Causes of Equine Cushing's Disease
Often, the disease, known by a variety of names including Equine Pituitary Gland Hyperplasia (EPGH), as well as Pituitary Pars Intermedia Dysfunction (PPID), results either from an adenoma (a benign tumor) or a benign hypertrophy (increase in cell size) in the pars intermedia. Whatever the reason, the pituitary gland increases in size and in some cases becomes so large as to actually press against other areas of the brain. This can lead to neurological problems and some horses have been known to suffer blindness or seizures.
Besides causing neurological problems (which often do not occur), the dysfunctional pituitary also has an effect on the adrenal glands, which are continuously stimulated to produce cortisol. The pituitary gland is an important organ in the endocrine system, which releases chemical signals to instruct various other organs in the body. These signals can take the form of hormones or proopiomelanocortin peptides (POMCs). In equine Cushing’s disease, the pituitary tumor causes the gland to remain active continuously and secrete POMCs non-stop.
In other cases, there is no cancerous growth within the pituitary gland. Instead, some scientists have found that there is a connection between dopamine production and equine Cushing’s disease, as well as abnormal enzyme activity and the disease. It is known that the pars intermedia is tightly governed by neurotransmitters, mainly dopamine, secreted by the hypothalamus. Dopamine inhibits the activity of the pars intermedia. Horses with Cushing’s disease seem to have dopamine-producing nerve cells that are “weak", or that seem to die easily. Studies are now being done as to why these cells are so prone to dying. The abnormal enzyme linked to the disease is 11-beta hydroxysteroid (HSD), which is found within cells and is responsible for making sure the concentration of cortisol remains within an acceptable range.