A group of veterinarians gathered at the Land O' Lakes Purina Mills headquarters in St. Louis, Mo., last fall to participate in discussions on subjects that ranged from Cushing's disease to proper nutrition for horses young and old. Nicholas Frank, DVM, PhD, Dipl. ACVIM, of the University of Tennessee, addressed the issues of Cushing's disease and equine metabolic syndrome in separate talks.

The specialists who discussed nutritional issues were Purina staff members Randel Raub, PhD, Karen Davison, PhD, J. Kathleen Young, PhD, and Mary Beth Gordon, PhD.

Cushing's Disease

This problem, Frank said, is caused by excessive production of adrenocorticotropin hormone (ACTH) by the pituitary gland (hyperadrenocorticism), which, in turn, increases secretion of cortisol from the adrenal glands.

This dysfunction of the pituitary gland is more common in older horses and shows up more frequently in ponies than in full-sized horses. When the malady strikes, laminitis is often a clinical manifestation. Many horses develop laminitis when grazing on pasture, and the disease is triggered by an increase in pasture grass sugar content, Frank said. Sugar concentration rises dramatically when the grass grows rapidly in the spring and summer after heavy rainfall, or when it is preparing for winter dormancy in the fall. This alters the grazing horse's large intestine flora, changes intestinal permeability, and leads to the release of triggering factors in the blood.

Frank told the group that horses with Cushing's might be predisposed to laminitis because they have weaker hoof tissues or altered blood flow as a result of hyperadrenocorticism. Some affected horses also are insulin-resistant, and this can further lower the threshold for laminitis.

Horses with Cushing's often have long hair coats that don't shed out, often sweat excessively, are lethargic, lose muscle mass, and are more susceptible to other diseases.

Pergolide, which increases production of the inhibitory neurotransmitter dopamine, is the drug of choice for treating Cushing's, he said. The drug functions by suppressing the activity of the adrenal cells and inhibits growth of tumors. Pergolide is administered orally at a daily cost of approximately $2.

Horses with Cushing's that also suffer from insulin resistance should be placed on a diet that contains less sugar, noted Frank.

A diet composed primarily of hay often is recommended. Pasture grazing can present a risk for insulin-resistant horses because the sugar content of pasture grass varies widely and can fluctuate with changes in rainfall and temperature. Limited grazing time or use of a grazing muzzle should be considered when managing insulin- resistant horses, he said, and some affected horses must be kept off grass altogether.

Equine Metabolic Syndrome

Equine metabolic syndrome (EMS) is an endocrine and a metabolism disorder, Frank said. Insulin resistance is the primary problem encountered with EMS, and this condition "significantly increases the risk of pasture-associated laminitis."

EMS occurs in horses and ponies; often animals that are 10 to 20 years of age develop this disorder, but it can strike younger animals. No published information is available concerning the prevalence of EMS by breed, he said, but at the University of Tennessee, it has been found to be most common in pony breeds, Morgans, Paso Finos, and Norwegian Fjords.

According to Frank, EMS often is first recognized when laminitis develops. The horse is usually kept on pasture and the episode occurs after the pasture has gone through a period of rapid growth in the spring or has entered dormancy in the fall.

Often horses suffering from EMS are described as being "easy keepers" because they seem to require fewer calories to maintain their body weight. Both genetics and diet can be involved, he said.

"Certain breeds or genetic lines may have undergone evolutionary adaptations to survive in harsher environments," Frank said, "and these horses or ponies can more efficiently convert poor-quality forage into energy. Under modern circumstances, these modifications are unnecessary and are likely to predispose the animal to obesity. Equine metabolic syndrome may therefore begin with the genetically susceptible horse or pony grazing on lush pasture or being fed large amounts of concentrate.

"As excess energy is stored as fat, adipose (fat) tissues expand through an increase in the number and size of adipocytes (a connective tissue cell specialized for fat synthesis and storage)," he said. "Cellular functions are negatively impacted and adipose tissues release factors called adipokines (which cause mobilization of a stored lipid) that act locally and enter the circulation. Some of these adipokines exert pro-inflammatory effects that may contribute to the development of laminitis."

