Since the 1960s when reports of equine pituitary adenomas and their systemic consequences appeared in the literature, our understanding of Cushing’s disease (pituitary pars intermedia dysfunction or PPID) and the insulin resistance it causes has grown tremendously. Despite this, there is still outdated information out there that does a real disservice.
photo from ecirhorse.org
Why do horses get Cushing’s? We don’t know. Not true. Horses get PPID because of oxidative damage to nerves in the hypothalamus of the brain. These nerves normally produce dopamine which controls the output of POMC-derived hormones, like ACTH, from the intermediate lobe of the pituitary. What we don’t know is why some horses develop this damage and others don’t – or at least they don’t develop it in severe form before they die of other causes.
Good general management (diet, farrier, teeth, wound care) is more important than medication. I would have to also consider that false. Those things are extremely important but they do nothing to control the hormonal upheaval that the disease causes or to prevent the consequences of that.
ACTH does not cause laminitis. ACTH levels alone are not predictive of laminitis. ACTH may be present in a form that is not highly biologically active. However, high ACTH and the other POMC derived hormones are directly responsible for causing or worsening the insulin resistance that eventually can result in laminitis. If they are not controlled, no amount of dietary management and hoof care will prevent the laminitis.
You shouldn’t expect a treated horse to live longer or even avoid PPID related problems. In a study of 217 diagnosed and treated horses followed for 4.6 years (only 44 were actually followed), 50% were euthanized for Cushing’s related problems. What you’re not told is that some of these horses were treated with cyproheptadine which has since been shown to be ineffective. There is also no mention of the dosage of pergolide used and whether or not it was sufficient to control the ACTH (and other hormones; ACTH is a marker for uncontrolled intermediate lobe hormonal output).
Because brand name pergolide is expensive, for many years it has been suggested that all you should expect to call a treatment “successful” is improvement in symptoms and/or blood work – not normalization. I personally set the bar much higher.
I have been closely following and working with PPID horses for 15 years. Laminitis is by far the worst consequence of PPID and the focus has been on preventing that. This takes the form of keeping ACTH in normal range (which usually IS possible with adequate dosing in the vast majority of cases) and a low sugar and starch, mineral balanced diet to further support control of insulin resistance. Hoof care is critical to rehabing the laminitis horse. Once stable, regular exercise is encouraged.
Good general horse care but no medication or serial testing to make sure medication dosing is effective is not adequate treatment in my book. In fact, it’s no treatment at all.
Eleanor Kellon, VMD