Seasonal and annual influence on insulin and cortisol results from overnight dexamethasone suppression tests in normal ponies and ponies predisposed to laminitis.
Authors: Borer-Weir, Menzies-Gow, Bailey, Harris, Elliott
Journal: Equine veterinary journal
Summary
# Editorial Summary Identifying ponies at risk of laminitis before clinical disease develops remains a significant challenge in equine practice, prompting investigation into whether overnight dexamethasone suppression testing could reliably distinguish laminitis-prone animals from their normal counterparts. Researchers conducted longitudinal testing over multiple seasons on seven normal ponies and five previously laminitic ponies, measuring serum insulin and cortisol responses to dexamethasone administration whilst animals were at pasture, with an additional hay-only feeding trial in spring 2010. Laminitis-prone ponies did show significantly elevated post-dexamethasone insulin concentrations compared to normal ponies during spring 2009 and summer 2008, but this difference disappeared in three other testing periods, and seasonal variation in insulin concentration proved substantial regardless of group status; cortisol suppression was also inconsistent, with abnormally elevated post-dexamethasone concentrations recorded in both groups during summer and autumn regardless of laminitis history. The practical implication is sobering: whilst abnormal insulin or cortisol responses to dexamethasone suppression testing may indicate metabolic dysfunction, the marked individual variation combined with seasonal and year-on-year fluctuations means this test cannot reliably predict laminitis susceptibility in individual animals, and results must always be contextualised within the animal's seasonal status and previous responses rather than interpreted against reference ranges alone.
Read the full abstract on PubMed
Practical Takeaways
- •Overnight dexamethasone suppression tests cannot be relied upon as a consistent screening tool to identify individual laminitis-prone ponies, as results vary significantly by season and year
- •A single abnormal insulin or cortisol response to dexamethasone should not be used alone to assess laminitis risk—interpret results within the context of the individual animal's history, current season, and repeat testing
- •Diet (hay vs pasture) does not explain seasonal variation in dexamethasone response, suggesting endocrine changes are driven by other seasonal factors
Key Findings
- •Insulin concentrations post-dexamethasone were significantly higher in previously laminitic ponies than normal ponies in spring 2009 and summer 2008, but not in other seasons or years tested
- •Cortisol concentrations post-dexamethasone were significantly higher in previously laminitic ponies in autumn 2009, with abnormal suppression in both groups during summer 2008 and autumn 2009
- •Significant seasonal and annual variation in insulin and cortisol responses occurred in both groups, limiting the test's utility for individual animal prediction
- •Wide interindividual variation in dexamethasone suppression test responses means abnormal results cannot reliably predict laminitis susceptibility in individual animals