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| Equine Metabolic Syndrome |
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 Horsemen have known for decades that obesity is an unhealthy condition in horses.
In olden times, overweight horses were not common because they were required to toil all day behind a plough or a herd of sheep or cattle. Their reward for a day’s work was suitable feed, but not enough to become too fleshy.
In this age, obese horses are more the rule rather than the exception. Despite efforts to keep horses in moderate body weight, well-meaning horse owners still offer high-calorie concentrates and hay. More times than not, the result is a plump middle-aged horse that is anything but healthy or athletic. He gasps for breath when subjected to mild exercise, and his limbs bear the brunt of unnecessary pounds.
Horses with equine metabolic syndrome (EMS) tend to be obese, insulin resistant, and predisposed to develop laminitis. Nutritional management of EMS cases is important to reduce body fat and the chances of laminitis. Grazing lush springtime pasture that is high in sugars and fructans is an important risk factor to induce laminitis in horses affected with EMS.
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Insulin Resistance: The Root
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Equine metabolic syndrome is characterized foremost by insulin resistance, defined as a peculiar physiological response to the ingestion of foods digested to glucose or other sugar molecules.
Abundant in certain feedstuffs commonly fed to horses, glucose causes a normal state of hyperglycaemia or elevated sugar in the blood. Hyperglycaemia prompts the release of insulin from the pancreas, which encourages the removal of glucose from the bloodstream by fat or muscle cells. Once in the cells, glucose can be put to work immediately to fuel exercise or growth, or stored as glycogen or fat for later use.
Insulin resistance implies that either the central tissue (liver) or the peripheral tissues (the skeletal muscle or the fat cells) are relatively insensitive to the action of insulin or that the quantity of insulin released by the pancreas in response to hyperglycaemia is diminished. This leaves glucose circulating in the blood. Because glucose levels do not drop, the pancreas continues to discharge insulin, leading to elevated concentrations of insulin in the bloodstream, known as hyperinsulinemia.
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What predisposes a horse to insulin resistance?
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Little is known on this front but responsibility might rest on genetic, gestational, and environmental factors. In humans, causative factors are well documented: aging, pregnancy, smoking, reduced physical activity, and obesity. In genetically susceptible humans, glucose intolerance can lead to noninsulin-dependent diabetes mellitus.
In the equine model, obesity appears to be related to the onset of metabolic syndrome. One suggested cause is that certain fat cells produce cortisol, among other hormones, which interferes with the ability of insulin to move the glucose into the cell. Because obese horses have more fat cells, more cortisol is produced and there is greater interference with insulin. This explains why weight reduction is effective in increasing insulin sensitivity.
Not all fat horses are insulin resistant. Current beliefs hold that horses whose fat cells produce high levels of leptin as well as cortisol are the ones prone to insulin resistance. Leptin is not causing insulin resistance but is found to be higher in horses that are insulin resistant.
Age and diet may be directly related to the development of equine metabolic syndrome. Age is thought to decrease the horse’s sensitivity to insulin. Meals high in starch and sugar cause significant spikes in blood glucose and insulin, and years of consuming such meals might lead to insulin resistance.
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Obesity-Associated Laminitis
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An overwhelming clinical sign of equine metabolic syndrome is laminitis, but not the disabling, painful disease related to gastrointestinal failure and endotoxemic insults. The laminitis exhibited by these obese, middle-aged horses tends to be mild. On occasion, so minimal are the laminitic episodes that knowledgeable, conscientious horse owners cannot vouch definitively for any clinical signs of lameness.
The hard evidence speaks a different tale, however. Abnormal hoof growth occurs. Dropped soles, unusual growth lines, and separation of the hoof at the white line are frequently observed. More damning, however, is the shifting of the pedal bone within the hoof, which is obvious upon radiography.
But what causes the laminitis? Over the years, the cause of laminitis in obese mature horses has been attributed to:
(1) endocrine disorders, namely hypothyroidism or high cortisol production;
(2) aggravation of a pre-existing laminitis caused by endotoxemia (over consumption of grain, for example); and
(3) mechanical inadequacy due to the stress of excessive weight on soft tissues of the leg.
