Researchers Evaluate Steps for Diagnosing Cushing's Disease

An earlier diagnosis for a horse often means a better prognosis for his recovery, but the reality with equine Cushing's disease has been that clinical signs are abundant before the diagnostic process begins. A team of researchers recently determined what they believe to be the ideal first step for diagnosing PPID early, however, and this could allow affected horses to receive treatment sooner and slow the progression of the debilitating disease.

The disease is caused by an enlargement of the pars intermedia of the pituitary gland, which is the central part of the pituitary gland located at the base of the brain. When the gland becomes enlarged, it secretes excessive amounts of adrenocorticotropin hormone (ACTH) which increases adrenal gland cortisol (stress hormone) secretion. A common condition in older horses, clinical signs include a long hair coat, delayed shedding of the winter hair coat, loss of muscle mass, increased drinking and urination, a pot-bellied appearance, and an increased susceptibility to infections. If left untreated, Cushing's horses generally experience chronic bouts of disease, a decline in health and comfort, and reduced quality of life.

There are several tests currently available to aid veterinarians in diagnosing PPID, measuring:

  • Resting adrenocorticotropic hormone levels (and keeping in mind that ACTH concentrations can be variable in an individual horse with pituitary dysfunction, it is important to get at least two blood samples);
  • Suppression of cortisol from dexamethasone administration (the dexamethasone suppression test); and
  • The change in ACTH concentrations in the blood after administering thyrotropin-releasing hormone (the TRH response test) to the horse.

However, veterinarians report that alternative tests for equine Cushing's are needed, particularly to diagnose the disease prior to the onset of potentially life-threatening conditions, such as laminitis. In an attempt to find a better PPID diagnostic method, Jill Beech, VMD, Dipl. ACVIM, of the Department of Clinical Studies at the New Bolton Center in Pennsylvania, and colleagues compared the ACTH responses in the TRH test to those in a domperidone response test. (Domperidone is the substance that stilumates secretion of ACTH.)

"We also... compared the α-melanocyte-stimulating hormone (αMSH) response with ACTH response to TRH administration, as some research has suggested that measuring the former should be more accurate for diagnosing PPID," said Beech. "αMSH is produced by the pituitary gland like ACTH but is more specific for the pars intermedia."

Key findings of the study were:

  • The domperidone stimulation test did not appear as accurate as the TRH stimulation test; and
  • Measuring αMSH was not a superior method for measuring ACTH levels.

The authors' findings indicate that measuring ACTH from several basal (resting) blood samples is the first logical step for diagnosing PPID.

"If the ACTH levels are very high and other potential influential factors have been eliminated from consideration, this is very good evidence that the horse or pony has PPID," advised Beech. Levels of ACHT and αMSH have been documented to vary seasonally, however the reason behind the difference remains unclear.

The authors caution, however, that even if the concentrations on testing are normal, the horse might still have the condition, so he should be kept under close observation for further signs of the disease. In this case the authors attest that the ACTH response to TRH administration appears to be the best test for diagnosing the condition.

According to Beech, "if the horse shows classical signs of PPID, endocrine testing is probably not necessary for making the diagnosis," but it's a better scenario to diagnose the condition before it advances to a less treatable stage.

The study, "α-melanocyte-stimulating hormone and adrenocorticotropin concentrations in response to thyrotropin-releasing hormone and comparison with adrenocorticotropin concentration after domperidone administration in healthy horses and horses with pituitary pars intermedia dysfunction," was published in the May 15, 2011 edition of the Journal of the American Veterinary Medical Association. The abstract is available online.

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