Causes of Laminitis (AAEP 2008)

Progression of obesity and insulin resistance exacerbates laminitis risk. As a horse gets fatter, insulin sensitivity decreases. Insulin resistance worsens with chronic obesity, making a horse more susceptible to laminitis and less tolerant of triggering events.
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At the 2008 AAEP Convention, which was held Dec. 6-10 in San Diego, Calif., Nicholas Frank, DVM, PhD, Dipl. ACVIM, associate professor in the Department of Large Animal Clinical Sciences at the University of Tennessee's College of Veterinary Medicine, presented a thorough review of precipitating causes of laminitis. Obesity is a primary predisposing factor, with some horses and breeds having a genetic susceptibility. Individuals might have efficient energy metabolism and/or nutrient digestion or absorption, or they might have an insatiable appetite. Diet and management practices that lead to obesity include overfeeding (particularly grain), lack of exercise, and varied perceptions of what is considered "good" body condition. In the wild horses would have scarcer forage supply during the winter, so weight would fluctuate with season; in domestic life horses are often fed more in the winter, thereby eliminating seasonal weight loss.

Equine metabolic syndrome describes a syndrome of obesity with regional fat deposits in the neck, prepuce or udder, rump, and subcutaneous tissues. There can be a genetic and/or breed predisposition coupled with insulin resistance (IR) and high risk for laminitis. Bouts of laminitis are not always clinically apparent other than visible divergent growth rings of the hoof or radiographic evidence of coffin bone rotation.

Part of the equine metabolic syndrome picture includes insulin resistance, which describes reduced response to insulin to take up glucose in the tissues (skeletal muscle, fat, and liver). Frank explained that the body's capacity to store fat is finite; then fatty acids are mobilized for fat storage in other tissues where it isn't as well-tolerated. There it disrupts actions of insulin, particularly in skeletal muscle and liver. It is thought that certain fat deposits, like a cresty neck, are metabolically active, with fat cells secreting chemical mediators that increase insulin resistance. Equine Cushing's disease might also be accompanied by insulin resistance.

Studies have shown that IR raises the risk of pasture-associated laminitis in ponies. Insulin slowly dilates blood vessels, so an insulin-resistant horse might experience vessel constriction. This phenomenon might connect insulin resistance with laminitis due to changes in vascular tone in vessels in the feet; then a horse has less ability to adapt when experiencing a triggering event. One such trigger might be alterations in the composition of pasture grass coupled with lowered threshold due to insulin resistance. Frank again stressed that obesity is a harmful state for the horse

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Written by:

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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