By: Molly Kaplan, DVM
A: While there are a number of tests that veterinarians have used in the past to test for Equine Cushing’s Disease or Pituitary Pars Intermedia Disorder (PPID), today there are two main blood tests that veterinarians will use due to improved sensitivity and safety, namely the Resting ACTH test and the ACTH Thyrotropin Releasing Hormone (TRH) Response test.
Cushing’s Disease is largely due to degeneration of dopamine-producing neurons in the brain. The decreased levels of dopamine are unable to properly regulate the pituitary gland, resulting in the excessive release of hormones and neurotransmitters. Cushing’s Disease may also be associated with a benign tumor of the pituitary gland (pituitary adenoma) that produces excessive hormones and neurotransmitters. Adrenocorticotropic Releasing Hormone (ACTH) is of most concern regarding this disease, as it floods to the adrenal glands and stimulates the release of excessive blood cortisol. The excessive blood cortisol leads to the clinical signs commonly associated with a horse affected by Cushing’s, such as a long curly haircoat, increased drinking and urination, behavioral changes, chronic infections such as foot abscesses and muscle wasting.
Depending on if your horse is displaying early or late signs of Cushing’s Disease and given the time of the year, your veterinarian will choose one of the two following blood tests–Resting ACTH or the ACTH TRH Response test.
The Resting ACTH test is historically one of the most common ways to test for Cushing’s Disease. It requires one blood sample and can be tested at any time during the year. ACTH is normally produced in small amounts by every horse at all times of year and it is normally elevated in the fall, as it is a hormone that is intimately involved in your horse’s preparation for the winter month (picture the winter coat!), but a horse affected by Cushing’s Disease has an excess in ACTH production at all times of the year. The Resting ACTH test is the test your veterinarian might choose if the horse is showing classic, late term Cushing’s-like signs (shaggy hair coat, pot belly, muscle wasting, increased urination and drinking). Because the Resting ACTH level may periodically fall within normal limits for a horse only mildly affected by Cushing’s Disease (resulting in a “false negative” test), your veterinarian may recommend performing an ACTH TRH Response test instead.
Recently the Equine Endocrinology Group (EEG) has recommended performing the ACTH TRH Response test over the Resting ACTH test because it is a more sensitive test for diagnosing early or mild Cushing’s Disease. The process for this test is fairly simple–a baseline ACTH blood sample is collected before administering a small dose of TRH to your horse. A second ACTH blood sample is collected 10 minutes after administering the TRH. Once the TRH is injected into the horse, the TRH stimulates the horse affected by Cushing’s Disease to produce abnormally high concentration of ACTH, but does not affect the ACTH levels of a normal, healthy horse. One limitation of this new test however is that there are no seasonal reference intervals for the fall months so testing should be avoided from mid-July to late October.
Starting in 2018, Three Oaks Equine Veterinary Services will be switching from the Resting ACTH test to the ACTH TRH Response test in the Senior Wellness Program. Based on the recommendations of the Equine Endocrinology Group and the improved sensitivity of the test, the ACTH TRH Response test is a better means for both identifying and managing a horse with Cushing’s Disease. If you have any questions about the different Cushing’s tests or if you feel that your horse may be displaying signs of Cushing’s Disease, please contact either Dr. Jennifer Wright or Dr. Molly Kaplan.