This is where insulin resistance enters the picture. Frank described insulin as a hormone secreted by the pancreas that stimulates the uptake of glucose by the tissues when sugar is abundant, such as after feeding. When insulin resistance occurs, the tissues fail to respond to insulin and, in the process, fail to utilize the glucose in the bloodstream.

The goals for managing horses with EMS should be twofold: first, to reduce body fat mass to improve insulin sensitivity, control the adverse effects associated with obesity, and lower adipokine production, and second, to lower the risk of laminitis by improving insulin sensitivity and reducing the likelihood of sudden changes in bacterial flora within the large intestine.

Horses with EMS are like people with diabetes and, therefore, sugar should be avoided. Exercise also is highly important, Frank told the group.

Fat Horses

Gordon said that during the past 10 years, the amount of research in the field of endocrinology as it relates to body composition and exercise has increased dramatically. One of the things learned is that horses, like an increasing number of people in the United States, are becoming obese.

Gordon said the USDA has estimated that 4.5% of the horses in this country are obese. Many veterinarians, she said, place the number much higher than that.

While much has been learned about the function of hormones correlated with body composition--such as leptin, ghrelin, and adiponectin, which have been shown to play a role in food intake, glucose/insulin metabolism, and energy balance--more studies are needed to fully understand how the hormones function.

"More studies are necessary to determine if certain concentrations of hormones can serve as risk factors for future disease or illness in horses," she said. "Additionally, hormonal manipulation may be employed to treat appetite disturbances or obesity and insulin sensitivity problems."

Also highly important in the overall equation, Gordon said, is exercise. She said studies in humans and horses have proven that exercise can increase insulin sensitivity. "Therefore," she noted, "in today's world of overweight horses that may have reduced insulin sensitivity, exercise is an important component in helping to combat both insulin resistance and excessive weight gain."

Gordon then had this to say: "Overall, a critically important issue of horse health is ensuring that a horse is not too fat, yet not too thin--just right. So why don't many horses do a good job of regulating their intake naturally or maintaining a theorized healthy 'set point'? One understandable supposition is that horses are genetically designed to utilize relatively poor- quality feedstuffs and store fuel sources very well as a protective mechanism for surviving in the wild. Today, as many horses are more intensely managed and offered good- quality, higher-caloric feedstuffs, horse owners essentially overfeed their horses. Therefore, it is our responsibility to ensure that horses receive the appropriate amount of calories for their lifestyle to remain healthy."

She added that feeding obese and underweight horses can be difficult because some overweight horses appear to resist weight loss while consistently thin horses resist weight gain. Obviously, these types of horses should undergo thorough veterinary examinations to rule out diseases or conditions contributing to the problem.

Weight loss should be slow and steady, she said, with a program taking as long as several months before the desired level is reached. Horses should be monitored with weight tapes, and body condition scoring should be done weekly to provide proper information that is used in adjusting diet and exercise regimens.

Young, Growing Horses

During the conference, Raub addressed the trials and tribulations of feeding young, growing horses.

He opened his presentation by saying: "Although much knowledge has been gained regarding the nutrient requirements of the young, growing horse, there are still many unanswered questions concerning optimum nutrition for growth and the avoidance of developmental orthopedic disease (DOD)."

Lending to the confusion is the fact that man's increasing ability to recognize DOD might be contributing to the perception that the problem is increasing.

Even the terms can be confusing to the layman. Osteochondrosis, osteochondritis dissecans, and osteochondrosis dissecans have all been used interchangeably, Raub said, "and with some confusion."

Osteochondrosis can be considered the disease, he said, osteochondritis, the inflammatory response, and ostechondrosis dissecans, the condition when a piece of cartilage has actually avulsed--or come loose from--the surface of a joint.

In 1986, the term developmental orthopedic disease was introduced as a heading for all orthopedic problems in growing horses, Raub said, and acquired limb deformities, physitis, subchondral cystic lesions, flexural deformities, cuboidal bone malformation, and juvenile arthritis all fall under the DOD heading.

There definitely appears to be a genetic factor involved in the risk for DOD, Raub told his listeners, but there is much in this area that is not known.

Nutrition also appears to play a significant role, he said. It is important to increase the amount of nutrients available to the broodmare during the last third of gestation and during lactation to help ensure proper fetal growth and development, but what isn't known for certain is how the intricacies of broodmare nutrition might affect the skeletal soundness of the foal.

