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

The research article investigates the seasonal effect on insulin and cortisol responses in normal and laminitis-prone ponies using an overnight dexamethasone suppression test. The findings show that these physiological responses differ depending on the season and individual animal, which limits the effectiveness of the dexamethasone suppression test for predicting laminitis susceptibility.

Research Objective and Methods

• The aim of the research was to establish whether there is an increased insulin response to dexamethasone, a type of corticosteroid medication, in ponies that have previously experienced laminitis, a painful inflammatory condition of the tissues (laminae) bonding the hoof wall to the pedal bone in the hooves of horses.
• The research was conducted as a prospective longitudinal study. The subjects included seven normal ponies and five ponies that had previously experienced laminitis.
• The study involved performing overnight dexamethasone suppression tests during various seasons while the ponies were at pasture. In spring 2010, the test was also carried out after the ponies had only consumed hay for three weeks.
• Both cortisol and insulin levels were tested before and after administering dexamethasone. For analysis, linear mixed models were used.

Results

• The results showed differences in insulin concentrations between the previously laminitic group and the normal ponies, although these differences were not found during all seasons. Specifically, insulin concentrations were higher in the laminitis-prone group in spring 2009 and summer 2008.
• The diet, based on hay consumption, showed no distinct effect on insulin levels.
• As for cortisol, it demonstrated higher concentrations post-dexamethasone in the previously laminitic group than in normal ponies in autumn 2009, and in both groups the concentrations were above the reference range in summer 2008 and autumn 2009.
• All individual ponies showed insufficient cortisol suppression in all seasons.

Conclusions

• The research found evidence of significant variation in insulin and cortisol responses to dexamethasone between the study groups during certain seasons but not consistently throughout all years.
• The study concludes that the usefulness of the dexamethasone suppression test to predict laminitis susceptibility in individual animals is limited due to the wide interindividual differences in response and seasonal and yearly variation.
• The researchers caution that any interpretation of abnormal insulin and cortisol reactions to dexamethasone should consider the individual animal’s characteristics and the variations explored in this research.