The Effect of Season and Breed on Hypothalamic-Pituitary-Adrenal Axis Hormones, Metabolic Hormones, and Oxidative Markers in Ponies and Horses.
Authors: Vaughn Sarah Alison, Lemons Margaret B, Hart Kelsey A
Journal: Journal of veterinary internal medicine
Summary
# Editorial Summary Welsh ponies and Quarter horses exhibit distinct seasonal and breed-specific endocrine profiles that have important implications for clinical interpretation of metabolic and stress-related hormone testing. Researchers measured plasma ACTH, insulin, leptin, cortisol fractions, and oxidative markers (dROMs and plasma antioxidant capacity) in 34 Welsh ponies and 14 Quarter horses during May and October sampling periods, using linear mixed effects modelling to assess breed and seasonal effects. Ponies demonstrated markedly elevated autumn ACTH concentrations compared with horses (p<0.001), alongside significantly higher insulin levels in autumn versus spring (p<0.001) and versus horses in autumn (p<0.001), whilst total cortisol paradoxically decreased in ponies between seasons. Horses showed improved plasma antioxidant capacity in autumn compared with spring, suggesting breed-specific differences in seasonal oxidative stress adaptation. These findings underscore that diagnostic thresholds and endocrine interpretation protocols cannot be universally applied across breeds, and clinicians should account for both seasonal timing and breed-specific physiology when evaluating ACTH, insulin dysregulation risk, and metabolic health—particularly given ponies' well-documented predisposition to equine metabolic syndrome and their distinct HPA axis responsiveness.
Read the full abstract on PubMed
Practical Takeaways
- •Breed differences significantly affect HPA axis and metabolic hormone interpretation—reference ranges for ponies should not be extrapolated from horses, particularly for ACTH and insulin in fall months
- •Ponies show elevated insulin and ACTH in fall/winter, suggesting heightened metabolic and endocrine challenges during these seasons that may increase laminitis and metabolic disease risk
- •Consider seasonal timing when performing endocrine testing in both ponies and horses, as results vary substantially between spring and fall sampling
Key Findings
- •Fall plasma ACTH concentration was significantly higher in ponies compared with horses (p<0.001)
- •Insulin concentrations in ponies were significantly higher in fall compared with spring (p<0.001) and compared with horses in fall (p<0.001)
- •Total cortisol concentration was significantly lower in ponies in fall compared with spring (p=0.05)
- •Plasma antioxidant capacity was higher in horses in fall than spring (p=0.01), with correlations between oxidative status and endocrine hormones varying by season and breed