We know horses and ponies with PPID (pituitary pars intermedia dysfunction, formerly known as equine Cushing’s disease) typically have higher-than-normal levels of specific hormones—namely ACTH, or adrenocorticotropic hormone—circulating in their bloodstreams. But researchers in Australia have recently confirmed certain breeds, even when healthy, harbor higher ACTH concentrations than others, as well.

Because PPID horses have elevated blood ACTH concentrations, veterinarians often use a baseline ACTH concentration test to diagnose the condition. But PPID isn’t the only thing that causes ACTH levels to rise, which is important for practitioners to keep in mind when evaluating test results.

“There are multiple factors that are known to influence ACTH levels, including season, latitude, diet, stress, exercise, and illness,” said Nicholas Bamford, BVSc (Hons), PhD, Dipl. ACVIM (LAIM), a senior lecturer at the Melbourne Veterinary School, in Victoria, Australia. “There is also evidence that breed can influence ACTH levels, so we conducted this study to further investigate the effect of breed on ACTH levels in horses and ponies.”

The study included 399 healthy horses and ponies (127 Thoroughbreds, 131 Shetland ponies, and 141 non-Shetland ponies, mostly Welsh or Welsh-type, Bamford said), none of which were exhibiting lameness or PPID clinical signs or had a history of PPID-related treatments. From each horse, the research team evaluated two blood samples: One collected within two weeks of the autumn equinox (because studies have shown ACTH levels increase in autumn, regardless of PPID status) and one within three weeks of the spring equinox.

“Our study confirmed that ponies, especially Shetland ponies, had much higher ACTH levels in autumn when compared to Thoroughbred horses, while ACTH levels were similar among breeds during spring,” Bamford said.

This, he said, suggests practitioners should use extra care when interpreting test results, particularly in pony breeds during fall.

“Since healthy ponies can demonstrate high ACTH levels during autumn, measurement of ACTH should only be performed when there is a clinical suspicion of PPID that warrants investigation,” he said. “The measurement of ACTH as part of a general health check in a pony without clinical signs of PPID could be problematic because a one-off high ACTH result in an otherwise healthy pony during autumn will be difficult to interpret. If a high ACTH result is obtained during autumn for a pony, it could be worth rechecking in spring to confirm that ACTH levels remain high.”

And while it’s still not clear why ACTH levels are higher in the autumn, this finding provides researchers with a potential direction for future research opportunities, Bamford said.

“We know that ponies and horses evolved from different lineages, so the observed differences in ACTH are likely to reflect natural patterns of pituitary hormone secretion that would have prepared ‘thrifty’ pony breeds for harsh winters and seasonal shortage of food,” he explained. “Future work to further investigate these differences in other horse breeds, such as Arabians or Warmbloods, would be interesting.”

The study, “Investigation of breed differences in plasma adrenocorticotropic hormone concentrations among healthy horses and ponies,” was published in The Veterinary Journal in July, 2023.