Thymus atrophy is an efficient marker of illicit treatment with dexamethasone in veal calves: Results from a triennial experimental study.

Glucocorticoids, used in a wide range of pathologies thank to their therapeutical properties, are also illegally used as growth-promoters in animal breeding even if the European Union regulates their use to protect consumers' health from the adverse effects of residues in food. The first aim of the study was to establish the applicability of two histological parameters - atrophy and cortex-medulla ratio - to detect glucocorticoids misuse in calves. The second aim was to concurrently test the potentiality of both parameters to discriminate between treated and untreated animals. One hundred and seventy-two male Friesian veal calves were raised for six months and divided into two groups: Group A (106 calves) was given dexamethasone per os for twenty days (0.4mg/day), Group B (66 calves) used as control. Thymic samples were microscopically examined. Fat infiltration was evaluated and a degree of atrophy, ranging from 1 to 3 (mild, moderate, severe) was attributed; thymic cortex-medulla ratio was calculated too. Fisher's exact test and a Wilcoxon-Mann-Whitney test were performed to investigate the differences in thymic atrophy and cortex-medulla ratio between the groups. Results demonstrate that the thymic atrophy grading was significantly increased in group A (p=0,006), whereas the cortex-medulla ratio was decreased (p<0,004) when compared to group B; moreover, the parallel testing with fixed degree of atrophy and cortex-medulla ratio cut-off thresholds optimize the sensitivity (90%) in the detection of glucocorticoids anabolic treatments. These data suggest that microscopic thymus analysis represent a valid tool for the screening and monitoring of glucocorticoid illicit treatments.