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Australian veterinary journal2020; 98(12); 616-621; doi: 10.1111/avj.13022

Circannual variation in plasma adrenocorticotropic hormone concentrations and dexamethasone suppression test results in Standardbred horses, Andalusian horses and mixed-breed ponies.

Abstract: To compare circannual plasma concentrations of adrenocorticotropic hormone (ACTH) and seasonal dexamethasone suppression test (DST) results between three different equine breed groups. Methods: Six Standardbred horses, six Andalusian horses and six mixed-breed ponies were followed over a 1-year period, during which time groups were managed identically. Blood samples were collected monthly (around the autumn equinox) or in every second month (other times of the year) for the determination of plasma ACTH concentrations using a chemiluminescent immunoassay. Overnight DSTs were performed quarterly, with suppression of plasma cortisol to below 27 nmol/L at 19 h considered a normal result. Results: Seasonal variation in plasma ACTH concentrations was present among all breed groups with, as expected, higher levels detected around the autumn equinox, from February to April (P < 0.001). Plasma ACTH concentrations were different between breed groups in March, with higher levels in Andalusians compared with Standardbreds (P = 0.048) and in ponies compared with Standardbreds (P = 0.010). Suppression of cortisol during the DST was normal for all animals in winter, spring and summer, but five Andalusians and three ponies returned abnormally high results in autumn, compared with zero Standardbreds. Conclusions: Higher plasma ACTH concentrations and more false-positive DST results were obtained during autumn in ponies and Andalusian horses when compared with Standardbred horses. Potential differences between breeds should be considered when interpreting test results for horses and ponies that are evaluated for pituitary pars intermedia dysfunction. Further work is recommended to establish population-based reference intervals and clinical cut-off values for ACTH in different equine breeds.
© 2020 Australian Veterinary Association.
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Publication Date: 2020-10-01 PubMed ID: 33001453DOI: 10.1111/avj.13022Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

The research article explores the seasonal variations in ACTH concentration and results of dexamethasone suppression tests among Standardbred horses, Andalusian horses, and mixed-breed ponies over a year. The study discovered that these equine breeds exhibit varied ACTH concentrations during seasons and respond differently to dexamethasone suppression tests, suggesting breed differences should be considered when diagnosing pituitary pars intermedia dysfunction in equine species.

Research Methods

  • The research involved three groups of equines – six Standardbred horses, six Andalusian horses and six mixed-breed ponies. These groups were observed over one year, and their management was identical throughout this period.
  • Blood samples were gathered from the equines monthly (specifically during the autumn equinox) or every second month at different times of the year. These samples were taken to measure plasma ACTH concentrations using a chemiluminescent immunoassay.
  • An overnight dexamethasone suppression test (DST) was conducted every quarter. The researchers considered a suppression of plasma cortisol to below 27 nmol/L at 19 h as a normal DST result.

Findings

  • The research observed a seasonal fluctuation in plasma ACTH concentrations in all the breed groups, with a notable increase around the autumn equinox, between February and April (P < 0.001).
  • There were distinctions between the breed groups’ plasma ACTH concentrations in March. The Andalusian horses and ponies exhibited higher levels than the Standardbred horses (P = 0.048 and P = 0.010 respectively).
  • During the DST, all animals showed normal suppression of cortisol in winter, spring, and summer. However, in autumn, five Andalusian horses and three ponies had abnormally high results compared to zero for Standardbreds.

Conclusion

  • The study concluded that the plasma ACTH concentrations and DST results in autumn were higher in ponies and Andalusian horses as compared to Standardbred horses.
  • The study suggests that breed differences should be taken into account when interpreting such test results for different breeds of horses and ponies, especially in terms of pituitary pars intermedia dysfunction.
  • The researchers recommend additional studies to establish population-based reference intervals and clinical cut-off values for ACTH in different equine breeds.

Cite This Article

APA
Bamford NJ, Harris PA, Bailey SR. (2020). Circannual variation in plasma adrenocorticotropic hormone concentrations and dexamethasone suppression test results in Standardbred horses, Andalusian horses and mixed-breed ponies. Aust Vet J, 98(12), 616-621. https://doi.org/10.1111/avj.13022

Publication

Australian veterinary journal
ISSN: 1751-0813
NlmUniqueID: 0370616
Country: England
Language: English
Volume: 98
Issue: 12
Pages: 616-621

Researcher Affiliations

Bamford, N J
  • Melbourne Veterinary School, The University of Melbourne, Parkville, Victoria, Australia.
Harris, P A
  • Equine Studies Group, WALTHAM Petcare Science Institute, Melton Mowbray, Leicestershire, UK.
Bailey, S R
  • Melbourne Veterinary School, The University of Melbourne, Parkville, Victoria, Australia.

MeSH Terms

  • Adrenocorticotropic Hormone
  • Animals
  • Breeding
  • Dexamethasone
  • Horse Diseases
  • Horses
  • Hydrocortisone
  • Pituitary Diseases / veterinary

Grant Funding

  • Australian Research Council
  • WALTHAM Centre for Pet Nutrition

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Citations

This article has been cited 5 times.
  1. Santamaria F, Schlagloth R, Valenza L, Palme R, de Villiers D, Henning J. The Effect of Disease and Injury on Faecal Cortisol Metabolites, as an Indicator of Stress in Wild Hospitalised Koalas, Endangered Australian Marsupials. Vet Sci 2023 Jan 16;10(1).
    doi: 10.3390/vetsci10010065pubmed: 36669066google scholar: lookup
  2. Horn R, Stewart AJ, Jackson KV, Dryburgh EL, Medina-Torres CE, Bertin FR. Clinical implications of using adrenocorticotropic hormone diagnostic cutoffs or reference intervals to diagnose pituitary pars intermedia dysfunction in mature horses. J Vet Intern Med 2021 Jan;35(1):560-570.
    doi: 10.1111/jvim.16017pubmed: 33368633google scholar: lookup
  3. Santamaria F, Young S, Valenza L, Schlagloth R, Henning J, Palme R. Case Study: A Dexamethasone Suppression Test in Koalas (Phascolarctos cinereus). Vet Med Sci 2026 Jan;12(1):e70812.
    doi: 10.1002/vms3.70812pubmed: 41548202google scholar: lookup
  4. Menzies-Gow NJ. Equine Pituitary Pars Intermedia Dysfunction. Vet Sci 2025 Aug 20;12(8).
    doi: 10.3390/vetsci12080780pubmed: 40872730google scholar: lookup
  5. Vaughn SA, Lemons MB, Hart KA. The Effect of Season and Breed on Hypothalamic-Pituitary-Adrenal Axis Hormones, Metabolic Hormones, and Oxidative Markers in Ponies and Horses. J Vet Intern Med 2025 Mar-Apr;39(2):e70047.
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