Equine metabolic syndrome

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Also known as Peripheral Cushings Disease and Equine Syndrome XThis 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 othersL Gray, The Horse News Report February 19 2007, accessed at [http://www.thehorse.com/viewarticle.aspx?ID=8963] at 1610 8th July 2007), tend to be more prone to EMS and insulin resistance, possibly as a survival mechanism (i.e. lay down fat when conditions are good).

Pathogenesis

In EMS, peripheral adipocytes (fat cells) synthesise adipokines which are analogous to cortisol, resulting in Cushings syndrome-like symptomsPJ Johnson, T Messer, E Kellon "Treatment of Equine Metabolic Syndrome" Horse Industry of Alberta 2004 Proceedings. Accessed at [http://www.albertahorseindustry.ca/hboc/2004/proceedings/laminitis.pdf] at 1625 on 8th July 2007 - however, cortisol levels are grossly normal [http://www.mvma.org/Proceedings/Equine/Equine3.htm] Proceedings of the Minnesota Veterinary Medical Association, Equine Metabolic Syndrome, 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, 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.EMS is also implicated in the development of laminitisKH 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; 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 ensueDS 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 - this may support the argument that EMS is an evolved survival traitPJ Johnson (Michigan Veterinary Medical Association Procedings "Endocrinopathic Laminitis - What is it and what should be done about it?", accessed at [http://www.michvma.org/documents/MVC%20Proceedings/Johnson1.pdf] at 1620 on 8th July 2007.

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[http://www.liphookequinehosp.co.uk/LabMetabolicsynd.htm] Liphook Equine Hospital, Metabolic Syndrome in Horses.Often, the initial presentation is as a laminitic 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., weight and obesity control by dietary restriction and exercisePJ Johnson "Metabolic Syndrome in Horses" at [http://www.safergrass.org/full%20articles/METSYNDeq.pdf]accessed at 2135 on 8th July 2007. 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. 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.

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