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). 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. Obesity is a harmful state for the horse.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. The longer a horse is obese, the more other tissues are affected.
Dietary changes amplify susceptibility to laminitis as a horse consumes seasonal increases in sugars and starches on pasture–a horse’s threshold is lowered when pasture grass challenges are the greatest. In horses older than 15 to 20 years of age, equine Cushing’s disease poses an additional risk since high levels of circulating cortisol (associated with equine Cushing’s disease) antagonize actions of insulin. For middle-aged and geriatric horses, it is important to control this disorder.
Laminitis triggers include exacerbation of an insulin resistance crisis due to increased starch and sugar in green grass or other stresses, such as diet and grain supplements, change in pasture, stress of transport, management or weather, disease, hospitalization, and/or surgery. The crisis might be related to seasonal hormonal changes. Another trigger develops in the intestine, particularly when a horse is turned out on new pasture with a sudden increase in starches, sugars, and fructans that creates a carbohydrate overload in the large intestine. This alters bacterial flora, lowers the pH, increases intestinal permeability, and results in endotoxemia, which increases multiorgan inflammation, increases vascular constriction, and induces IR.
Following are preventive strategies for avoiding these laminitis scenarios:
Manage and treat obesity and insulin resistance;
Treat equine Cushing’s disease and monitor regularly;
Provide consistent farrier care;
Regulate starch, sugar, and calorie intake;
Avoid sudden feed changes;
Gradually introduce a horse to new pasture; and
Treat endotoxemia and systemic inflammation.