One of the Table Topic discussions at the 2010 American Association of Equine Practitioners annual meeting, held Dec. 4-8 in Baltimore, Md., addressed equine endocrine disorders. Moderators and attendees discussed diagnostic testing for equine metabolic syndrome (EMS) and pituitary pars intermedia dysfunction (PPID). Horses with the EMS phenotype (easy keeper, regional adiposity, obesity, predisposition to laminitis) should be screened by measuring blood glucose and insulin concentrations after a short fasting period (leave one flake of hay after 10 p.m. and collect blood the next morning). Hyperinsulinemia (high blood insulin concentrations) further supports the diagnosis of EMS. Glucose concentrations usually fall within reference range in horses with EMS, but should still be measured to rule out diabetes mellitus. If glucose and insulin concentrations fall within reference range, but mild/early EMS is still suspected, a newly developed oral sugar test can be performed on the farm. This modified oral glucose tolerance test is performed by the owner giving their horse a dose of corn syrup (purchased at the grocery store) by mouth 60 to 90 minutes before the veterinarian arrives. A single blood sample is collected; horses with EMS will have higher glucose and insulin concentrations.

Table topic attendees recommended two diagnostic tests for PPID. Adrenocorticotropin hormone (ACTH) concentrations can be monitored as part of biannual wellness examinations in middle-aged and older horses. Only one or two blood samples are required for this test and high ACTH concentrations are detected in horses with PPID. Practitioners noted that ACTH concentrations normally rise in the late summer and fall (July to November), so the reference range is wider at this time of the year. As a rule of thumb, negative results (low concentrations) are reliable throughout the year, but positive results must be interpreted according to season. Middle-aged horses should be monitored for early signs of PPID, such as delayed shedding of the winter haircoat for a few additional weeks, loss of muscle mass, and switching from being an easy keeper to a hard keeper with respect to feeding. Veterinarians also recommended the dexamethasone suppression test for diagnosing PPID, and two additional tests were mentioned: the thyrotropin-releasing hormone (TRH) stimulation test and the oral domperidone challenge.

Equine metabolic syndrome first develops in younger horses, whereas PPID is an endocrine disorder of older horses. Horses with EMS might be predisposed to PPID and should be monitored for this endocrine disorder as they get older.

Attendees also discussed management of endocrine disorders. Most horses with EMS are obese, so this problem should be addressed through weight loss and exercise. Levothyroxine can be administered to obese horses for three to six months to accelerate weight loss while also restricting caloric intake is. The practitioners noted that insulin resistance also can occur in lean horses, and these cases are more difficult to manage. Owners must provide their horses with calories without exacerbating insulin resistance, and metformin is a medical option. Although metformin currently is used to manage insulin resistance in horses, further research is required to establish ideal dosages. Laminitis is usually triggered by changes in pasture grass abundance and composition, so insulin resistant horses should be turned out in drylots until insulin sensitivity improves. Grazing must be restricted for as long as insulin resistance persists, and pasture turnout should be slowly reintroduced once normal insulin sensitivity returns. Horses that were obese in the past are likely to return to this state when reintroduced to pasture, and EMS might have a genetic basis. A study is under way at the University of Minnesota to examine the genetics of EMS, and horse owners are encouraged to participate.

Pergolide is the drug of choice for treating PPID, and treatment should be instituted early in the course of the disease with the aim of slowing growth of the pituitary tumor. The pergolide dosage should be periodically reassessed as the horse ages.

This session was moderated by Nicholas Frank, DVM, PhD, Dipl. ACVIM, associate professor of large animal clinical sciences at the University of Tennessee, and Frank Andrews, DVM, MS, Dipl. ACVIM, director of the Louisiana State University Equine Health Studies Program.

About the Author

Nicholas Frank, DVM, PhD, Dipl. ACVIM

Nicholas Frank, DVM, PhD, Dipl. ACVIM, is a professor of large animal internal medicine and chair of the department of clinical sciences at the Cummings School of Veterinary Medicine, Tufts University, Massachusetts. His research interests include laminitis, metabolic disorders, PPID/equine Cushing’s, and many other internal-medicine related areas.

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