Equine Metabolic Syndrome, In Depth
While the two cases in this month’s Clinical Case Review are quite different in terms of severity of lameness at the time of presentation, as well as their current prognosis for soundness (good for case 1; poor-to-fair for case 2), they share common themes. First, both horses display the hallmarks of Equine Metabolic Syndrome (EMS) as a predisposing feature. They are both overweight; have crested necks; their body condition scores are higher than expected considering the low dietary intake being fed prior to laminitis; and they both lose weight reluctantly in spite of being fed meager rations. Additionally, both horses have elevated blood insulin but normal to high normal glucose. This is the case for most insulin resistant horses because pancreatic insulin secretion increases to normalize blood glucose levels.
Equine Metabolic Syndrome
Equine Metabolic Syndrome, described by Dr. Nicholas Frank in this Concensus Statement in the Journal of Veterinary Internal Medicine, should be suspected in any horse or pony that is predisposed to obesity. These animals are often described as “easy keepers” because weight gain occurs with pasture grazing and obesity persists after caloric restriction. Genetics are likely to play an important role in EMS, and Dr. Frank is collaborating with researchers at the University of Minnesota to examine the genetic basis of EMS. Two important components of EMS are hyperinsulinemia and insulin resistance (IR), and these problems are usually associated with increased adiposity in genetically predisposed horses and ponies. Fat accumulation in certain regions of the body (referred to as regional adiposity) serves as a signal to horse owners and veterinarians that these health problems are occurring. Expansion of the neck crest, appearance of fat pads close to the tail head, and swelling of the prepuce or mammary regions (adipose tissue expansion and decreased lymphatic return) are all hallmark signs of EMS. Animals with this appearance are at greater risk for developing laminitis.
Identification of EMS and prevention of laminitis should be our first goals. Horses and ponies with increased generalized or regional adiposity should be tested for hyperinsulinemia and IR to assess their endocrine status and risk of laminitis. It is easy to screen horses for hyperinsulinemia, so this test should be performed first. Testing is performed under short-term fasting conditions, which means that the horse is placed in a stall, arena, or dirt paddock away from grass and left with only one flake of hay after 10 PM the night before. A single blood sample is collected the next morning and submitted for measurement of glucose and insulin concentrations. Hyperinsulinemia is defined by a fasting insulin concentration > 20 µU/mL (divide pmol/L by 7 to convert units) and this is a positive result. The same horses are likely to have normal or high-normal glucose concentrations, but hyperglycemia is rarely detected. If persistent hyperglycemia is detected in the absence of systemic disease, this indicates the development of diabetes mellitus.
Fasting insulin concentrations are a useful screening test for detecting moderate to severe IR, but mildly affected animals might have normal results. However, the same animals can have difficulty metabolizing dietary sugars and develop pronounced hyperinsulinemia under fed conditions. A second test has therefore been developed to assess these animals – the oral sugar test (OST) is performed under the same fasting conditions as described before and involves the administration of corn syrup (Karo Light®) orally at a dosage of 0.15 mL/kg (75 mL for a 500-kg horse). A blood sample is then collected 60 to 90 minutes later and submitted for measurement of insulin concentrations. Research performed by Dr. Frank’s group has shown that an insulin concentration > 40 µU/mL at 75 minutes indicates that the horse suffers from IR. If neither of these tests provides a clear diagnosis, a combined glucose-insulin test is recommended. This test involves intravenous infusion of dextrose and insulin, with blood samples collected at regular intervals to assess glucose and insulin responses.
Two management plans have been developed for horses and ponies with EMS. The first plan is adopted in the majority of cases and focuses upon obesity and dietary sugar intake. This plan targets the genetically predisposed horse that has become obese as a result of unrestricted pasture grazing or overfeeding. Insulin resistance is associated with increased adiposity and a high-sugar diet in these cases. Key components of this plan include 1) reducing caloric intake, 2) increasing exercise, and 3) limiting pasture grazing. This plan is described in greater detail below. A second management plan has also been developed for subsets of EMS horses that are more severely affected. These are animals that remain obese in the face of caloric restriction, are still insulin resistant after returning to a lean body condition, or are lean when diagnosed with IR. In these cases, IR must be managed more intensively to lower the risk of laminitis, but it is not always possible to achieve normal insulin sensitivity. Intensive nutritional management is required, with low-starch/low-sugar pelleted feeds provided to maintain body condition and hay analysis performed to ensure that the non-structural carbohydrate (NSC) content of the diet is low (< 10% on a dry matter basis). Medications are prescribed to some of these patients and it is important to recognize that some affected animals are at risk for spontaneous laminitis. These persistently insulin-resistant horses should not be turned out on pasture and are ideally housed in large dirt paddocks with companions.
Our objective is to minimize sugar intake–first, to reduce the need for insulin and secondly, to aid in weight loss. We are particularly keen to reduce intake of soluble carbohydrates, including sugars and starches that challenge the glucose/insulin system and fructans that can contribute to the development of laminitis. Soaking the hay in cold water prior to feeding for 30-60 minutes can reduce sugar content, although results vary with different types of hay. And while it is difficult to select specific hays from vendors or to assay the sugar content of each type of hay, sugars can be reduced by avoiding bales that are known to have come from grasses that were exposed to drought conditions, intense sunlight, cold, those that were cut in the late afternoon or those that were fast drying – all circumstances that increase stored sugars. In a study performed at TCSVM by Costa, Hermida and Paradis, 52 different hays from barns in Massachusetts and Connecticut were analyzed by sampling 12 different bales in each batch. The non-structural carbohydrates (NSC), which includes the sugars and starches and is calculated by adding the Water Soluble Carbohydrates (WSC) and starches, varied from 7.9 to 24.2%, and only 12 samples had NSC less or equal to 10%. For horses with insulin resistance, hay with NSC content less or equal than 10% of dry matter is recommended.
