Written By Dr. Kris Hiney
In previous articles we have discussed some of the key strategies in preventing laminitis in the equine. Many of these have centered on grazing strategies which limit the horse’s access to pastures high in fructan content. Remember that fructans are carbohydrates which are enzymatically unable to be digested in the small intestine of the horse. These fructans pass into the hindgut of the horse where they are fermented by the microbial population, specifically gram positive bacteria. The production of certain organic acids and amines enhance the permeability of the gut wall allowing these and other endotoxins to enter the bloodstream of the horse and ultimately effect the circulation to the digit. However, it is not practical to simply right off all horses’ ability to graze. Rather, we should try and identify those individuals which may have a susceptibility to fructan content in the grass. With this month’s article, we will try to identify which individuals may be at risk, and other strategies that may be employed to reduce your horse’s risk.
While the outward appearance of your horse may give you an indication to whether they are susceptible to laminitis (See Carbohydrates III: Metabolic Syndrome), there may be more to it than just which horses are overweight. There certainly appears to be a genetic link to laminitis, with pony breeds leading the list of susceptible horses. Their comparatively thrifty genotype may make their utilization of carbohydrates and insulin sensitivity differ from breeds which typically do not possess these characteristics. For example, thoroughbreds, which typically have the reputation for being “harder keepers” do not experience the same rate of laminitis. However, the lifestyle and management of thoroughbreds may differ significantly enough to partially explain the decreased incidence of laminitis. Even within ponies, there does appear to be a decided link to genetics. In a study examining the pedigrees of an inbred herd of ponies, 37% of these ponies had experienced laminitic episodes. Of those, half had at least one parent which had also experienced laminitis. Even in controlled research trials which attempt to examine the effects of various carbohydrate loads on horses, wide variability exists between individuals. This leads to the supposition that individual variation, thus genetics, is at play. Thus, if you aware of your horse’s pedigree and know of relatives which have experienced laminitis, you might want to manage your own horse more carefully. Perhaps some day the genes which make a horse more susceptible to laminitis will be identified, and we can use genetic tests in developing management protocols.
As mentioned previously, development of obesity and insulin resistance certainly predisposes the horse to laminitis. One theory behind the development of laminitis in the insulin resistant horse is the glucose deprivation model. When a horse becomes insulin resistant, more and more insulin release is needed to elicit a normal tissue response. In essence, the tissues become “desensitized” to insulin. One of the key roles of insulin in the body is to allow cellular uptake of glucose. Due to the polarity of glucose, it cannot freely enter the cell without the presence of specialized protein transporters. Glut 4 is a protein transporter which is located internally in the cell until insulin binds to the cell membrane. Binding of insulin to the receptor causes a cascade of intracellular reactions to occur and initiates the translocation of Glut-4 to the cell membrane. The insulin insensitivity may result in Glut 4 no longer moving to the cellular membrane, and the inability of glucose to enter into the lamellar tissue of the foot, thereby starving it of glucose. A recent study looked at the presence of different glucose transporters found in skeletal muscle, the coronary band and lamellar tissue. Glut-4 is the insulin dependent transporter found primarily within muscle, while Glut 1 is found in other tissues which have non-insulin dependent uptake of glucose, such as the brain. While Glut 4 was heavily expressed in skeletal muscle, only Glut 1 was found within hoof tissues of both normal and insulin resistant ponies. Therefore, glucose uptake in the hoof is thought to be insulin independent and glucose deprivation within the hoof is unlikely to be the cause behind laminitis. However, in a subsequent study, laminitis was induced in normal healthy ponies using a hyperinsulinemia-euglycemia clamp technique. In this model, insulin is infused into the ponies at a constant rate, while glucose is infused at a sufficient rate to maintain euglycemia, or normal blood glucose levels. Therefore, it is not an absence of glucose which causes laminitis, but perhaps the sustained levels of insulin or other hormones which causes this disorder. This would certainly support the observation of the increased laminitis risk to the insulin resistant horse which suffers from hyperinsulinemia.
If owners wish to try and avoid the development of insulin resistance, the diet the horse receives may be critical. Diets which avoid high amounts of sugars and starches, and have a low glycemic response, result in less insulin release. For horses which still need a significant amount of calories, diets which are fat and fiber based and properly formulated, rather than those which provide a higher glucose or insulinemic response, may prevent the development of insulin resistance. Certainly just monitoring body condition in the horse may be the easiest way to avoid insulin resistance. Although if you ask any horse owner if that is easy you may get a different response! In addition, horses which receive regular exercise seem to be fairly protective against laminitis. However, it is difficult to know whether the exercise regimen aids in increasing insulin sensitivity, or is simply protective against obesity.
Many horse owners wonder if there is a magic pill or supplement that they can provide their horse in order to prevent laminitis. One approach is to reduce the gram positive, lactate producing bacteria which prefer to ferment sugars and fructans. Antibiotics are commonly used in the livestock industry in order to promote growth by shifting the microbial population within the gut. Some antibiotics select against gram positive bacteria, thus have been studied in the horse as a way to prevent laminitis. While this may work, the use of anti-biotics in livestock for growth promotion has been banned in the Europe Union over concerns of anti-biotic resistance. Similarly many in the United States have followed suit, searching for other ways to influence growth and increase immune status. The use of probiotics and prebiotics may influence the gut microflora in favor of less potentially problem causing bacteria. Ironically enough, short chain fructo-oligosaccharides have been demonstrated to improve insulin sensitivity, if not glucose levels, in obese horses. However, none of these methods have been proven to prevent laminitis. I would caution individuals to monitor diet, grazing patterns, and body condition first, before relying on supplements to prevent laminitis.