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,