Potential Biomarker for Equine Cushing's Identified

Lead a veterinarian to a horse with equine Cushing's disease and he or she will likely try to diagnose the horse by measuring blood levels of adrenocorticotrophic hormone (ACTH) in a single sample at a single point in time. Lead researchers from Pegasus Equine Diagnostics Ltd., in Nottingham, United Kingdom to the same horse and they'll measure ACTH levels over 24 hours at multiple times of the year and claim that ACTH is a biomarker for the disease.

Equine Cushing's disease (pituitary pars intermedia dysfunction, PPID) is a relatively common hormonal disturbance in older horses and ponies. Most affected horses have increased blood levels of ACTH, but because of normal, cyclical rhythms in the secretion of ACTH, blood levels can vary markedly over a 24-hour period and depending on the season. This makes diagnosing Cushing's disease challenging when only a single ACTH level is measured.

"While it is possible to diagnose equine Cushing's disease based on the ACTH concentration in a single blood sample, improper sample handling, stress, pain, and other factors can all affect the test results," relayed the researchers. "This is why reservations have been raised by experts in this field regarding the value of using ACTH to diagnose this disease."

Convinced of the merits of ACTH testing, the research team measured ACTH levels in healthy older horses over a 24-hour period and compared them to the ACTH levels in horses with Cushing's disease. ACTH was measured in blood samples collected in March, June, September, and December.

Key findings were:

  • ACTH levels were significantly higher in horses with Cushing's disease than in the healthy horses over a 24-hour period;
  • ACTH concentrations were significantly higher in September and December than in March and June in all horses; and
  • Fluctuations in ACTH levels were larger in horses with Cushing's compared to normal horses.

According to the study authors, "This is the first study that systematically evaluates ACTH levels of a 24-hour period over the course of four seasons."

Together, the data suggest that ACTH does indeed have a role in diagnosing Cushing's disease but that the reliability of ACTH as a biomarker--a compound that can measure the progress of disease or the effects of treatment on clinical outcome--is enhanced if more than a single sample is assessed at more than one time point per year.

The study, "The use of adrenocorticotrophic hormone as a potential biomarker of pituitary pars intermedia dysfunction in horses," was published in the July 2010 edition of The Veterinary Journal.

The abstract is available for free 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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