Diagnosing and Managing Endocrine Disorders in Senior Horses

More than 20% of aged horses are known to suffer from equine Cushing’s disease (also known as pituitary pars intermedia dysfunction, or PPID), a harmful endocrine condition that can carry with it a host of other dangerous health problems. To give our elderly equids their best chance at comfort, one researcher recently described best practices for diagnosis and treatment of this and other metabolic problems, and he recommended that veterinarians screen all aged horses for these disorders, confirming their magnitude.

Nicholas Frank, DVM, PhD, Dipl. ACVIM, reviewed the presentation, diagnosis, and medical management of aged horses with Cushing’s disease, equine metabolic syndrome (EMS), hyperinsulinemia (HI), and insulin resistance at the 2013 American Association of Equine Practitioners' Convention, held Dec. 7-11 in Nashville, Tennessee.

Disease Presentations

One of the challenges of recognizing equine endocrine disorders is that early signs are difficult to recognize and affected horses don't always exhibit the same signs. And while veterinarians see PPID most frequently in horses beyond 20 years old, early disease is now being recognized in horses as young as 10 years of age. EMS can occur in young or old animals, and it is most often associated with obesity and HI (or high circulating levels of insulin in the blood). Horses with HI seem to be more likely to develop laminitis than other horses, so diagnosing this problem is an important focus of testing for horses of all ages.

Frank, a professor of large animal internal medicine and chair of the department of clinical sciences at Cummings School of Veterinary Medicine at Tufts University, in N. Grafton, Massachusetts, described three possible variations of endocrine disease in aged horses:

  • The first type is a horse that has battled insulin problems its whole life and now has reached old age. These horses and ponies are genetically predisposed to EMS and experience a high insulin peak following a meal. They suffer from insulin resistance (meaning the horse is less sensitive to insulin, which makes it harder for the fat, muscle, and liver cells to transport the glucose out of the bloodstream and store it as glycogen); Frank refers to HI and insulin resistance collectively as insulin dysregulation. “In horses with EMS, obesity is a modifying factor that causes the insulin system to work harder, no matter the age of the horse,” Frank said, so it’s crucial to feed obese horses low-energy diets, restrict their pasture access, and give them ample exercise.
  • The second type occurs in aged horses with PPID and normal insulin status. Many PPID horses have normal insulin status and are no more likely to develop laminitis than healthy aging animals. Manage these individuals with pergolide and a normal diet for aged horses, including senior feeds.
  • The third type is an aged horse with both PPID and EMS. These horses are genetically predisposed to insulin dysregulation and obesity, and have developed PPID as they have grown older. Some affected horses suffer from a combination of obesity, insulin dysregulation, and PPID and require a weight loss program, low-sugar diet, and pergolide. Other horses are lean, but battle insulin problems in addition to PPID; a low-sugar diet that provides adequate calories, combined with pergolide treatment, is recommended for these animals. .

Diagnostic Options

Methods for confirming disease vary, depending on the genetic predisposition of the horse. Currently there is no genetic test for EMS; however, the oral sugar test (OST) is a simple, dynamic test for insulin dysregulation that is easily conducted in the field and is sensitive enough to detect early stages of endocrine disease. Administering a bolus of Karo corn syrup orally after an overnight fast stimulates insulin production in the horse. The veterinarian compares blood glucose and insulin concentrations in samples taken before syrup administration and then 60 and 90 minutes later.

Horses with early stages of PPID typically show subtle signs of disease, including decreased performance, delayed shedding in areas of the body, and loss of muscle mass along the topline. Easy keepers become horses with greater calorie demands to maintain body condition. Frank said that in early stages of PPID, the thyrotropin-releasing hormone (TRH) stimulation test is especially useful. In this test, the veterinarian collects a blood sample before administering TRH intravenously and drawing blood again 10 minutes later. This hormone causes the pituitary gland to release adrenocorticotropin hormone (ACTH), so these concentrations will be higher in PPID horses than in normal ones.

Management Techniques

Frank said that managing EMS medically in older horses is difficult at best. If the animal is obese and diet and exercise interventions aren’t successful, he suggests veterinarians prescribe oral thyroid hormone (such as Thyro L) for three to six months to speed up a horse’s metabolism to stimulate weight loss. “This only works if calories are also restricted,” he stressed.

Another drug that could help EMS horses is metformin, which can increase insulin sensitivity, especially if given orally 30 to 60 minutes prior to eating. Frank said that preliminary research findings suggest that metformin works at the intestinal level to block spikes of insulin and glucose following a meal. “Metformin is used when weight control efforts and/or PPID signs (if present) are controlled, yet the horse continues to show high insulin responses and persistently suffers from laminitis,” advises Frank.

Meanwhile, pergolide remains the “gold standard” for PPID treatment. Frank recommends slowly introducing an increasing dose so that the horse will be less likely to experience the loss of appetite that occurs in 30% that start out on a full maintenance dose.

Take-Home Message

In summary, endocrine problems are common in elderly equids, Frank said. By screening all equine seniors for such problems, like PPID and HI, veterinarians can pick out problem horses quickly and implement treatment protocol.

"Endocrine disorders can be successfully managed in aged horses through appropriate husbandry and medical treatment and we can now expect well managed older horses to live into their 30s," he concluded.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.