At Fox Creek Veterinary Hospital, we take the health of the whole horse seriously. This includes assessing hair coat quality, hoof health, and body condition score at each visit. These are some of the markers of endocrine health. As horses age, they become increasingly susceptible to endocrine issues, including Cushing’s disease. Overweight horses are often affected by Equine Metabolic Syndrome. True hypothyroidism is rare in horses, but low thyroid hormone levels can be seen in conjunction with other diseases. We offer nutritional consultation and blood test diagnosis for various endocrine conditions to help you manage your horse’s hormonal health.
Read on for an article about hyperinsulinemia, a common link between several equine endocrine disorders.
What is hyperinsulinemia (and why is it relevant)?
Cushing’s Disease. Equine Metabolic Syndrome. “Easy Keepers.” You’ve probably heard the buzzwords thrown around, and you may know what all of these conditions have in common: a propensity for affected horses to develop laminitis, a painful and potentially life-threatening condition resulting in inflammation of the feet and disruption of the coffin bone’s attachment to the hoof. But why?
Cushing’s (more accurately termed Pituitary Pars Intermedia Dysfunction, or PPID) is a disease primarily seen in middle-aged and older horses. Clinical signs include chronic laminitis, a long haircoat that fails to shed, abnormal sweating, increased urination (with subsequent excessive consumption of water), greater susceptibility to infections, and regional adiposity (deposition of fat in specific areas of the body, most commonly the crest of the neck, but also the topline and sheath/mammary area, which in PPID can occur even in the face of generalized weight loss and muscle wasting).
Equine Metabolic Syndrome (EMS) is a multifaceted disorder characterized by inflammation, generalized obesity, and regional adiposity. Not all overweight horses have EMS, but for those that do, the risk for laminitis is greatly increased. While PPID results from abnormal hormone secretions caused by a tumor in the brain’s pituitary gland, the causes for EMS are less well understood, but a combination of genetic predisposition and environmental/management factors are suspected.
Although they may coexist in the same horse, PPID and EMS are two distinct clinical conditions with different risk factors, causes, and symptoms. So what is their common link to laminitis? The most important universal factor, as it turns out, is hyperinsulinemia, which simply means excessive insulin concentrations in the blood. Insulin is a hormone secreted in the pancreas, normally in response to a meal. It stimulates uptake of sugar from the blood (absorbed through the intestine after eating) by organs such as the liver, and therefore plays a major role in metabolism and sugar regulation.
Insulin Resistance (IR) is common in horses affected by EMS and PPID. This means that insulin no longer works as effectively, because the body’s tissues are no longer as sensitive to its actions. Therefore, more insulin release by the pancreas is required to have the same effect, and hyperinsulinemia may develop. However, studies on horses with EMS have shown that hyperinsulinemia may occur even in the absence of IR, making it the primary problem, with IR developing secondarily as a response to the increased insulin load. The exact reasons for the development of hyperinsulinemia are unknown, but current work is focused on investigating the link between hormone-secreting cells within the intestine which can trigger the pancreas to secrete insulin.
Hyperinsulinemia is dangerous for horses because excessive insulin concentrations are “toxic” to the feet. Insulin is hypothesized to stimulate growth of the hoof laminae, which causes them to stretch, and their attachments to weaken. Laminitis subsequently develops, with the weakened laminae allowing the coffin bone to sink or rotate, and the hoof wall to become distorted. Successful treatment of laminitis can be difficult and costly, and even if the horse recovers and returns to soundness, it may always have some residual hoof deformity, in addition to being predisposed to future laminitic episodes. Ideally, laminitis should be prevented before it ever develops, in order to spare equine suffering and the financial/emotional burden placed on the owner. While laminitis prevention is often easier said than done, managing risk factors plays a major role, and this can start with testing your horse for the metabolic disorders that predispose to it.
