Equine metabolic syndrome
Encyclopedia
Also known as Peripheral Cushings Disease and Equine Syndrome X.
It is an illness
affecting Equidae
(horses
and related animals).
This is an area of much new research and is increasingly believed to have a major role in conditions such as laminitis
. Although many factors (including cortisol
metabolism and adipocyte
(fat cell) turnover) are involved, the primary cause is insulin resistance
. This is similar to type II diabetes in humans, where the action of insulin
is impaired, despite often elevated concentrations.
Ponies and horse breeds that evolved in relatively harsh environments with only sparse grass (such as domesticated Spanish Mustangs and Peruvian Pasos, among others), tend to be more prone to EMS and insulin resistance, possibly as a survival mechanism (i.e. lay down fat when conditions are good). Such animals are sometimes called "easy keeper
s."
levels are grossly normal, and there is no pituitary dysfunction.
In addition, omental
adipocytes may produce the enzyme 11β-hydroxysteroid dehydrogenase type 1, which "regenerates" active cortisol
from its inactive metabolite, cortisone
, which may induce a form of Cushings whereby normal cortisol levels have an exaggerated action due to a longer systemic Half-life
.
In addition, a hormone termed resistin
has been shown to be produced in adipose tissue, which is a known cause for insulin resistance.
EMS is also implicated in the development of laminitis
; however, recent research suggests that the situation is more complex, in that "compensated insulin resistance is essentially physiological and health sustaining", and only when this compensatory mechanism fails does laminitis ensue - this may support the argument that EMS is an evolved survival trait.
, acyclic and generally lethargic.
Often, the initial presentation is as a laminitic
displaying signs of Cushings, but with a normal pituitary-adrenal axis.
and obesity control by dietary restriction and exercise. Substantial clinical improvement can be achieved with as little as 5-10% weight reduction. In addition, antioxidant
treatment (e.g. Vitamin E
or Chromium
supplementation) have been suggested.
Pharmacological treatments tend to be symptomatic. The condition cannot be cured, but can be managed. Some veterinarians have reported success with pergolide
and cyproheptadine
, although the mechanism of any such action is unclear. Trilostane has also been used extensively as hydroxysteroid-dehydrogenase inhibitor, to prevent overproduction of cortisol. Exogenous thyroid hormone
administration has also been used, on the rationale that thyroid hormones facilitate insulin-mediated glucose
absorption by cells - however, they also facilitate glucose uptake by the gut, and in depth cost/benefit studies have not as yet been published.
It is an illness
Illness
Illness is a state of poor health. Illness is sometimes considered another word for disease. Others maintain that fine distinctions exist...
affecting Equidae
Equidae
Equidae is the taxonomic family of horses and related animals, including the extant horses, donkeys, and zebras, and many other species known only from fossils. All extant species are in the genus Equus...
(horses
Horse
The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...
and related animals).
This is an area of much new research and is increasingly believed to have a major role in conditions such as laminitis
Laminitis
Laminitis is a disease that affects the feet of ungulates. It is best known in horses and cattle. Symptoms include lameness, and increased temperature in the hooves...
. Although many factors (including cortisol
Cortisol
Cortisol is a steroid hormone, more specifically a glucocorticoid, produced by the adrenal gland. It is released in response to stress and a low level of blood glucocorticoids. Its primary functions are to increase blood sugar through gluconeogenesis; suppress the immune system; and aid in fat,...
metabolism and adipocyte
Adipocyte
However, in some reports and textbooks, the number of fat cell increased in childhood and adolescence. The total number is constant in both obese and lean adult...
(fat cell) turnover) are involved, the primary cause is insulin resistance
Insulin resistance
Insulin resistance is a physiological condition where the natural hormone insulin becomes less effective at lowering blood sugars. The resulting increase in blood glucose may raise levels outside the normal range and cause adverse health effects, depending on dietary conditions. Certain cell types...
. This is similar to type II diabetes in humans, where the action of insulin
Insulin
Insulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
is impaired, despite often elevated concentrations.
Ponies and horse breeds that evolved in relatively harsh environments with only sparse grass (such as domesticated Spanish Mustangs and Peruvian Pasos, among others), tend to be more prone to EMS and insulin resistance, possibly as a survival mechanism (i.e. lay down fat when conditions are good). Such animals are sometimes called "easy keeper
Easy keeper
An easy keeper, easy doer or good doer is a livestock animal that can live on relatively little food. The opposite of an easy keeper is a hard keeper , an animal that is prone to be too thin and has difficulty maintaining adequate weight.Easy keepers tend to be found most often in breeds...
s."
