Impact of antimicrobial drug usage measures on the identification of heavy users, patterns of usage of the different antimicrobial classes and time-trends evolution.

Several measurement units are available to quantify antimicrobial usage in veterinary medicine, to obtain diverse measures such as the weight of active substance used, the live weight (LW) treated, the fraction of animals exposed, the number of treatments recorded or the cost represented. These measures can be applied to study practices variability between farms, to characterize patterns of usage of the different antimicrobial classes or to follow evolution of antimicrobial usage with time. An investigation was carried out to specifically explore the influence of measurement units on the conclusions obtained from such studies. Antimicrobial exposure was explored in a sample of turkey and chicken broiler flocks, using six different units [kg of active compound, treatments, days of administration, kg of LW treated, animal daily dose to treat 1 kg of LW (ADD(kg)) and euros] to compare flocks usage variability and patterns of use of the different antimicrobial classes. Time-trends evolutions of macrolides usage in turkey broilers, characterized by percentage of flocks exposed and LW treated, were also compared. In all analyses, the measure gave different results without equivalences, highlighting the necessity for care in choosing the measurement unit and caution in interpreting the figures obtained.