Testing different breeds for Cushings (PPID)

The first thing we learned was the time of year affected the normal values of commonly used tests for Cushing’s. In the fall normal values were higher. We started adjusting the interpretation based on the time of year and felt this worked pretty well. This paper reports that these fall changes in the normal values are also breed dependent. In this study both Andalusians and ponies had higher increases in normal values than Standardbreds in the fall.
DrO

Circannual variation in plasma adrenocorticotropic hormone concentrations and dexamethasone suppression test results in Standardbred horses, Andalusian horses and mixed-breed ponies
Aust Vet J. 2020 Oct 1.
Authors
N J Bamford 1 , P A Harris 2 , S R Bailey 1
Affiliations

1 Melbourne Veterinary School, The University of Melbourne, Parkville, Victoria, Australia.
2 Equine Studies Group, WALTHAM Petcare Science Institute, Melton Mowbray, Leicestershire, UK.

Abstract

Objective: To compare circannual plasma concentrations of adrenocorticotropic hormone (ACTH) and seasonal dexamethasone suppression test (DST) results between three different equine breed groups.

Methods: Six Standardbred horses, six Andalusian horses and six mixed-breed ponies were followed over a 1-year period, during which time groups were managed identically. Blood samples were collected monthly (around the autumn equinox) or in every second month (other times of the year) for the determination of plasma ACTH concentrations using a chemiluminescent immunoassay. Overnight DSTs were performed quarterly, with suppression of plasma cortisol to below 27 nmol/L at 19 h considered a normal result.

Results: Seasonal variation in plasma ACTH concentrations was present among all breed groups with, as expected, higher levels detected around the autumn equinox, from February to April (P < 0.001). Plasma ACTH concentrations were different between breed groups in March, with higher levels in Andalusians compared with Standardbreds (P = 0.048) and in ponies compared with Standardbreds (P = 0.010). Suppression of cortisol during the DST was normal for all animals in winter, spring and summer, but five Andalusians and three ponies returned abnormally high results in autumn, compared with zero Standardbreds. Conclusion: Higher plasma ACTH concentrations and more false-positive DST results were obtained during autumn in ponies and Andalusian horses when compared with Standardbred horses. Potential differences between breeds should be considered when interpreting test results for horses and ponies that are evaluated for pituitary pars intermedia dysfunction. Further work is recommended to establish population-based reference intervals and clinical cut-off values for ACTH in different equine breeds.