Often horse owners are unaware of problems that affect their horse’s health in less obvious ways. Endocrine diseases are classic examples. As a horse owner, it is important to be aware of two of the common endocrine diseases Cushing’s disease (PPID) and Insulin Resistance (IR), and understand that they usually manifest in subtle ways. Changes in a horse’s appearance that are caused by these diseases are often missed, and mistaken for normal.
The endocrine system is the hormonal, regulatory system of the body. Endocrine glands are tiny organs which secrete hormones and substances into the bloodstream as necessary to maintain control of body systems. These organs include the hypothalamus, pituitary gland, thyroid gland, parathyroid glands, adrenal glands, pineal body and the reproductive organs. The pancreas is also on this list as it has both hormone production and digestion duties. Hormones and their actions affect a broad range of essential life functions. For the horse with PPID or IR, the metabolism is effected which regulates energy, protein, fat and mineral use, as well as helping to reduce inflammation.
Equine Cushing’s Disease (now called Pituitary Pars Intermedia Dysfunction- PPID )
Cushing's syndrome is caused by the malfunction of the pituitary gland in the brain. The pituitary gland dysfunction in horses stems from overgrowth, meaning hyperplasia or tumor, of certain cell types within the gland’s pars intermedia. These cells are hyperactive or present in high numbers, and thus lead to production of abnormally high levels of many pituitary hormones. This list includes adrenocorticotropic hormone (ACTH), While not all of the effects of these hormones are known, some like ACTH are better understood. Specifically, ACTH overstimulates a horse’s cortisol synthesis by the adrenal glands. This hyper-cortisolemic state leads to the long list of outward problems in the affected animal. The more common signs observed in PPID-affected animals are:
- Failure to shed fully
- Long, sometimes curly, haircoat – this can begin as long hairs along the jaw line and “feathers”
near the fetlocks
- Chronic infections
- Repeated laminitis episodes, sometimes with associated hoof abscesses
- Excess or inappropriate sweating
- Increased water intake and urination, called polyuria/polydipsia (PU/PD)
- Lethargy
- Loss of muscle mass (later in process, if horse is untreated) – typically noticed over the back
and hind quarters, as well as the “pot-bellied” appearance
- Infertility or lack of estrus cycles
- Abnormal mammary gland development
The veterinary community has not yet determined the cause and effect for every PPID sign. One understood consequence is the effect of long-term high blood cortisol concentrations. Cortisol is called a “stress” hormone and leads to immune system suppression over time. This suppression can result in chronic infections in PPID-affected horses or ponies. In addition, chronic elevations in cortisol are thought to be related to a state of high insulin secretion and greater potential for insulin resistance to develop.
The most disheartening and damaging clinical sign of PPID is laminitis. Typically, this complication has a slow, insidious onset. Hoof changes and pain develop gradually and are often intermittent, but typically leading to chronic foot pain and anatomic changes. This disease is thought to be related to cortisol and other hormonal effects on glucose (aka sugar) action on the tissues of the hoof wall. Once a diagnosis has been made, the challenge for owners of PPID horses is to find and develop a “team” of people to aid in managing, not curing, both PPID and laminitis. This team ideally includes one’s veterinarian, a farrier, and the horse’s caretakers.
Additionally, an integral part of PPID management is medical treatment. In all cases, the general approach to treatment is to make the horse as comfortable as possible and to slow down the advancement of the condition as much as is practical. The most widely used drug is pergolide. This is the generic name of the drug which is a dopamine agonist. That substance is naturally produced in all of our bodies, and it serves to decrease some of the hormone production by the pituitary gland. Today, with our improved diagnostic and treatment options, Cushing's horses are living longer and enjoying a better quality of life.
Insulin resistance (a/k/a Peripheral Cushings Syndrome, which has been given the name Equine Metabolic Syndrome – EMS)
Insulin Resistance causes the body to become less sensitive to insulin, preventing proper control of blood sugar levels. Diets that are higher in carbohydrates exacerbate this state because they stimulate further insulin production when eaten. Insulin Resistance is related to Cushing's Disease, however horses suffering from insulin resistance tend to become obese very easily and may have abnormal fat deposits in the neck, shoulders, loin, sheath, above the eyes and around the tail head even when the rest of the body appears to be in normal condition.
A major concern with Insulin Resistance is that it appears to be linked to pasture-associated laminitis in horses and ponies. The mechanism by which laminitis associated with IR occurs is not completely understood but may be triggered by sugar and starch in the diet of susceptible individuals. Management of horses and ponies with IR focuses on therapy for laminitis and dietary adjustments with the aim of limiting the stimulus for insulin production. Unfortunately, because of the abundance of high carbohydrate-content commercial feed options, diet supervision can be extremely difficult for owners.
In order to decrease this stimulus for insulin secretion, an alternate feeding regimen with a low glycemic index is recommended for these animals. Glycemic index signifies the degree to which a certain food raises blood sugar and insulin levels in the body. Molasses-based diets, such as sweet and senior feeds, as well as oats and barley have high glycemic indexes. Low glycemic index feeds include Bermuda grass hay, rice bran and beet pulp. Other hays, such as timothy and alfalfa, have moderate glycemic indexes. An important recommendation is to feed grass hay or other feed sources which are low in water-soluble carbohydrates (WSC) or non-structural carbohydrates (NSC).
The goals in feeding a horse with an endocrine disorder are to decrease body fat in obese horses and to avoid feeds that will worsen insulin resistance. Establishing an appropriate feeding plan for a horse with an endocrine disorder (PPID/Cushings disease, Equine Metabolic Syndrome, and/or Insulin Resistance) can be challenging and is tailored to each horse’s specific situation. As a result, it is important to discuss your horse’s endocrine status and nutritional needs with your veterinarian and to closely monitor your horse’s body condition, appetite and presence of any foot soreness in those horses that are at high risk of developing laminitis. Regular health check-ups are critical to long-term care. This includes the performance of basic blood work, dental care and regular foot/hoof trimming and care.
By being on the lookout for the signs of these diseases, there is an opportunity for diagnosis and treatment, and in so doing, an opportunity to improve the quality of life for your horse. If your horses are showing even subtle signs of either of these diseases, contact your veterinarian promptly for the best diagnosis and treatment options.
Reference links:
http://www.aaep.org
http://www.equineelders.org/metabolic.htm
http://www.thalequine.com/articles.html
http://equisearch.com/horses_care/health/anatomy/endocrinesystem_062907/index1.aspx
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