My horse, Atticus, is starting to shed out his winter coat. To me, this is one of the most reliable indicators that spring is on its way (despite the fact that we currently have six inches of snow on the ground). However, one of the horses at my barn is not shedding her exceptionally shaggy coat at all yet. Is she like the groundhog who sees its shadow, thereby predicting six more weeks of winter? No, unfortunately it is more likely that she is developing a disease called pituitary pars intermedia dysfunction (PPID), more commonly known as equine Cushing’s disease.
Horses with PPID may enjoy the benefits of their long coats in the winter, but they are actually suffering from a very serious disease. PPID can also lead to increased thirst and urination, insulin resistance and the high blood sugar levels that result, repeated infections, and laminitis — a potentially fatal condition characterized by inflammation and disintegration of the tissues that connect a horse’s hoof to the deeper structures of the foot.
PPID is caused by a benign tumor of the pituitary gland located at the base of the brain. The tumor over-secretes melanocyte-stimulating hormone (MSH) and adrenocorticotropic hormone (ACTH), which together produce the symptoms described above. The disease is most commonly diagnosed in older horses; most are in their late teens or twenties.
Horses with advanced PPID are easy to diagnose — finding an inappropriately long, oftentimes curly coat in an older horse is generally sufficient. More vague symptoms require laboratory testing, and there is no one perfect test. Cortisol, ACTH, MSH, and insulin levels can be measured via a simple blood draw or a dexamethasone suppression or thyrotropin releasing hormone test run, but all can be influenced by outside forces such as stress, obesity, and another condition known as equine metabolic syndrome. Interestingly, the timing of many of these tests can have a big effect on their results. Cortisol levels are normally high in the morning and low in the evening. Also, equine ACTH and MSH levels are naturally much higher in the fall than at other times of the year, probably to get horses ready for cold temperatures and a decrease in forage availability in winter.
Treatment for PPID is essentially symptomatic and supportive, with the goal of keeping affected horses comfortable for as long as possible. The drug pergolide can be helpful as can management interventions like shaving a horse’s long coat when the weather warms and staying on top of farrier and routine veterinary care.
I was in a bit a quandary as to how to handle the situation with Atticus’s herd mate when I met up with the mare and her owner on a trail ride last week. The horse’s condition is really none of my business. I’m neither her doctor nor her owner’s friend (don’t think badly of me, I simply had never met her before), but I didn’t want the horse to suffer, either. I chatted with the woman for a few moments, and eventually said something inelegant along the lines of, “She sure is fuzzy.” Her owner replied, “Yeah, I’m worried she may be developing Cushing’s.”
Phew, for once I didn’t have to be the bearer of bad news.
Dr. Jennifer Coates