Estimating antimicrobial usage based on sales to beef and dairy farms from UK veterinary practices

Background: Accurate estimation of antimicrobial use (AMU) is importantin assessing reduction of agricultural AMU. This cross-sectional study aimedto evaluate several approaches for estimating AMU at the herd level and toreport on AMU for beef and dairy farms in Scotland.Methods: Pharmaceutical sales data for 75 cattle herds (2011–2015) werescreened for antimicrobial products and aggregated by herd and year. Severaldenominators for usage estimates were calculated and compared for theirsuitability at the herd level.Results: Themedian totalmass of active ingredient sold per kg of bovine livestockwas 9.5 mg/kg for beef herds and 14.3mg/kg for dairy herds. The ‘highestpriority critically important’ antimicrobials (HPCIA) were by total massof active ingredient, 10.6% of all sales; by total defined daily dose veterinary(DDDVet), 29.8% and by DCDvet, 20.0%. These are the first estimates of AMUfor beef cattle in the UK, and for cattle of any kind in Scotland.Estimates of herd-level usage based on population correction unit (PCU)weresensitive to low values for PCU for specific herd-years due to their demographiccomposition.Conclusion: Pharmaceutical sales data can provide useful estimates of AMU,but estimating usage per PCU is not appropriate for comparing groups of cattlewith different demographic compositions or for setting herd-level targets.Total mass of active ingredient per kilogram of livestock is more stable andhence suitable than PCU-basedmethods for assessing AMU at the herd level