminitis. Diet restrictions and specialized hoof care are typically required to allow the horse to lead a comfortable life (Feeding Horses for the Prevention and Management of Laminitis).
Obviously laminitis can be a devastating disease for the owner, and most would strive to prevent the disease, rather than address a symptomatic horse. If we look more closely at the causative factors in these alterations to circulation of the horse’s digit, we may be able to do a better job at identifying horses at risk.
We will begin with a review of pasture associated laminitis, which has been addressed in previous articles (Feeding Horses for the Prevention and Management of Laminitis and Carbohydrates III: Metabolic Syndrome). Remember that pastures grow more rapidly at certain times of the year, and may store their energy as different types of polysaccharides depending on the species of the plant. Frequently fructans are noted as the culprit in causing laminitis. Fructans are found in greater quantities in cool season grasses, as well as during periods where photosynthesis is favored over plant growth. As fructans contain beta bonds, which are not digested enzymatically by the equine small intestine, they pass through the tract and arrive at the large intestine where they then undergo bacterial fermentation. Other types of carbohydrates may act similarly, including sugars and starches which escape the small intestine undigested (the classic carbohydrate overload model of the horse in the grain bin), as well as other types of carbohydrates that may be rapidly fermented (pectins, resistant starches etc). (For more information on carbohydrates, please revisit Carbohydrates: Definitions and Relationship to Equine Diseases.)

In this first model of carbohydrate/rapidly fermentable fiber overload, too much of this rapidly fermentable material reaches the hindgut of the horse. These feedstuffs favor the proliferation of a particular bacterial population. These bacteria produce more lactate as their excretory waste. Excess lactate production lowers the pH of the hindgut which allows the mucosal cell wall to become permeable. In addition, too low of a pH stresses the bacteria causing them to either die or release endotoxins. Furthermore, altering the bacteria’s environment also changes their metabolism, releasing vasoactive amines into the hindgut. As the gut wall becomes more permeable, these toxins and amines are able to cross the mucosal wall and enter the bloodstream of the horse, where they then can exert their effects at the level of the hoof.

Horses which need to be restricted from pasture typically include ponies, as they are highly susceptible to laminitis.  In addition, any horse that has a history of laminitis, or has been diagnosed with PPID (pituitary pars intermedia disfunction)(Carbohydrates III: Metabolic Syndrome) should be grazed with care.  Horses with elevated insulin levels, or insulin resistant horses, also have a greater sensitivity to pasture associated laminitis, due to the influence of insulin on the vasculature of the horse.  Hyperinslulinemia increases the production of endothelin-1, and down regulates the production of nitric oxide.  A decrease in nitric oxide production has been linked to an elevation of homocysteine.  Incidentally, elevations in blood homocysteine are also linked to heart disease in humans. There has even been a suggestion, although with no scientific data to support this theory, that excessive supplementation of sulfur amino acids to horses with insulin resistance is unwarranted.  Typically sulfur amino acids (ie methionine) are included in hoof supplements, and homocysteine originates from methionine metabolism.   To further confuse the issue, it does appear that individuals are more susceptible to this disease, even if they are the same age, sex, breed and are managed the same as non-effected individuals.

Obviously preventative measures aimed at reducing carbohydrate related laminitis issues center on diet management. Certainly it is not practical or even advisable to state that all horses must be kept away from pastures. However, knowledge of which horses are susceptible to pasture associated laminitis is key. Once these individuals are identified, they should be placed on a principally harvested forage diet. The forage chosen should have very little rapidly fermentable material. However, horse’s which do not have these susceptibilities can continue to be managed with relative ease.
Next month we will continue to discuss laminitis at a more detailed tissue level, and address further strategies to limit your horse’s chances of acquiring this disease.