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Validation of a low-dose ACTH stimulation test in healthy adult horses

Allison J. Stewart BVSc, MS, DACVIM, DACVECC1, Ellen N. Behrend VMD, PhD, DACVIM2, James C. Wright DVM, PhD, DACVPM3, Linda G. Martin DVM, MS, DACVECC4, Robert J. Kemppainen DVM, PhD5, Katherine A. Busch DVM6, and R. Reid Hanson DVM, DACVS, DACVECC7
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  • 1 Department of Clinical Sciences, College of Veterinary Medicine, Auburn University, Auburn, AL 36849.
  • | 2 Department of Clinical Sciences, College of Veterinary Medicine, Auburn University, Auburn, AL 36849.
  • | 3 Department of Pathobiology, College of Veterinary Medicine, Auburn University, Auburn, AL 36849.
  • | 4 Department of Clinical Sciences, College of Veterinary Medicine, Auburn University, Auburn, AL 36849.
  • | 5 Department of Anatomy, Physiology and Pharmacology, College of Veterinary Medicine, Auburn University, Auburn, AL 36849.
  • | 6 Department of Anatomy, Physiology and Pharmacology, College of Veterinary Medicine, Auburn University, Auburn, AL 36849.
  • | 7 Department of Clinical Sciences, College of Veterinary Medicine, Auburn University, Auburn, AL 36849.

Abstract

Objective—To determine the lowest ACTH dose that would induce a maximum increase in serum cortisol concentration in healthy adult horses and identify the time to peak cortisol concentration.

Design—Evaluation study.

Animals—8 healthy adult horses.

Procedures—Saline (0.9% NaCl) solution or 1 of 4 doses (0.02, 0.1, 0.25, and 0.5 μg/kg [0.009, 0.045, 0.114, and 0.227 μg/lb]) of cosyntropin (synthetic ACTH) were administered IV (5 treatments/horse). Serum cortisol concentrations were measured before and 30, 60, 90, 120, 180, and 240 minutes after injection of cosyntropin or saline solution; CBCs were performed before and 30, 60, 120, and 240 minutes after injection.

Results—For all 4 doses, serum cortisol concentration was significantly increased, compared with the baseline value, by 30 minutes after administration of cosyntropin; no significant differences were detected among maximum serum cortisol concentrations obtained in response to administration of doses of 0.1, 0.25, and 0.5 μg/kg. Serum cortisol concentration peaked 30 minutes after administration of cosyntropin at a dose of 0.02 or 0.1 μg/kg, with peak concentrations 1.5 and 1.9 times, respectively, the baseline concentration. Serum cortisol concentration peaked 90 minutes after administration of cosyntropin at a dose of 0.25 or 0.5 μg/kg, with peak concentrations 2.0 and 2.3 times, respectively, the baseline concentration. Cosyntropin administration significantly affected WBC, neutrophil, and eosinophil counts and the neutrophil-to-lymphocyte ratio.

Conclusions and Clinical Relevance—Results suggested that in healthy horses, administration of cosyntropin at a dose of 0.1 μg/kg resulted in maximum adrenal stimulation, with peak cortisol concentration 30 minutes after cosyntropin administration.

Contributor Notes

Supported by the Birmingham Racing Commission.

Presented in abstract form at the 9th World Equine Veterinary Association Congress, Marrakesh, Morocco, January 2006, and at the Auburn University College of Veterinary Medicine Phi Zeta Research Day, Auburn, Ala, November 2006.

The authors thank Chanda Moxam, Tony Wolfe, Christopher Schreiber, Nicholas Smith, and Heather Smith for technical assistance.

Address correspondence to Dr. Stewart (stewaaj@auburn.edu).