Equine metabolic syndrome

Also known as Peripheral Cushings Disease and Equine Syndrome X

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 impared, despite often elevated concentrations.

Ponies and breeds that evolved in relatively harsh environments with only sparse grass (such as domesticated Spanish Mustangs and Peruvian Pasos, among others^[1]), tend to be more prone to EMS and insulin resistance, possibly as a survival mechanism (i.e. lay down fat when conditions are good).

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Pathogenesis

In EMS, peripheral adipocytes (fat cells) synthesise adipokines which are analogous to cortisol, resulting in Cushings syndrome-like symptoms^[2] - however, cortisol levels are grossly normal^[3], and there is no pituitary dysfunction. In addition, omental adipocytes may produce the enzyme 11β hydroxysteroid dehydrogenase, which "regenerates" active cortisol from its inactive metabolite, cortisone^[2], 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 (fat), which is a known cause for insulin resistance^[3].

EMS is also implicated in the development of laminitis^[4]; 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^[5] - this may support the argument that EMS is an evolved survival trait^[6].

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 polydipsic, acyclic and generally lethargic^[3][7].

Often, the initial presentation is as a laminitic displaying signs of Cushings, but with a normal pituitary-adrenal axis^[7]

Treatment

The basis of treatment is managing the horse's adipocyte level - i.e., weight and obesity control by dietary restriction and exercise^[8]. Substantial clinical improvement can be achieved with as little as 5-10% weight reduction^[6]. 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^[2]. 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^[6].

References

1. ^ L Gray, The Horse News Report February 19 2007, accessed at [1] at 1610 8th July 2007
2. ^ ^{a b c} PJ Johnson, T Messer, E Kellon "Treatment of Equine Metabolic Syndrome" Horse Industry of Alberta 2004 Proceedings. Accessed at [2] at 1625 on 8th July 2007
3. ^ ^{a b c} [3] Proceedings of the Minnesota Veterinary Medical Association, Equine Metabolic Syndrome
4. ^ KH Treiber, DS Kronfeld and Raymond J. Geor (2006)"Insulin Resistance in Equids: Possible Role in Laminitis", The American Society for Nutrition J. Nutr. 136:2094S-2098S
5. ^ DS Kronfeld, KH Treiber, TM Hess, RK Splan, BM Byrd, W Burton Staniar and NW White (2006) "Metabolic Syndrome in Healthy Ponies Facilitates Nutritional Countermeasures against Pasture Laminitis" The American Society for Nutrition J. Nutr. 136:2090S-2093S
6. ^ ^{a b c} PJ Johnson (Michigan Veterinary Medical Association Procedings "Endocrinopathic Laminitis - What is it and what should be done about it?", accessed at [4] at 1620 on 8th July 2007
7. ^ ^{a b} [5] Liphook Equine Hospital, Metabolic Syndrome in Horses
8. ^ PJ Johnson "Metabolic Syndrome in Horses" at [6]accessed at 2135 on 8th July 2007