According to Raub, feeding for growth starts at conception; thus, broodmares should be fed a balanced diet throughout gestation/lactation so that they maintain a body condition score of 5-7 on a scale of 1 to 9.

The role of genetics and nutrition can be confusing. For example, Raub pointed out that Thoroughbreds from 1989 to 1990 grew slightly heavier and taller compared to Thoroughbreds from 1958 to 1976. Was this a result of breeders breeding for heavier horses, or did it result from changed nutrient levels in the feeding programs?

One thing appears to be certain based on Raub's evaluations--the young horse with a rapid rate of growth is more at risk for DOD than the youngster with a moderate growth rate.

Raub offered these observations:

  • Genetics is the primary determinant factor for risk of DOD.
  • Meet as much of a growing horse's nutrient needs as possible with good-quality roughage (test for forage quality).
  • Feed for moderate or less growth rates.
  • Provide abundant free exercise or a well-constructed and controlled forced exercise program.
  • Feed concentrates that are research-based, well formulated, and manufactured under the highest quality control standards.
  • For horses with DOD, reduce caloric intake, but maintain adequate amounts and quality of protein, vitamin, and mineral intake.

Equine Performance Nutrition

Davison introduced her subject matter this way: "A winning performance is the culmination of sourcing good genetics, properly managing the development and training, getting excellent veterinary care, and providing appropriate nutrition. Anyone can be lucky enough to win once in a while, and horses, indeed, have tremendous heart. Even so, luck and heart can only go so far. When nutrition and health care are lacking, the ability to consistently win is greatly impaired."

Providing fuel for the equine system, she said, involves converting chemical energy into mechanical energy. She described the equine body as having three "fuel tanks" of stored substrate to supply the chemical energy--body stores of fat, glycogen, and protein. All three can be metabolized to produce adenosine triphosphate (ATP) that fuels muscle cells to do the mechanical work. The primary fuels, she emphasized, are glycogen and fat.

Nutrients requirements increase with accelerated workloads. Davison said the National Research Council recommendations state that daily energy intake be increased by 25% for light work, 50% for moderate work, and 100% for intense exercise.

"If the diet does not provide sufficient energy to meet the increased demands associated with these athletic activities," Davison declared, "loss of body condition and decreased performance will ensue."

Assessing dietary intake by body condition scoring is the best way to determine if that horse is in peak condition, Davison said. A body condition score between 4 and 5 is ideal for most athletic horses, she told the group. In addition to energy and protein, vitamin and mineral requirements also increase with increasing workloads, she said. However, she cautioned, just because the right amount is highly important doesn't mean that more is better; oversupplementation can be detrimental rather than helpful.

Nutritional Supplements

Young picked up on that last point where Davison left off. She told the group a survey by the USDA indicated that 69% of responding horse owners add supplements to their horses' diets, ranging from vitamin/mineral supplements to joint supplements and those designed to enhance performance.

In one catalog that she viewed, Young said, more than 90 dietary supplements were offered for sale.

According to Young, an important question for a horse owner is, "Does the supplement work?" she said. "Even if a supplement is proven to be absorbed from the horse's digestive tract and transported through the body, is there evidence that the supplement provides any improvement or benefit to the horse's performance or heath?"

Some of the supplements aren't properly absorbed and thus are of little or no benefit. For example, Young explained, research has shown that some orally supplemented glucosamine and chondroitin sulfate products are not wholly absorbed in the digestive tract, so they wouldn't be able to reach the joints for any therapeutic effect.

"When it comes to nutrition," Young said, "sometimes more is better, sometimes more is worse, and sometimes more is just more. There are certain amounts of all nutrients that the body requires for optimum function and performance. If the diet is deficient in a nutrient, health and performance will suffer. If the nutrient is added to the diet, performance will continue to improve until the nutrient requirement is met. However, there comes a point at which the continued addition of that nutrient will actually begin to hinder health and performance, and in some cases, oversupplementation can become toxic to the horse."

In general, she said, the best option is to provide a diet that contains all of the essential nutrients in the correct concentrations and balances.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.

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