Development of an Enhanced Histopathological Approach to Detect Low-dose Dexamethasone Illicit Treatment in Veal Calves

Dexamethasone is one of a number of synthetic corticosteroids illegally used to promote growth in food-producing animals. Since these low-level drug cocktails evade detection by currently available chemical methods, simple biological indicators that can aid in laboratory analysis are needed. In an attempt to devise an accurate biological method that could detect illicit drug treatment in food-producing animals, we characterized microscopic morphologic alterations of the thymus in veal calves administered low-dose dexamethasone versus control animals. For this purpose, 122 male calves were farmed for 6 months in controlled condition: 81 animals were orally administered dexamethasone (0.4 mg day(-1)) for 20 days during the sixth month and the remaining 41 were kept as control. Urine samples were collected systematically during the treatment period, the suspension period and at the slaughterhouse. All animals were slaughtered 10 per day starting from 10 days after the last dexamethasone administration and the thymus was sampled for histological examination. The difference between the two animal groups was evaluated by means of a non-parametric test of hypothesis. No residues were detected in the urines collected since the third day after the last administration, whereas morphometric analysis of the thoracic thymus revealed a significant decrease in the cortex:medulla ratio in the treated animals (p<0.0005). We can conclude that this histological approach offers encouraging prospects as a screening method to overcome current limitations in controlling growth promoter abuse.