by Aimee Eggleston, DVM
October 23rd, 2015
Too often, when asked by a client of mine to answer this seemingly simple question, does my horse have Cushing's Disease, the answer was, I don't know. Until recently, diagnosing horses with equine pituitary pars intermedia dysfunction (PPID), commonly referred to as Cushing's Disease, has been difficult. The blood testing available to equine veterinarians has been unreliable, giving inaccurate results. Too often horses afflicted with with the disease were not identified by the blood tests. Horses would go untreated, left to suffer the progressive, debilitating effects of Cushing's Disease – as their owners and veterinarians proceeded in the dark.
In 2011, a a group of leading veterinarians and researchers in the field of equine endocrinology, the Equine Endocrinology Group (EEG), developed a new, more reliable, test for Cushing's Disease. The test is called the Thyrotropin-Releasing Hormone (TRH) Stimulation Test. In 2013, the EEG released their findings and recommendations which focused on this test as central to reliably diagnosing Cushing's Disease.
The development of the TRH Stimulation Test is exciting and important news for horse owners. We now have a reliable blood test for Cushing's Disease, and one, importantly, that can detect the presence of the disease early. Previous blood tests for diagnosis were known to either produce “false negative” diagnoses, where a horse that did have Cushing's Disease was reported as “normal” on the blood test, or not identify a horse as positive until they were in a more advanced stage of the disease. As Cushing's Disease is a progressive disease, early diagnosis, and therefore earlier treatment, is imperative. Early treatment helps mitigate the myriad effects of the disease and can help prevent some of the worst consequences of the disease.
Cushing's Disease is caused by enlargement of the middle section of the pituitary gland which is located at the base of the brain. This section of the pituitary gland is important in regulating several circulating hormones which have important effects on many organ systems of the horse's body. Cushing's Disease primarily affects an increase in a hormone we commonly refer to as Adrenocorticotropic Hormone (ACTH), which then thereby causes an increase in the circulating hormone, cortisol, a steroid. Slowly over time more and more circulating steroids are released into the horse's body thereby causing greater effect on numerous systems of the body.
The TRH Stimulation Test is a reliable, easy to perform, blood test. The TRH Stimulation test is performed in two parts: a baseline blood sample is pulled, TRH is injected intravenously, and a second blood sample is collected 10 minutes later. Both blood samples are then submitted to the laboratory to test the ACTH level of each sample. If a horse is afflicted with Cushing's disease, the second sample, taken after the injection of TRH, will show a markedly elevated level of ACTH. Parameters for normal response in horses have been established by the EEG for the months December through June, so the test is most appropriately performed during those seven months.
It is important to identify Cushing's Disease early in order to provide the best chance of preventing the worst effects of the disease. Horse owners and veterinarians cannot rely on clinical signs alone for diagnosis of Cushing's Disease or even to identify suspect horses. The stereotypical wooly hair coat is only the most well known of the clinical signs of the disease, but may not be present in horses with Cushing's Disease. A horse in the early stages of Cushing's Disease may have no or only subtle clinical signs. Because of this fact the TRH Stimulation Test represents a significant step forward.
Cushing's Disease generally affects older animals, so if you have a senior horse (16+) – with or without symptoms – talk with your veterinarian about TRH Stimulation Test.
To learn more about the clinical signs of Cushing's Disease, visit: