Equine Metabolic Syndrome

By: L. D. Shamis DVM, MS
   
As with people, over the last few decades, we are seeing an increasing number of overweight horses.  Historically, the vast majority of horses more were “working” animals being fed high energy diets to address their daily “work” requirements.  It is generally believed that the increased frequency of obesity is the result sedentary lifestyle coupled with inappropriately rich supply of dietary energy in horses.

Along with exercise intolerance, veterinarians have long recognized an association between laminitis and varying degrees of obesity.  Attempts to understand the cause of laminitis have been challenging.  Recently, new information is now revealing the striking similarity between insulin resistance in humans known as Metabolic Syndrome and what we heretofore referred to by many different names (hypothyroidism, Cushing’s Syndrome, Syndrome X etc).  Current understanding of the condition has led to the name Equine Metabolic Syndrome.

Obesity appears to be related to the onset of Metabolic Syndrome.  It is now known that some fat cells have the ability to produce certain hormones among which is cortisol.  It should be noted that in Cushing’s Syndrome, the increased cortisol comes from the adrenal glands and not from fat cells.  Cortisol directly increases circulating glucose and also appears to decrease the body’s cells response to insulin.  This is called insulin resistance (insensitivity).  Thus, the more fat cells, the more potential cortisol produced leading to greater the inhibition of cellular response to insulin.

We now know that the primary manifestation of EMS appears to be insulin resistance.   Insulin is normally secreted by the pancreas in response to elevations in circulating blood sugar levels after eating.  It acts by directing cells in body tissues to take in glucose to be used for cellular energy needs.  Insulin resistance is defined as the inability of cells in body tissue to appropriately respond and therefore cannot take in glucose.  This leads to chronically high blood glucose levels.  It is now hypothesized that this inappropriately high glucose level leads to “glucotoxicity” in the cells lining the small blood vessels  of the lamina of the foot.  These small vessels constrict thereby impeding circulation  leading to cellular damage and laminitis.  The exact mechanisms of this action are still unclear.

Affected horses are usually 15 years or older but can be younger.  Many of these horses are considered “easy keepers” by their owners.    Horses and ponies of all breeds have been identified with this syndrome.  However, Morgans, Arabians, Paso Finos, Peruvian Pasos, domesticated Mustangs, and warmbloods appear to be predisposed to the syndrome.  Abnormal external fat accumulation is typical in many (but not all) affected horses.  Common areas are the crest of the neck, the rump, above the tail head, and over the shoulders.  Many geldings may have enlarged penile sheaths due excess fatty deposits.  It is not uncommon for affected horses to be chronically or intermittently sore footed.  A telltale sign may be divergent growth lines on the hoof wall.
There are many tests that will help differentiate EMS from Cushing’s and other diseases.  If you suspect your horse has EMS, it is best to contact your veterinarian immediately to discuss the appropriate course of action.

Currently, treatment is directed at increasing exercise and dietary changes by decreasing the percentage of sugar and starch content.  Ideally, hay should contain no more than 10% sugar/starch.   Affected horses generally do best on Bermuda grass hay along with an appropriate vitamin/ mineral supplement.  Avoiding grains in any form, “complete” or “senior” feeds, carrots, apples, molasses, fresh grass is important as they contain high levels of sugar and starch.   There are no generally accepted medications to treat this problem.  In some instances, it may be helpful to utilize a thyroid supplement in order to increase metabolic rate short term,   particularly when laminitis is active.   It is however not meant to replace changes in diet or exercise when the horse is more comfortable.  

If so inclined, you can get hay analysis for starch and sugar content at:

The Dairy One Laboratory in Ithaca, N.Y.  www.dairyone.co

                                                   
Evaluating your horse’s body condition every few months can be done with the help of the Henneke Body Score table.

Learn more at: http://www.umext.maine.edu/onlinepubs/htmpubs/1010.htm