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What is the best method to estimate antimicrobial use in livestock?

Measuring antimicrobial use in livestock can be complex because of the varying weights of animals at point of treatment, different ways of measuring and the different ways in which each livestock sector operates. For a broad picture, overall levels of use are approximated to annual sales data collated by the Veterinary Medicines Directorate, as reported in its Veterinary Antimicrobial Resistance and Sales Surveillance (VARSS) reports. However, because many products are licensed for use in many different species (for example, in both sheep and cattle, or both pigs and poultry), sales data don’t show what each livestock sector is using individually. Hence the different sectors have been collected increasing amounts of usage data annually, and this is now also checked and reconciled by the Veterinary Medicines Directorate and reported annually as well. For example, p27 in the 2017 VARSS report describes how the various datasets are gathered and calculated, and p8 shows a summary of usage per species.  Most measurements take the form of mg/kg, or mg/PCU (PCU being a standardised kg) but sometimes they are reported as DDDVet (Defined Daily Dose) or DCDVet (Defined Course Dose. Tools like the antibiotic calculator launched by the University of Nottingham and AHDB help vets and farmers with their calculations as well.