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

Background: Accurate estimation of antimicrobial use (AMU) is important
in assessing reduction of agricultural AMU. This cross-sectional study aimed
to evaluate several approaches for estimating AMU at the herd level and to
report on AMU for beef and dairy farms in Scotland.
Methods: Pharmaceutical sales data for 75 cattle herds (2011–2015) were
screened for antimicrobial products and aggregated by herd and year. Several
denominators for usage estimates were calculated and compared for their
suitability at the herd level.
Results: Themedian totalmass of active ingredient sold per kg of bovine livestock
was 9.5 mg/kg for beef herds and 14.3mg/kg for dairy herds. The ‘highest
priority critically important’ antimicrobials (HPCIA) were by total mass
of 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 AMU
for beef cattle in the UK, and for cattle of any kind in Scotland.
Estimates of herd-level usage based on population correction unit (PCU)were
sensitive to low values for PCU for specific herd-years due to their demographic
composition.
Conclusion: Pharmaceutical sales data can provide useful estimates of AMU,
but estimating usage per PCU is not appropriate for comparing groups of cattle
with different demographic compositions or for setting herd-level targets.
Total mass of active ingredient per kilogram of livestock is more stable and
hence suitable than PCU-basedmethods for assessing AMU at the herd level.