Testing for Equine Cushing’s Disease

New diagnostic guidelines may help find equine Cushing's Disease in its early stages, when signs are mild.
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Every horse in the barn has started to shed—except yours, and you’ve started to worry. Could he be developing equine Cushing’s disease? Delayed shedding can be one sign of that. New testing guidelines may make it easier for your veterinarian to find out.

Delayed shedding can be a sign of Cushing’s disease. © Paula da Silva/arnd.nl

Delayed shedding can be a sign of Cushing’s disease. © Paula da Silva/arnd.nl

Equine Cushing’s, also known as pituitary pars intermedia dysfunction, affects many older horses, most age 15 or more. It involves the pituitary gland, a master gland that orchestrates the production of the hormones that regulate body functions. The disease causes a range of problems, including abnormal hair growth, sweating and fat distribution; weight loss, muscle wasting and lethargy; excessive drinking and urination; lowered resistance to chronic infections and laminitis.

The risk of laminitis, which can be life threatening, makes identifying and managing PPID especially important. The new diagnostic guidelines were developed by a group of veterinarians and researchers with expertise in equine endocrinology. These guidelines are meant to help spot the disease in its early stages, when signs are mild—maybe just delayed shedding or decreased athletic performance.

“Our collective research has shown that horses can often develop this disease earlier in life, yet earlier clinical signs don’t always translate into positive test results,” says Nicholas Frank, DVM, professor and chair of the Department of Clinical Sciences at the Cummings School of Veterinary Medicine at Tufts University and coordinator of the Equine Endocrinology Group. The group’s other members include vets Frank Andrews of Louisiana State University, Andy Durham of Liphook Equine Hospital in Britain, Dianne McFarlane of Oklahoma State University and Hal Schott of Michigan State University.

The group recommends that horses with signs of PPID have one of two blood tests. One measures resting adrenocorticotropin hormone, which is produced by the pituitary gland. This test has been used to detect PPID, but in the early stages of the disease horses can have normal resting ACTH concentrations.

The other test, a thyrotropin-releasing hormone stimulation test, is especially useful in those cases. The veterinarian takes a baseline blood sample, injects TRH intravenously and collects a second blood sample 10 minutes later. TRH causes the pituitary gland to release more hormones and ACTH concentrations rise to higher levels in horses with the disease.

Previously, horses with signs of PPID were often given an overnight dexamethasone-suppression test. For this test, the veterinarian draws a baseline blood sample late in the day, gives the horse an injection of the steroid dexamethasone and returns the next morning to take a second blood sample. Dexamethasone causes the production of the hormone cortisol to decrease in healthy horses but not those with PPID. The group no longer recommends the DST because research shows that it’s no better at detecting PPID than other tests, and some people worry that dexamethasone could trigger laminitis.
Horses’ hormone levels fluctuate, and that complicates testing. ACTH levels are normally higher from August to October than from November to July, so season-specific reference ranges should be used when diagnosing PPID using resting ACTH concentrations. Season also affects the TRH stimulation test, and, currently, it can only be used from December to June, since cut-off values haven’t been established for the rest of the year.

There is no cure for PPID, but many horses improve with daily doses of the drug pergolide. Some horses with this condition also develop insulin resistance, which adds to laminitis risk. These horses have exaggerated insulin responses to starches and sugars in feed and forage, and they need a special diet. Additional blood tests can assess insulin status.

This article originally appeared in the March 2014 issue of Practical Horseman.

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