The Horse

MAR 2019

The Horse:Your Guide To Equine Health Care provides monthly equine health care information to horse owners, breeders, veterinarians, barn/farm managers, trainer/riding instructors, and others involved in the hands-on care of the horse.

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YOUR GUIDE TO THE 2018 AAEP CONVENTION A33 The Horse | AAEP Wrap-Up March 2019 STACEY OKE, DVM, MSC CLAIR THUNES, PHD Testing for Endocrine Diseases in Middle-Aged and Older Horses M iddle-aged and older horses are at greater risk of developing debilitating endocrine disor- ders such as insulin dysregulation (ID, abnormal blood insulin levels) and equine metabolic syndrome (EMS), which are best treated as early as possible. A Michi- gan State University (MSU) vet says it's essential that owners know the signs of these disorders and have their vets screen for them during routine visits. Elizabeth Tadros, DVM, PhD, Dipl. ACVIM, assistant profesor of endocrinol- ogy at the MSU Veterinary Diagnostic Laboratory, in Lansing, described these diseases and tests for diagnosing them. Insulin dysregulation is the culmination of interactions between genetic, physi- ological, and environmental risk factors and can plague horses of any age, said Tadros. The biggest concern when man- aging ID is the impact of high levels of circulating insulin (hyperinsulinemia) on hoof health, because these horses are at an increased risk of developing laminitis (the painful separation of the laminae that connect the coffin bone to the hoof wall), she said. Therefore, monitoring and controlling ID in at-risk horses is vital. Insulin dysregulation occurs alone or alongside clinical signs associated with other diseases. It's one of three compo- nents that constitute EMS, the other two being obesity and endocrine-related laminitis. It also affects 30-60% of horses with pituitary pars intermedia dysfunc- tion (PPID, equine Cushing's disease). Endocrine tests can be static or dynam- ic. Static tests measure resting hormone concentrations and represent endocrine function at one point in time, whereas dynamic tests measure the response to a challenge (such as oral glucose adminis- tration) over time. Tadros said vets should use dynamic ID tests—such as the oral sugar test (OST), in-feed oral glucose test, and insulin tolerance test—in conjunction with the thyrotropin-releasing hormone stimulation test (TRH) to diagnose PPID and gain a comprehensive diagnostic pic- ture of endocrine status. Once diagnosed, she added, practitioners can use static tests to monitor progress over time. Tadros said it's important to consider the horse's clinical response (how he appears) to treatment and management compared to his biochemical response— what's going on inside his body. Some horses might show clinical improvement, but their endocrine test results still aren't normal. At that point, vets and owners can work together to make management changes to control ID—the most impor- tant goal when managing horses with endocrine disorders. Diagnosing ID: Consider the Season As many as 89% of horses with lami- nitis develop it because of insulin-related endocrine disorders, so it's important to monitor for these problems and make diagnoses early. Time of year can impact blood-insulin levels, though, complicating diagnosis for other endocrine diseases, such as PPID. A University of Kentucky (UK) research group recently analyzed how season affects one type of endocrine test and visual indicators that could help veterinarians make a diagnosis. Erica Macon, MS, a PhD candidate under Amanda Adams, PhD, at UK's Gluck Equine Research Center, in Lexington, examined seasonal effects on morphometric—body shape and size— measurements and insulin responses to the OST in horses with and without ID. The measurements she and her team con- sidered were body weight, body condition score (BCS), and cresty neck scores. At the start of the study, Macon and her colleagues examined the endocrine history, insulin levels, and adrenocorti- cotropic hormone (ACTH) levels of 22 horses (11 horses with ID and 11 without ID) in their mid- to late-teens. "This is important to rule out cases of PPID," Macon said. "PPID could affect the results of the OST, so we want it to be clear that horses only have one metabolic disorder." They maintained the horses on pasture with hay as needed and collected morpho- metric measurements and OST samples in February, April, August, and October. The OST involves collecting baseline insulin and glucose samples (T0) after a grain-fast and then administering corn syrup orally at a body-weight-determined dose. Macon and team collected addition- al blood samples 60 minutes later (T60) for glucose and insulin analysis. Macon's team found no significant difference in the ID and normal horses' body weights across seasons. But when looking at seasons individually, they noticed BCS and cresty neck scores were significantly higher in the affected horses than in the normal horses. When ISTOCK.COM Endocrine Disease & Other Old-Horse Issues

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