Equine Cushing’s disease, technically known as pituitary pars intermedia dysfunction (PPID), is a degenerative hormonal disease of middle-aged and senior horses. To completely understand the disease, a little background in anatomy and hormonal cycles is
Equine Cushing’s disease, technically known as pituitary pars intermedia dysfunction (PPID), is a degenerative hormonal disease of middle-aged and senior horses. To completely understand the disease, a little background in anatomy and hormonal cycles is helpful. First, the equine pituitary is located near the base of the brain, and is composed of 3 lobes or sections. The “pars intermedia” is one of those sections. The pituitary produces a variety of hormones that regulate a wide range of body functions, including milk production, control of growth rate, body temperature, and heart rate, and reacting to stress. The pars intermedia is the portion of the pituitary in charge of producing Adrenocorticotropic Hormone (ACTH), which stimulates the adrenal gland to produce cortisol. Cortisol helps control the use of sugar, protein, and fats in the body and helps the body deal with stress.
Equine Cushing’s disease develops when the pituitary begins uncontrolled production of ACTH and other hormones. This unregulated hormone production eventually leads to very high cortisol levels, which in turn leads to high blood sugar and abnormal fat deposits throughout the body.
How can I tell if my horse has it?
The disorder is most commonly diagnosed in horses greater than 15 years of age. The typical signs of PPID include a long hair coat that sheds late or not at all, muscle wasting, and obesity, with bulging eyes, cresty neck, and fat deposits along the topline. The horse may also suffer from a loss of muscle tone along the topline due to high cortisol levels in the blood. The ribs may be showing, but the horse may have a rounded abdomen, giving it the appearance of being overweight. The high cortisol concentrations predispose the horse to complications such as chronic laminitis, infertility, and chronic infections, both parasitic and bacterial.
- If horse is IR plus PPID, double risk of laminitis
- Start testing IR horse for ppid at 10 or 12
- Early detection ppid
- Delay shed
- Increase calorie easy to hard
- Loss of muscle mass along topline
- Deterioration in glucose and insulin values (exacerbation of IR)
Basal ACTH and Insulin Testing. While the increases in basal ACTH are comparatively small in PPID horses versus a-MSH concentrations, the test has good sensitivity and specificity (Table 1). The basal ACTH test does not require multiple farm visits like the DST, for optimal sensitivity the samples must be processed within 3 hours of collection or frozen for shipment.15
Overnight Dexamethasone Suppression Test. The DST utilizes the negative feedback loop exerted on ACTH secretion by glucocorticoids. Melanotropes are normally insensitive to glucocorticoid feedback, and this insensitivity allows melanocyte-induced steroidogenesis in PPID animals to be differentiated from corticotrope-maintained steroidogenesis in normal animals. The premise of the DST is that administration of dexamethasone suppresses the secretion of ACTH and consequently cortisol in normal animals and also in those with PPID, but it does not significantly impact the production of cortisol as a result of POMC synthesis (Figure 3).
Timing of the DST is essential because of the normal circadian pattern of glucocorticoid secretion. Dexamethasone must be administered before the normal diurnal increase in ACTH to attain maximal suppression of