The effect of month and breed on plasma adrenocorticotropic hormone concentrations in equids.
Authors: Durham A E, Potier J F, Huber L
Journal: Veterinary journal (London, England : 1997)
Summary
# Editorial Summary: Breed and Seasonal Effects on Equine ACTH Concentrations Interpreting plasma adrenocorticotropic hormone (ACTH) as a diagnostic marker for pituitary pars intermedia dysfunction (PPID) requires understanding the considerable natural variation in this hormone across different horse and pony populations. Durham and colleagues examined plasma ACTH data from ten breeds—including Arabians, Thoroughbreds, Warmbloods, donkeys, and several native pony breeds—to characterise how breed and seasonal factors interact to influence baseline ACTH concentrations. A clear circannual rhythm emerged, with ACTH nadiring in April and peaking in September, with the seasonal increase apparently triggered by lengthening daylight in late April/early May and declining once daylength dropped below 12 hours post-equinox. Importantly, inter-breed differences were substantial: Arabians and donkeys demonstrated significantly elevated ACTH concentrations from May through November compared to other breeds, whilst Shetland ponies and Welsh breeds showed elevation only during the July-to-November window, suggesting breed-specific physiological thresholds to photoperiod changes. For practitioners diagnosing suspected PPID, these findings underscore the critical need to contextualise ACTH results within both the patient's breed and the calendar month of testing, as failing to account for these variables risks both false-positive diagnoses in breeds with naturally higher ACTH and potentially missing disease in affected individuals with less pronounced seasonal elevation.
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Practical Takeaways
- •Breed significantly influences plasma ACTH concentrations; use breed-specific reference intervals when interpreting ACTH results for PPID diagnosis rather than universal cut-offs
- •Timing of ACTH testing matters: test during consistent months (preferably September peak or April nadir) and be aware that some breeds remain elevated longer into winter
- •Daylight duration appears to be the primary driver of seasonal ACTH variation, suggesting that environmental management and lighting cannot be ignored when diagnosing PPID in at-risk horses and ponies
Key Findings
- •Plasma ACTH concentrations show a clear circannual pattern with nadir in April and peak in September across equid breeds
- •Arabian horses and donkeys had significantly higher ACTH concentrations than other breeds, primarily from May to November
- •Shetland ponies and Welsh breeds showed relatively elevated ACTH only from July to November
- •Annual ACTH increase commences in late April/early May in response to increasing daylight and decreases after late September equinox when daylength falls below 12 hours