Seasonal and annual influence on insulin and cortisol results from overnight dexamethasone suppression tests in normal ponies and ponies predisposed to laminitis

REASONS FOR PERFORMING STUDY: A simple, accurate test for identifying individual animals at increased risk of laminitis would aid prevention. Laminitis‐prone ponies have a greater serum insulin response to dexamethasone administration than normal ponies in the summer, but the response during different seasons is unknown. OBJECTIVE: To test the hypothesis that previously laminitic ponies have a greater insulin response to dexamethasone than normal ponies, which is present during all seasons. STUDY DESIGN: Prospective longitudinal study. METHODS: Overnight dexamethasone suppression tests were performed on 7 normal ponies and 5 previously laminitic ponies in spring 2009 and 2010, summer 2008 and 2010, autumn 2009 and winter 2008, while the ponies were at pasture. In spring 2010, a dexamethasone suppression test was performed after the ponies had been fed only hay for 3 weeks. Serum cortisol and insulin concentrations pre‐ and post dexamethasone were measured. Linear mixed models were used to analyse the data. RESULTS: Insulin concentrations pre‐ and post dexamethasone were significantly higher in previously laminitic ponies than in normal ponies during spring 2009 and summer 2008, but there was no difference between groups in spring 2010, summer 2010, autumn 2009 or winter 2008. Insulin concentration varied significantly with season. Diet had no apparent effect on insulin concentration pre‐ or post dexamethasone in spring 2010. Cortisol concentrations post dexamethasone were significantly higher in previously laminitic ponies than in normal ponies in autumn 2009, with concentrations above the reference range (<25 nmol/l) in both groups in summer 2008 and autumn 2009. Individual ponies had insufficient cortisol suppression in all seasons. CONCLUSIONS: There were significant differences between groups in insulin and cortisol concentrations post dexamethasone during some seasons, but this was not present in all years. Wide interindividual variation in response limits the usefulness of a dexamethasone suppression test for predicting the susceptibility of an individual animal to laminitis. POTENTIAL RELEVANCE: Abnormal insulin and cortisol responses to dexamethasone must be interpreted in the light of the individual animal, seasonal and annual variation reported here.