Cushing’s disease is a hormonal disease resulting from a benign tumor in the pituitary gland, and causing the disproportionate release of the hormone adrenocorticotropin or ACTH. This hormone fuels an increase in the quantity of cortisol hormones that are released into the bloodstream from the adrenal glands. This excess of these cortisol hormones leads to the typical Cushing’s disease clinical signs which include a long, shaggy, curly hair coat that doesn’t shed during the warm weather months, weight loss coupled with abnormal fat deposits over parts of the body, increased drinking and urinating, increased susceptibility to infections, and chronic laminitis. Some theorize that the clinical signs of Cushing’s disease are due to the pressure by the tumor on the hypothalamus, an area of the brain that produces hormones that controls the release of hormones by the pituitary gland. Affected horses are usually between the ages of 19 and 21 years. Ponies seem to be more susceptible than larger horses, and some studies would indicate that mares are more susceptible than gelding or stallions.
Veterinarians will often suspect Cushing’s disease just by the outward clinical signs, although a variety of blood tests can be used to confirm the diagnosis. The most common test is measurement of ACTH levels in the blood, ACTH levels in horses with Cushing’s disease having been reported to be approximately six times higher than those measured in normal horses. This test does require very careful handling of the blood samples. On the other hand, the most reliable test in the diagnosis of Cushing’s disease is the low-dose dexamethasone suppression test or LDD test. In this test, first the horse’s resting cortisol levels in the blood are measured, and then a dose of corticosteroid medication call dexamethasone is administered. In a normal horse, the dexamethasone suppresses the normal release of cortisol, which results in a lowered or “suppressed” resting cortisol measurement. However, in a horse with Cushing’s disease, the resting cortisol levels will remain normal since the normal feedback mechanisms that control the release of cortisol will not respond to the administered dexamethasone.
Since laminitis is a potential complication of dexamethasone administration, many veterinarians are reluctant to perform the LDD test on a horse that is already experiencing symptoms of laminitis lest it worsens the condition. In these cases blood tests can be used to help confirm the diagnosis.
Good basic care is the main element of a treatment plan for a horse with Cushing’s disease. This means clipping the hair coat during the hot weather months and making sure it does not becomes matted. Since Horses with Cushing’s disease are prone to dental problems due to their propensity to infections, regular dental care is very important. Laminitis being one of the chief complications of Cushing’s disease, special attention to foot care is necessary to minimize any chance of this problem occurring. A proper diet is also a requisite for keeping a horse suffering from Cushing’s disease as healthy as possible. Some recommend feeding hay with less that 10% total sugar plus starch
For now, there are two medications that can help treat the symptoms of Cushing’s disease: pergolide mesylate and cyproheptadine, the latter being less effective than pergolide. In some cases, pergolide and cyproheptadine have been used in combination for maximum effectiveness, especially in horses that have developed a resistance to pergolide over a period of 2 to 3 years.
In most cases, Cushing’s disease is a fairly benign condition with a few problems that can be easily managed. Nevertheless, when laminitis becomes a major component of the disease, it can then become life threatening being an extremely painful condition that can be difficult to control. When a horse has Cushing’s disease, the laminitis that accompanies it is often resistant to treatment and can easily spiral out of control.
The high levels of circulating cortisol in the horse’s system can also lead to complications since it causes the horse to be susceptible to infections. In particular, skin infections that are exacerbated by the long hair coat and excessive sweating that accompanies Cushing’s disease may require extra attention as well as any infections in the periodontal tissues that can lead to dental problems, including tooth loss and even sinus issues.