Obese horses should initially receive hay in amounts equivalent to 1.0-1.5% of bodyweight. Looking ahead, we will need to limit both horses’ access to green grass, for example, by using a grazing muzzle or limiting grazing to ½ – 1 hour once or twice daily. We can also help by mowing the grass frequently or by grazing it intensively to keep it in a constantly growing state (which uses sugars rather than letting them be stored). Grazing horses late at night and early in the morning can also help since some stored sugars will have been used overnight yet insufficient time will have passed to provide new sugars to replace them. While neither of our cases will need concentrates in their immediate futures, the likelihood of laminitis can generally be reduced in at-risk horses by avoiding feeds that represent a high glycemic load: Sweet feed is definitely to be avoided. Low-sugar/low-starch pelleted feeds are available for purchase and rice bran and alfalfa pellets can be used as alternatives but there is no need to feed any grain to the obese horse. Finally, feeding small quantities of food frequently throughout the day is preferable to feeding just once or twice because it helps moderate the need for insulin.
In animals that are non-responsive to dietary changes, one might add the following medications: levothyroxine sodium (Thyro L) and metformin. Thyro L, a thyroid supplement, has long been used in the obese horse in an attempt at weight reduction, and Dr. Frank’s research has shown that it is helpful in hastening weight lose in some horses. The current dosage recommendation is 48 mg/day for horses weighing 450-525 kg (1,000-1,150 pounds). Metformin is a drug used in humans with insulin resistance. It has been used safely in horses and is indicated in lean horses that suffer from IR to increase insulin sensitivity. The current recommended dosage is 30 mg/kg three times daily. However, the absorption of the drug may vary between individuals.
These two cases illustrate two extremes in podiatry interventions. Case 1 was comparatively straightforward. Our goals at Tufts’ Hospital for Large Animals were to mechanically assist the horse by using a light (aluminum), wide-web shoe, polymeric support putty for the frog and caudal sole, and pads. Collectively, these interventions helped dissipate loading forces during weight-bearing across the entire hoof. They also dissipate the potentially destructive shockwave that propagates up the limb when the hoof first strikes the ground. Taking advantage of the shoe’s 3-degree wedge angle reduces the tensile force that the flexor tendon (DDFT) exerts on the coffin bone, reducing the likelihood of rotation. Finally, using a rolled toe and placing the shoe as far back as possible on the hoof eases break-over, the tension exerted through the DDFT to achieve heel lift-off. Placing the shoe far back on the foot also allows the horse to shift his center of gravity toward the rear limbs via postural change. This ultimately reduces the load experienced by the front hooves.
Case 2 required a more aggressive podiatry regimen. On presentation, the horse was experiencing a severe acute laminitic crisis. Fore- and hind-limbs were affected, and the fore-limb coffin bones were rotated and sinking. Initial management was directed at reducing tension in the DDFT of all 4 feet, this time using roughly 10-degree heel elevation with Redden Modified Ultimate® (RMU) (Nanric Inc, Lawrenceberg, KY) glue-on shoes in combination with polymeric support putty. This combination also shifts weight-bearing toward the back part of the foot. The rockered toe of the RMU also eases break-over. The horse’s lameness improved clinically in response. Venograms were particularly useful in this case, not only because they clearly identified damage to the forelimb vasculature at the time of presentation, but, more importantly, they revealed worsening circulation by day 18, even though clinical lameness was improved. Under these circumstances, tenotomy (transection) of the deep flexor tendon was indicated. Mechanically, tenotomy reduces the pull of the DDFT on the coffin bone, effectively reducing further rotation and sinking secondary to delamination of the sensitive and insensitive laminae. Derotation of the coffin bone should always be carried out in conjunction with the tenotomy. Its goal is to realign the coffin bone with the ground surface such that its palmar angle lies at about 5 degrees, which is that found normally. This optimizes the mechanical environment of the coffin bone, prompting maximum blood flow through the damaged hoof vasculature. This, in turn, speeds regrowth of hood capsule, particularly the sole. Our practice is to immediately support the hoof using Four Point Tenotomy Railbar Shoes (Nanric Inc, Lawrenceberg, KY) with polymeric putty distributing load across the caudal sole and frog. Usually, the toe of the hoof capsule has to be moderately elevated relative to the (hoof) bearing surface of the rail shoe if the shoe is to achieve our goal of being parallel to the solar surface of the coffin bone.
Both horses will require moderate (case 1; ~3 – 6 months) to long term (case 2; 12+ months) specialist care to optimize outcome. Case 1 will likely require the Trac-me shoes and padding in conjunction with restricted activity for 1-3 months as the damaged hoof tissues heal. Thereafter, a gradual return to regular shoes and activity may be attempted. Dietary management will be implicit to a successful outcome. Case 2 will require much closer veterinary and farrier oversight. The horse needs exercise immediately following the tenotomy to prevent recontraction of the DDFT but at the same time the foot cannot be over-used if more tissue damage is to be avoided. Consider starting with 3-4 minutes of walking 2-3 times daily. The rail-shoes will likely be retained for at least 3 months until it is clear that the horse is stabilized. The risk of complication, such as hoof absessation, is always present and owners should only pursue management of this type of severe case if they are fully committed both emotionally and financially. Medical management in this instance includes use of acepromazine (vasodilator and sedative), phenylbutazone (analgesic), ranitidine and, for the time being, doxycycline (antibiotic and matrix metalloproteinase inhibitor). This patient was transitioned from Tufts’ hospital environment to a rehabilitation facility following derotation surgery.