When a horse is undergoing diagnostics for suspected pituitary pars intermedia dysfunction (PPID, formerly known as equine Cushing’s disease), the veterinarian must consider many factors, including test results, age, history, time of the year, and clinical signs.
Amanda Adams, PhD, associate professor at the University of Kentucky’s Gluck Equine Research Center, in Lexington, described PPID testing methods and other diagnostic considerations at the 2022 American College of Veterinary Internal Medicine (ACVIM) forum, held June 23-25, in Austin, Texas.
“Signs suggestive of early stage PPID include regional hypertrichosis/delayed shedding, loss of topline muscle, change in attitude/lethargy, and decreased performance,” Adams told The Horse. “Advanced PPID signs include generalized hypertrichosis, abnormal sweating, topline muscle atrophy, exercise intolerance, rounded abdomen, polyuria/polydipsia (excessive urination and drinking, respectively), and recurrent infections.”
Age is also a factor when making an accurate diagnosis: Around 20% of horses over 15 have PPID, and that number rises to 30% in horses over 30 years old. Therefore, Adams recommends testing all horses aged 15 and older if they’re showing signs consistent with PPID.
Adams said relying on clinical signs alone to make a diagnosis can result in missed cases. A horse can show few signs in the summer and fall, for instance, while displaying more prominent ones in the winter or spring. Therefore, evaluating both clinical signs and diagnostic test results is ideal.
“Because PPID is a progressive degenerative disease, early detection is really critical,” she said. “(Catching cases early is) quite important for starting appropriate treatment in the early stage … to help slow down the progression of the disease.”
Veterinarians consider two methods to be gold-standard diagnostic tests for PPID. They can either measure baseline plasma adrenocorticotropic hormone (ACTH) concentrations or perform a thyrotropin-releasing hormone (TRH) stimulation test and subsequently measure ACTH concentration. Adams said she typically prefers TRH stimulation testing for horses suspected of being in the early stages of PPID. If the horse has more advanced PPID, she might recommend baseline ACTH testing, which can yield significant results. Veterinarians no longer recommend oral domperidone challenge testing or combined dexamethasone suppression tests. She said ACTH stimulation testing, diurnal cortisol rhythm testing, and baseline cortisol rhythms are not valid methods for testing a horse for PPID.
To get the most accurate results, said Adams, owners and veterinarians should “be consistent with what diagnostic test is being used, as this will be helpful in monitoring clinical signs of disease and for response to treatment over time.”
The timing of testing can influence results. If the horse has been stressed, exercised, or trailered recently, delay testing by at least 30 minutes. The presence of laminitis or pain, said Adams, can also influence results. Avoid sedating the horse within 48 hours of testing.
Horses can have access to forage before testing but must be grain-fasted, said Adams. Samples should not be collected within 12 hours after a grain meal.
Adams suggests keeping blood samples refrigerated or on ice packs, due to ACTH’s instability; temperature can affect ACTH levels by 5-12%. If the samples are kept cool, research has shown they will retain a stable ACTH level for 36 hours. Ideally, samples should be centrifuged within two to four hours of collection and frozen. Avoid multiple freeze-thaw cycles, and ship samples overnight if local testing isn’t available.
“Samples collected from a horse should be sent to credible diagnostic lab that uses appropriate and validated assays for measuring ACTH,” said Adams, adding that “results are not interchangeable from lab to lab,” because laboratories use different reference ranges and processes. Therefore, send a horse’s routine samples to the same lab for the most accurate diagnostics.
ACTH levels are naturally highest in horses (with or without PPID) in the fall, mid-range in August, and lowest in December through June and July and November. Keep this in mind when testing so you can interpret test results accurately. Adams suggests performing routine diagnostic testing for monitoring at least twice per year, in the fall and nonfall seasons, in addition to working with a credible diagnostic lab that uses the currently recommended seasonal cutoff ranges for ACTH.
Insulin dysregulation (ID) can occur concurrently with PPID. If the veterinarian suspects a horse has ID based on clinical signs, they should test for that condition, as well.
If a veterinarian is testing a horse for both PPID and insulin dysregulation, they should perform PPID testing first. They can take samples immediately before an oral sugar test but not within the ensuing 12 hours. Because many horses that are PPID positive can also have insulin regulation challenges, “it’s a good idea to look at the insulin status of the horse while you are diagnosing PPID,” said Adams.
Identifying PPID early is ideal for the horse’s long-term well-being. “Lack of considering all factors together and relying on diagnostic testing alone could result in misdiagnosis and/or inappropriate diagnosis and unnecessary lifelong treatment,” Adams said. “Also, not incorporating diagnostic testing could result in lack of diagnosing PPID, especially in early disease stages. Thus, incorporating clinical signs of disease along with diagnostic testing is important.”
Keeping season in mind and testing ACTH levels prior to insulin dysregulation testing will produce the most accurate results. Most importantly, said Adams, consider the whole picture of the horse, including age, clinical signs of disease, and bloodwork when diagnosing PPID.