The good news is that horses are living longer. Regular veterinary care and a more thorough understanding of equine nutrition and husbandry has added years to the equine lifespan. The bad news is that more horses are developing diseases associated with old age. One of the most common is Cushing’s disease.
Although Cushing’s is the common name for this disease, it is more accurately referred to as pituitary pars intermedia dysfunction (PPID) because it differs in cause from the Cushing’s disease seen in humans and dogs. In horses, the illness is usually caused by a tumor on the pituitary gland, which results in an over-secretion of the hormone pro-opiomelanocortin (POMC). In normal horses, POMC is released by the pituitary gland in small amounts to help the body cope with stress, both physical and psychological. But when too much of this hormone is released, a variety of symptoms can result.
Horses with Cushing’s can have one or many symptoms of the disease. These include increased thirst and urination; a heavy, long hair coat that doesn’t shed in the spring; obesity; a pot-bellied appearance and a sagging back, due to loss of muscle tissue; loss of muscle in some areas of the neck and haunches; extra fat deposits over the eyes; a large, cresty neck, especially thickened on the dorsal topline; chronic laminitis; immune system dysfunction, such as chronic infections, periodontal disease, skin infections, and especially hoof abscesses; lethargy; infertility and lack of estrus cycles in mares; and abnormal mammary gland development.
“A long hair coat that won’t shed out—called hirsutism—is the most common sign of PPID,” says Kate Gentry-Running, DVM, a practicing veterinarian with an emphasis in equine integrative medicine, located in Tolar, Tex.
“Approximately 80 percent of all PPID horses have hirsutism. In fact, hirsutism in an aged horse is the most common tip-off that PPID is lurking.”
One of the most distressing symptoms of PPID is chronic laminitis. More than 50 percent of horses with PPID founder repeatedly. In fact, Cushing’s disease is the most common cause of laminitis among horses in a primary care veterinary practice, according to a study by the University of Pennsylvania School of Veterinary Medicine.
Testing for Cushing’s
Cushing’s is diagnosed primarily by symptoms, although two clinical tests are available that can confirm the disease: the dexamethasone suppression test and the plasma ACTH measurement test.
According to “Equine Endocrinology: Cushing’s Disease and Metabolic Syndrome,” by Christie Malazdrewich, DVM, Diplomate ACVIM, University of Minnesota College of Veterinary Medicine, the dexamethasone suppression test is an overnight protocol in which a pretreatment blood sample is collected in the late afternoon, after which a low dose of dexamethasone (a synthetic steroid hormone) is administered by intramuscular injection. A second sample of blood is collected the following day at around noon, and both samples are submitted for measurement of plasma cortisol. In normal horses, administration of dexamethasone stimulates a negative-feedback response that suppresses secretion of cortisol from the adrenal glands, yielding a much lower concentration in the second blood sample. In horses suffering from Cushing’s disease, however, the negative-feedback response is blunted and a lesser degree of suppression, if any, is observed.
Many owners and veterinarians fear, however, that administration of dexamethasone might increase the risk of laminitis in a horse that is already predisposed to its development, according to Malazdrewich, so many veterinarians prefer a test that takes a measurement of plasma ACTH instead. This test involves collection and analysis of a blood sample.
The pituitary gland in horses affected with Cushing’s often secretes excessive amounts of ACTH into the bloodstream. This test is somewhat less accurate than the dexamethasone suppression test, and blood samples must be handled carefully to avoid degradation of ACTH and falsely low-measured values. Stress and pain due to other conditions may also result in falsely elevated values.
Cushing’s can’t be cured, but it can be managed. In fact, it’s extremely important to treat the condition, because ignoring it can prove fatal.
“It’s important to treat PPID to prevent chronic laminitis, not to mention other health issues,” says Gentry-Running. “Horses suffering from this condition don’t feel good, and don’t want to work. Also, if you’re trying to breed a mare with PPID, she’s going to be a real reproductive challenge unless this problem is treated. She won’t cycle normally, if at all, and she’ll be much more prone to uterine infections that are difficult to resolve.”
One of the treatments for Cushing’s is use of a drug called pergolide, which was previously used to treat Parkinson’s disease in humans. Another drug, cyproheptadine, is not as effective as pergolide, but can be used in conjunction with pergolide if pergolide isn’t as effective alone.
Horses suffering from Cushing’s disease also need special care to keep them comfortable.
“It’s very important that owners keep horses with PPID body clipped in warm weather,” says Gentry-Running. “You may have to clip them down in the early spring, long before you’d clip normal horses. If you live in a mild winter climate, you may even need to do a partial clip in these horses in the winter to keep them comfortable.”
Dentistry and hoof care are especially important for horses with Cushing’s. “Be absolutely meticulous about dentistry and foot care because of the much higher incidence of periodontal disease and laminitis in these horses,” says Gentry-Running.
Horses with Cushing’s disease should also be managed with diet. Because the illness can cause a horse to be insulin-resistant and have high blood sugar levels, it’s important to carefully control what the horse eats. For starters, starches and sugars should be eliminated from the horse’s diet. This means no sweet feed or commercial feeds high in carbohydrates. Also, be careful with grazing a horse with Cushing’s on a late summer or early fall pasture because, in some areas, older grass can have a much higher starch or sugar content than fresher, green grass. In fact, late summer and early fall show the highest incident of laminitis in pastured horses with Cushing’s disease because of the sugar and carbohydrate content of late-season grasses.
Certain hays and grasses are naturally higher in sugar and carbohydrates as well. Avoid small-grain hays such as barley and oat, as well as ryegrass, fescue and bromegrass in the pasture.
It’s important to keep weight off a horse with Cushing’s disease because the excess weight can increase the likelihood of laminitis and other problems.
“Overweight horses have more of a probability of developing insulin resistance, and extra weight is hard on sore feet,” says Gentry-Running.
All this extra care is worthwhile. With proper treatment and management, many Cushing’s horses lead happy, productive lives.