Equine Cushings disease is also known as Pituitary Pars Intermedia Dysfunction because tumors form in the middle portion of the pituitary gland. The presence of these tumors does not seem to cause any problems. However, they stimulate the release of pituitary hormones. One of these is ACTH (Adrenal Cortical Stimulating Hormone) which causes endogenous release of glucocorticoid steroids. Another hormone release is β-endorphin. This release results in several clinical signs including hirsutism, laminitis, increased drinking and urination, weight loss and narcolepsy. Hirsutism refers to a long hair coat often with a curly appearance and a lack of shedding. Other symptoms include muscle wasting, docility, decreased pain response, increased appetite and recurrent infections.
While this is typically a disease of older horses, it has been reported in horses as young as seven years old. Symptoms may vary based on length of disease and amount of hormones secreted. The symptom most commonly recognized is a long hair coat and failure to shed at an appropriate time. These horses may fail to shed or be late shedders.
The increase in glucocorticoids can have many effects. These steroids tend to suppress the immune system making affected horses more susceptible to infections. Glucocorticoids are also implicated in laminitis. This may be directly or in combination with increases in blood glucose leading to insulin resistance. Insulin resistance is associated with Equine Metabolic Syndrome (EMS) another cause of laminitis.
Docility, narcolepsy and decreased pain response are associated with the release of β-endorphins. Endorphins are the hormones that are associated with pain control. Part of their effect can be dulling the nervous system. Owners will describe these horses as being hard to get their attention. Some affected horses may exhibit signs that would be consistent with senility such as walking off from feed.
This disease may not be easy to detect especially in the early stages. These horses are often kept outside making evaluation of changes in drinking and urination difficult. The symptoms associated with mentation often have an insidious onset.
Many tests have been used to diagnose Equine Cushings. A baseline cortisol level has been used but is of limited value. Cortisol levels vary normally during the day. Unless a baseline is grossly elevated, its meaning is unclear. Furthermore, PPID horses can have normal baseline levels. To get around this, two other tests are often used. The first is the Overnight Dex Suppression test. Blood is drawn in the evening and dexamethasone is administered. Blood is again drawn the next morning. Normal horses will have decreased cortisol levels. Cushings horses will fail to suppress cortisol levels. This test has the drawback of using dexamethasone which is also a glucocorticoid. There is some risk of pushing a horse over the edge and into laminitis. The TRH stimulation test is the most current test. Blood is drawn and TRH (Thyroid Releasing Hormone) is given. Blood is drawn again thirty minutes later. PPID horses will have an increase in cortisol levels while normal horses will not.
Several treatments have been used for Cushings. The current medication most often used is pergolide which is marketed as Prascend™. This drug interferes with the abnormal secretion of hormones. It has a wide dose range. Treatment is usually begun at the low end and adjusted based on resolution of clinical signs and further testing as needed. It should be noted that it can take up to six weeks for the full effects of treatment to be realized.