Equine Cushing’s Disease and Insulin Sensitivity

Equine Cushing’s disease, or pituitary pars intermedia dysfunction (PPID), affects up to one-third of older horses, and many older horses also have insulin sensitivity. Does this mean that all older horses with Cushing’s disease have a reduced sensitivity to the effects of the hormone insulin? According to the latest research in this field*, the two medical conditions do not necessarily go hoof-in-hoof.

Equine Cushing’s disease is caused by a benign growth in the pituitary gland. Signs of disease include abnormal hair growth, muscle atrophy, laminitis, and a decreased sensitivity to insulin.

Impaired insulin sensitivity occurs when a horse does not respond to the effects of insulin after being fed a meal. This means that more and more insulin is secreted by the horse to control blood sugar levels. High insulin levels can contribute to laminitis. Insulin is also involved in the pathways of protein metabolism and can potentially contribute to the muscle atrophy that is common in horses with Cushing’s disease.

Although previous studies found differences in sugar (glucose) and insulin dynamics in horses with Cushing’s disease and horses that did not, those studies compared older Cushingoid horses with younger non-Cushingoid horses. In the new study, researchers were wise to “match” the horses and compared horses older than 15 years with Cushing’s to horses older than 15 years without Cushing’s disease. Interestingly, absolutely no differences in either insulin sensitivity or muscle development/degradation were identified.

This information contributes to the body of knowledge on equine Cushing’s disease and how to best manage the condition, which can be challenging.

*Mastro, L.M., A.A Adams, and K.L. Urschel. 2015.  Pituitary pars intermedia dysfunction does not necessarily impair insulin sensitivity in old horses. Domestic Animal Endocrinology. In Press.