testing for endocrine diseases in older horses
Thanks to advances in equine health care, more horse owners find themselves in the position of managing middle-aged and older horses with careers extending well in to their 20s. This older equine population is at greater risk of developing debilitating endocrine disorders such as insulin dysregulation and equine metabolic syndrome, all of which are best treated as early as possible. As such, one Michigan State University (MSU) researcher says it’s essential that owners be aware of the signs that suggest their horses might be suffering from these conditions and to have their veterinarians screen for them during routine visits.

At the 2018 American Association of Equine Practitioners Convention, held Dec. 1-5, in San Francisco, California, Elizabeth Tadros, DVM, PhD, Dipl. ACVIM, assistant professor of endocrinology at the MSU Veterinary Diagnostic Laboratory, in Lansing, gave a rundown on these diseases and described commonly used tests for diagnosing them.

Insulin dysregulation (ID), which can plague horses of any age, is a metabolic derangement that can manifest in several ways, including:

  • Higher-than-expected insulin levels in horses that have been fasted (fasting hyperinsulinemia);
  • Higher-than-expected insulin levels after a meal (postprandial hyperinsulinemia);
  • Exaggerated responses to oral or intravenously introduced carbohydrates;
  • Tissue insulin resistance; and
  • Dyslipidemia (elevated blood lipid, or fat, levels).

Horses with ID might have one or a combination of these characteristics.

Insulin dysregulation is the culmination of interactions between genetic, physiological, and environmental risk factors for the disorder, said Tadros. The biggest concern when managing ID is the impact of high levels of circulating insulin (hyperinsulinemia) on hoof health, because these horses are at increased risk of developing laminitis (the painful separation of the laminae that connect the coffin bone to the hoof wall), she said. Researchers believe hyperinsulinemia activates a laminar growth factor receptor in the hoof, resulting in weakened, stretched laminar tissue, said Tadros. Therefore, monitoring and controlling ID in at-risk individuals is vital to preventing laminitis.

Insulin dysregulation can occur alone or alongside clinical signs associated with other diseases. It is one of three components that constitute equine metabolic syndrome (EMS), the other two being obesity and clinical or subclinical (apparent or inapparent) endocrine-related laminitis. Insulin dysregulation also affects 30-60% of horses with pituitary pars intermedia dysfunction (PPID, or equine Cushing’s disease).

Endocrine tests can be static or dynamic in nature. 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 administration) over time. Tadros said veterinarians should use dynamic ID tests—such as the oral sugar test, in-feed oral glucose test, and insulin tolerance test—in conjunction with the thyrotropin-releasing hormone stimulation test (TRH) for diagnosing PPID to gain a comprehensive diagnostic picture of a horse’s endocrine status. Once diagnosed, she added, veterinarians can use static tests to monitor a condition’s progress over time.

Tadros said it’s important to consider the horse’s clinical response—how he appears on the outside—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 within the normal ranges. At that point, vets and owners can work together to make additional management changes to control the horse’s ID—the most important goal when managing horses with endocrine disorders.