Investigation of breed differences in plasma adrenocorticotropic hormone concentrations among healthy horses and ponies.
Authors: Bamford N J, Stewart A J, El-Hage C M, Bertin F R, Bailey S R
Journal: Veterinary journal (London, England : 1997)
Summary
# Plasma ACTH Concentrations Show Significant Breed and Seasonal Variation in Healthy Horses and Ponies When diagnosing pituitary pars intermedia dysfunction (PPID), veterinarians rely on elevated plasma ACTH concentrations as a key diagnostic marker, yet multiple factors—including breed and season—can influence these values in clinically normal animals. Bamford and colleagues prospectively measured ACTH concentrations in 399 healthy horses and ponies across three breed groups (Thoroughbreds, Shetland ponies, and other pony breeds) using blood samples collected around the autumn and spring equinoxes, with plasma ACTH quantified by chemiluminescent immunoassay. In autumn, Shetland ponies demonstrated markedly higher ACTH concentrations than Thoroughbreds (2.67-fold higher), whilst non-Shetland ponies were also significantly elevated (1.55-fold higher than Thoroughbreds); however, these breed differences disappeared entirely in spring, with all groups showing similar concentrations. The findings underscore that breed-specific and season-specific reference intervals are essential for accurate interpretation of ACTH results, particularly in autumn when diagnostic thresholds for PPID may be incorrectly applied to pony breeds using horse-derived reference values. Practitioners should adjust their diagnostic approach according to breed and time of year to avoid both false-positive diagnoses in healthy ponies and missed cases of PPID.
Read the full abstract on PubMed
Practical Takeaways
- •When screening for PPID using autumn ACTH measurements, use breed-specific reference intervals rather than universal cutoffs—Shetland and other pony breeds naturally have higher ACTH than Thoroughbreds
- •Spring ACTH testing may be more reliable for cross-breed comparisons since seasonal differences are eliminated and reference intervals converge
- •Breed differences in baseline ACTH are substantial enough to cause misdiagnosis if not accounted for, particularly for pony breeds in autumn
Key Findings
- •In autumn, Shetland ponies had 2.67-fold higher ACTH concentrations than Thoroughbreds (95% CI 2.33-3.08; P<0.001)
- •In autumn, non-Shetland pony breeds had 1.55-fold higher ACTH concentrations than Thoroughbreds (95% CI 1.35-1.77; P<0.001)
- •No significant differences in ACTH concentrations among breed groups were detected in spring (all P>0.05)
- •Breed-specific reference intervals for ACTH are necessary in autumn but not spring for accurate PPID diagnosis