Medical implications of obesity in horses--lessons for human obesity

Abstract

There is growing recognition that obesity is common and represents a significant detriment to the health of companion animals in a manner similar to that by which it is affecting the human population. As is the case for other species, obesity appears to promote insulin resistance in horses and it is through this pathophysiological process that many of the adverse medical consequences of obesity are being characterized. Equine medical conditions that have been described in the context of obesity and insulin resistance differ from those in humans. Chronic human conditions that have been attributed to obesity and insulin resistance, such as atherosclerosis and diabetes mellitus, are rarely described in obese horses. Significant current interest is centered on the recognition that insulin resistance plays a role in the pathogenesis of laminitis, a potentially severe and debilitating cause of lameness in the equine species. Other equine medical conditions that are more likely in obese, insulin-resistant individuals include hyperlipemia (hepatic lipidosis) and developmental orthopedic disease (osteochondrosis). Pituitary pars intermedia dysfunction (equine Cushing's syndrome) represents another common endocrinopathic condition of older horses associated with insulin resistance. This review presents an introductory overview of the present understanding of obesity and insulin resistance and how these conditions may be associated with disease conditions in horses.

Keywords: Cushing's; horse; insulin resistance; laminitis; nutrition; obesity; pituitary gland.

Figure 1.

(A) Typical physical appearance of an obese pony. The body condition score for this pony is 9 out of 9 (maximal obesity score = 9). (B) Typical physical appearance of a thin horse. The body condition score for this pony is 2 out of 9 (maximal obesity score = 9).

Figure 2.

Diagrammatic representation of the equine gastrointestinal tract. Note the extent and the capacity of the equine large intestine in contrast to its size and extent in humans. For purposes of diagrammatic clarity, the descending colon was excluded. RVC, right ventral colon; LVC, left ventral colon; LDC, left dorsal colon; RDC, right dorsal colon.

Figure 3.

Physical appearance of the thickened neck that is commonly recognized as a sign of underlying insulin resistance.

Figure 4.

(A) Low-powered, light microscopic appearance of the normal healthy hoof lamellar interface. The epidermal component has been stained brown and the dermal connective tissue component appears blue. Note the regular appearance of the interdigitating hoof lamellae that serves to facilitate attachment of the distal phalanx to the overlying hoof wall. HLI, hoof lamellar interface. (B) Low-powered, light microscopic appearance of the hoof lamellar interface obtained from a horse affected by chronic laminitis. The epidermal component has been stained magenta and the dermal connective tissue component appears blue. Note the irregular appearance of the interdigitating hoof lamellae. The lamellar structure has been distorted and attenuated. The net result of the structural remodeling process is a weakening of the attachment interface. HLI, hoof lamellar interface.

Figure 5.

(A) Physical appearance of the hoof of a horse affected by chronic laminitis. Note the prominent ridges at the surface of the hoof (“growth lines”) that tend to converge toward the front of the hoof. This physical appearance is characteristic of chronic laminitis. Broadening of the white line zone is a feature of chronic laminitis but it is evident when viewing the solar aspect of the hoof (not depicted here). (B) Physical appearance of a normal equine hoof.

Figure 6.

Physical appearance of a pony affected by pituitary pars intermedia dysfunction (equine Cushing's syndrome) resulting from the unfettered secretion of POMC peptides from the intermediate lobe of the pituitary gland. Note that the hair coat has failed to shed out (inappropriate hirsutism) and that the front feet are affected by chronic laminitis.

Figure 7.

Graph depicting the response of the plasma glucose concentration to administration of an IV bolus of glucose and an IV injection of insulin (combined insulin glucose test; details provided in the text). The graph depicts the response of two different animals. Insulin sensitivity is generally assessed as satisfactory for those individuals in which the plasma glucose concentration returns to its baseline value within 45 minutes of the outset of the protocol. In this case, horse A had poor insulin sensitivity and horse B had normal insulin sensitivity.

Figure 8.

(A) A horse instrumented with a continuous interstitial glucose monitor. Note that the instrument has been secured to the patient's head collar. (B) A horse instrumented with a continuous interstitial glucose monitor. Note that the probe has been inserted into the subcutaneous tissue on the right side of the horse's face.