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Equine Group Recommends a Switch to TRH Testing for PPID

Posted: December 10, 2013, 3:30 p.m. EDT

The Equine Endocrinology Group, or EEG, is recommending that the overnight dexamethasone suppression test (DST) be replaced with the more reliable thyrotropin-releasing hormone (TRH) stimulation test as a diagnostic tool for equine pituitary pars intermedia dysfunction, or PPID.

Since 2011, the EEG has recommended that horses exhibiting signs of PPID be tested for adrenocorticotropin hormone (ACTH) in the pituitary gland or that the DST be used.

The shift toward TRH testing by the group of equine veterinarians and endocrinologists is based on:
• Recent research showing that the DST is no better at detecting PPID than other tests.
• Horse owners’ concerns about dexamethasone inducing laminitis, a painful condition affecting the feet of horses that can lead to death.

Although PPID is most often associated with older horses, which exhibit elevated hormones in their blood, younger horses are getting PPID, too.

Horse
Although PPID is most often associated with older horses, they can also develop the disease early in life. Shutterstock

"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,” said Nicholas Frank, DVM, Dipl. ACVIM, chair of Clinical Sciences at Tufts University’s Cummings School of Veterinary Medicine and group coordinator for the EEG.

Research by Donaldson et al. (2004) gives the age range of horses suspected of having PPID from 3 to 28 years, with a median age of about 15.

First identified in the 1930s and called equine Cushing’s disease, the condition was renamed PPID in the 1990s.

The disease presents a litany of clinical signs, including hirsutism, hyperhidrosis, sinusitis, sole abscesses, muscle wasting, extreme thirst, frequent urination, periodontal disease, skin infections, swayback, potbelly, laminitis and pneumonia.

The TRH test is especially useful when horses with early PPID have normal resting ACTH concentrations. This test causes the pituitary gland to release more hormones and ACTH concentrations increase to a higher level in horses with PPID.

The TRH test only requires a baseline blood sample, injecting TRH intravenously and collecting a second blood sample 10 minutes later. 

The current recommendations are that the TRH test should only be used between December and June, which is the only period in which cut-off values have been established. 

"As research on PPID advances, we are identifying practical ways to improve early detection and diagnosis,” Dr. Frank said.

Once diagnosed, PPID can be successfully managed using pharmaceuticals. If left untreated, symptoms are likely to increase and become more severe over time. 

In addition to Frank, the EEG includes Drs. Frank Andrews, DVM, Dipl. ACVIM, Louisiana State University; Andy Durham, BVSc, Dipl. ECEIM, Liphook Equine Hospital in the United Kingdom; Dianne McFarlane, DVM, Ph.D., Dipl. ACVIM, Oklahoma State University; Hal Schott, DVM, Ph.D., Dipl. ACVIM, Michigan State University.   

For more on PPID diagnostic guidelines, visit Sites.Tufts.Edu/EquineEndoGroup/.

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