Remember Seasonal Variation in Cushing's Testing

Diagnosing equine Cushing's disease (pituitary pars intermedia dysfunction, or PPID) in horses or ponies not showing the "full blown" disease is complicated and frustrating.

"To accurately diagnose PPID in the early stages of disease, monitor the progression of the condition, or response to treatment, we require accurate endocrine tests," said Jill Beech, VMD, Dipl. ACVIM, from the Department of Clinical Studies at the New Bolton Center in Pennsylvania.

"If we don't know what normal is, then we can't identify abnormal, and if we don't take into account seasonal variation when we are testing animals, we could make an incorrect diagnosis."
--Dr. Jill Beech

A variety of tests are currently used to test horses for PPID including the dexamethasone suppression test, and adrenocorticotropin hormone (ACTH) concentration in blood samples. Insulin is also sometimes measured in conjunction with ACTH as horses and ponies with PPID can be insulin resistant. The potential value of measuring alpha-melanocyte-stimulating hormone (alpha-MSH) has been investigated, although there is currently no commercially available test.

"One problem with measuring ACTH and alpha-MSH is that concentrations appear to be affected by season," said Beech. "If we don't know what normal is, then we can't identify abnormal, and if we don't take into account seasonal variation when we are testing animals, we could make an incorrect diagnosis."

Levels of these hormones also appear to differ between horses and ponies.

To evaluate how ACTH and alpha-MSH concentrations varied seasonally and how this could affect endocrine testing and monitoring, Beech and colleagues compared levels of these hormones in 13 normal ponies, 14 clinically normal horses, and seven ponies and eight horses with PPID. The study included eight different photoperiods (February 13 to October 18) to determine the effect of season on these hormone levels.

Key findings in this study were:

  • There is a seasonal variation in both ACTH and alpha-MSH blood levels that is more evident in ponies than horses.
  • These variations should be taken into account when testing or monitoring horses or ponies for PPID;
  • Reference ranges for ACTH and alpha-MSH probably should be adjusted for season;
  • Normal ranges of ACTH, alpha-MSH, and insulin appear slightly different for horses and ponies and these differences should be considered when interpreting laboratory test results.

"This study revealed that ACTH and alpha-MSH concentrations in blood from horses and ponies with and without PPID changed depending on the season, but insulin did not seem to be affected," relayed Beech.

She did caution, however, that larger numbers of animals should be studied to fully evaluate seasonal effect on insulin.

The study, "Evaluation of plasma ACTH, alpha-melanocyte-stimulating hormone, and insulin concentrations during various photoperiods in clinically normal horses and ponies and those with pituitary pars intermedia dysfunction," was published in the September 15 edition of the Journal of the American Veterinary Medical Association. The abstract is available on PubMed.  

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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