There are several tests for hyperinsulinemia. Most commonly, blood is simply drawn at baseline and submitted to a lab for measurement of insulin levels. Another common test is the oral sugar challenge. Here, the horse is fasted overnight, and blood is collected in the morning to test for basal insulin levels. The horse is then administered an oral dose of sugary syrup, and the blood insulin test is repeated one hour later. Hyperinsulinemic horses may have an elevated insulin concentration at baseline, and also will have an exuberant response to the sugar challenge. Other testing can specifically target PPID independent of hyperinsulinemia. We commonly test for endogenous ACTH (adrenocorticotropic hormone, a substance which is elevated in affected horses due to abnormal secretion in the pituitary). ACTH levels can also be measured following stimulation by another hormone, TRH (thyrotropin-releasing hormone), although many alternative tests exist, with variable sensitivity. With all of this testing, it is important that the horse is in its home environment, calm, and not currently affected by other disease, as stress and inflammation can interfere greatly with results. In addition, when evaluating a horse suspected to have an endocrine disorder, the importance of good veterinary physical examination with thorough evaluation of the history cannot be overstated.
Managing horses with hyperinsulinemia consists of reducing the primary condition and avoiding risk factors. There are no “miracle drugs” to cure or prevent hyperinsulinemia. Metformin, a drug designed for human diabetics, has shown some promise, but has not been fully evaluated in horses. Levothyroxine is a synthetic thyroid hormone analog. While it does not directly address hyperinsulinemia, it can help with weight loss on a short term basis by boosting the metabolic rate. For horses who have been diagnosed with PPID, pergolide is a useful medication which can address some of the abnormal hormone production in the brain that predisposes to clinical symptoms.
Exercise has been shown to help reverse hyperinsulinemia. For sound horses (not affected by acute laminitis), exercise can be very beneficial at restoring tissue sensitivity to insulin. It can also help with weight loss, which is another important goal for overweight horses. However, it is important not to “starve” an overweight horses in order to achieve rapid weight loss, because in addition to putting the horse at risk for gastric ulcers and stress, it can result in breakdown of the body’s fat stores for energy (this can be very damaging to the liver, which has to process the mobilized fat). As with many things, moderation is best. Eating a sugary meal, including pasture grass, is a major trigger for insulin release, which is compounded in horses prone to hyperinsulinemia. Utilizing grazing muzzles, dry lots, and slow hay feeders are excellent ways to reduce food intake while still permitting natural behaviors. The use of sugary grains should also be avoided, and most horses will do well on a good forage source supplemented with a ration balancer (such as Purina Enrich) to meet their basic vitamin/mineral requirements. When feeding hay, it can be beneficial to have it evaluated, trying to reduce the content of non-structural carbohydrates (simpler sugars that are associated with greater laminitis risk). Access to pasture grass, especially in the spring and fall (when it is extra “lush”), but also after times of stress (such as drought, when the damaged plants may be storing extra sugars), should be minimized, and horses should be gradually acclimated to pastures, rather than turned out for long grazing spells after a long hiatus. The website safergrass.org hosts many useful articles regarding feeding and pasture management for horses prone to laminitis.
Clearly, hyperinsulinemia is a complicated condition, but one that should not be taken lightly. Your veterinarian can work with you to discuss your individual horse’s risk factors, diagnosis, and ideal management. With proper care, your horse’s laminitis risk can be minimized, which means a better quality of life for him, and more days in the saddle (and a lot less worry!) for you.
How does food affect the feet?
A meal is consumed (1). Nutrients, including sugars, are absorbed via the intestines (2). Nutrient absorption triggers insulin release by the pancreas (3). Insulin travels systemically in the bloodstream, triggering uptake of sugar by the liver and other organs. It also acts at the feet (4), where it is postulated to stimulate receptors in the laminae, the structures which attach the hoof to the coffin bone. The laminae are triggered to grow (5), which stretches them, weakening their attachments and causing pain. Eventually, coffin bone rotation and/or sinking can occur, and the hoof is left deformed (6).