Pathogenesis
In EMS, peripheral adipocytes (fat cells) synthesise adipokines which are analogous to cortisol, resulting in Cushings syndrome-like symptoms. However, cortisolCortisol
Cortisol is a steroid hormone, more specifically a glucocorticoid, produced by the adrenal gland. It is released in response to stress and a low level of blood glucocorticoids. Its primary functions are to increase blood sugar through gluconeogenesis; suppress the immune system; and aid in fat,...
levels are grossly normal, and there is no pituitary dysfunction.
In addition, omental
Omentum
Omentum may refer to:* greater omentum* lesser omentum...
adipocytes may produce the enzyme 11β-hydroxysteroid dehydrogenase type 1, which "regenerates" active cortisol
Cortisol
Cortisol is a steroid hormone, more specifically a glucocorticoid, produced by the adrenal gland. It is released in response to stress and a low level of blood glucocorticoids. Its primary functions are to increase blood sugar through gluconeogenesis; suppress the immune system; and aid in fat,...
from its inactive metabolite, cortisone
Cortisone
Cortisone is a steroid hormone. It is one of the main hormones released by the adrenal gland in response to stress. In chemical structure, it is a corticosteroid closely related to corticosterone. It is used to treat a variety of ailments and can be administered intravenously, orally,...
, which may induce a form of Cushings whereby normal cortisol levels have an exaggerated action due to a longer systemic Half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
.
In addition, a hormone termed resistin
Resistin
Resistin also known as adipose tissue-specific secretory factor or C/EBP-epsilon-regulated myeloid-specific secreted cysteine-rich protein is a cysteine-rich protein that in humans is encoded by the RETN gene....
has been shown to be produced in adipose tissue, which is a known cause for insulin resistance.
EMS is also implicated in the development of laminitis
Laminitis
Laminitis is a disease that affects the feet of ungulates. It is best known in horses and cattle. Symptoms include lameness, and increased temperature in the hooves...
; however, recent research suggests that the situation is more complex, in that "compensated insulin resistance is essentially physiological and health sustaining", and only when this compensatory mechanism fails does laminitis ensue - this may support the argument that EMS is an evolved survival trait.
Symptoms
EMS horses tend to become obese very easily and, even when starved down, may have abnormal fat deposits in the neck, shoulders, loin, above the eyes and around the tail head, even when the rest of the body appears to be in normal condition. Some cases may become pot-bellied, perhaps polydipsicPolydipsia
Polydipsia is a medical symptom in which the patient displays excessive thirst. The word derives from the Greek πολυδιψία, which is derived from πολύς + δίψα...
, acyclic and generally lethargic.
Often, the initial presentation is as a laminitic
Laminitis
Laminitis is a disease that affects the feet of ungulates. It is best known in horses and cattle. Symptoms include lameness, and increased temperature in the hooves...
displaying signs of Cushings, but with a normal pituitary-adrenal axis.
Treatment
The basis of treatment is managing the horse's adipocyte level - i.e., weightand obesity control by dietary restriction and exercise. Substantial clinical improvement can be achieved with as little as 5-10% weight reduction. In addition, antioxidant
Antioxidant
An antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons or hydrogen from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions. When...
treatment (e.g. Vitamin E
Vitamin E
Vitamin E is used to refer to a group of fat-soluble compounds that include both tocopherols and tocotrienols. There are many different forms of vitamin E, of which γ-tocopherol is the most common in the North American diet. γ-Tocopherol can be found in corn oil, soybean oil, margarine and dressings...
or Chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
supplementation) have been suggested.
Pharmacological treatments tend to be symptomatic. The condition cannot be cured, but can be managed. Some veterinarians have reported success with pergolide
Pergolide
Pergolide is an ergoline-based dopamine receptor agonist used in some countries for the treatment of Parkinson's disease....
and cyproheptadine
Cyproheptadine
Cyproheptadine , sold under the brand name Periactin, is a first-generation antihistamine with additional anticholinergic, antiserotonergic, and local anesthetic properties.- Indications :...
, although the mechanism of any such action is unclear. Trilostane has also been used extensively as hydroxysteroid-dehydrogenase inhibitor, to prevent overproduction of cortisol. Exogenous thyroid hormone
Thyroid hormone
The thyroid hormones, thyroxine and triiodothyronine , are tyrosine-based hormones produced by the thyroid gland primarily responsible for regulation of metabolism. An important component in the synthesis of thyroid hormones is iodine. The major form of thyroid hormone in the blood is thyroxine ,...
administration has also been used, on the rationale that thyroid hormones facilitate insulin-mediated glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
absorption by cells - however, they also facilitate glucose uptake by the gut, and in depth cost/benefit studies have not as yet been published.