Regardless of the cause, laminitis is the result of changed circulation to the laminae, the interconnected layers of tissue that insure the integrity of the hoof. In obese horses, researchers believe that insulin insensitivity and vascular spasms may incite changes in the endothelial tissue of the laminae. On a physiological level, this concurs with the circulation problems observed in human patients with noninsulin-resistant diabetes mellitus. Despite well- founded theories, a definitive cause for obesity-associated laminitis remains elusive.
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Diagnosis
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At this time, diagnosis of equine metabolic syndrome is based on description and physical characteristics, results of glucose-tolerance testing, and elimination of similar conditions.
Description and physical characteristics. Affected horses are usually between the ages of 8 and 18, though numerous patients have fallen outside this range. Horses and ponies of nearly all breeds have been diagnosed, though Morgans, Peruvian Pasos, Paso Finos, domesticated Spanish Mustangs, and warmbloods appear to be especially predisposed to the syndrome. As a group, ponies tend to become overweight more readily than horses and are often inclined to suffer from laminitis.
What's more telling than either age or breed of the patient is distribution of exterior body fat. Areas of unusual fat accumulation include the top of the neck (commonly called the crest), over the shoulders, and the rump (including deposits over the croup and just above the tailhead). Significant fat sometimes settles in the sheaths of geldings or around the teats of mares, so much so that they may appear swollen. Affected broodmares show unusual oestrous cycling, which makes them incredibly difficult to get pregnant.
Anecdotal evidence by owners is also instrumental in diagnosing equine metabolic disease. Owners frequently describe their horses as easy keepers, finding it virtually impossible to reduce the weight of these horses by calorie restriction alone. Many report that high-calorie feeds such
as grain are not being fed.
Results of glucose-intolerance testing. Veterinarians often perform an oral or intravenous glucose tolerance test on horses they suspect to be insulin resistant. Following the administration of glucose, insulin and glucose responses are measured and compared against the responses of normal horses. This test should be performed on a fasted animal so glucose from a recent meal does not shade the results of the assessment.
According to some equine nutritionists, the only truly effective method of diagnosing insulin resistance is the 'euglycemic hyperinsulinemic clamp'. The procedure is complicated, time- consuming, and can be expensive. Because of these limitations, veterinarians typically diagnose on clinical signs alone.
Elimination of similar conditions. In the past, EMS was often mistaken for other metabolic disorders such as hypothyroidism or Cushing's syndrome.
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Nutritional Management
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In most cases, a weight-loss diet begins by removing concentrates from the horse's ration and reducing grazing time on lush pasture.
If possible, exercise should also be increased.
Forage needs can be met by mature hay with low energy content that still provides some gut fill. As a general rule for weight loss, hay or other forages should not be fed at more than 1.5% of body weight. Forage can be fed in hay nets or hay bags with small holes to slow feed intake.
Feeding forage in three or four small meals during the day will also help prevent boredom and stable vices.
It is important to ensure that the diet continues to provide essential amino acids so that there is no loss of lean body mass. Whenever dietary intake is reduced, it is advisable to feed a vitamin and mineral balancer pellet to ensure adequate intake of other nutrients.
Some horses may consume up to 5% of their body weight per day if they are given free-choice access to forage. It is important to weigh feed and forage so that the owner can accurately know how much is being fed and dietary changes can be made accordingly.
Increased exercise and turnout must be balanced with reduced pasture intake, especially if pasture is lush and fast-growing. Grazing muzzles have been shown to be very effective to reduce pasture intake and still allow the horse to exercise. Simply reducing turnout time may not reduce pasture intake, especially in ponies. In one study, obese ponies did not lose weight when turned out for 12 hours per day, probably because they ate more while turned out. Another study showed that ponies could eat 40% of their expected daily dry matter intake or 1% of their body weight when turned out onto pasture for only 3 hours.
With dietary restriction and increased exercise, some horses may have noticeable weight loss within 2 to 3 months. Others, however, can be extremely difficult to trim down despite a restricted diet. Research in ponies and other species indicates that a loss of 0.5% of body weight per week is reasonable.
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