Measuring antibiotic use

General principles

The European Union methodology for calculating levels of use or sales of antibiotics in different species uses a Population Correction Unit (PCU). The PCU is a theoretical unit of measurement developed by the European Medicines Agency (EMA) in 2009 and adopted across Europe. This standardised unit allows data to be easily aggregated or compared. It is also the method adopted in the UK for ease of standardising sales data reports to the EU (such as contained in the annual ESVAC report). The PCU takes into account a country’s animal population over a year, along with the estimated weight of each particular species at the time of treatment with antibiotics. The VMD has published a guide to understanding the PCU.

While the PCU used in this way works at a ‘macro’ level, it doesn’t always help those on-farm or in veterinary practices to understand at a management level what is happening regarding antibiotic use. While the PCU may work well in a species which completes its production cycle and is slaughtered within a year, it does not work so well when production cycles last over a year, or when breeding, laying or milking animals which have a continuing cycle of production are involved. For example, the PCU for beef cattle is based on slaughter cattle, which does not apply to calf rearing enterprises, store cattle and suckler herds, all of which sometimes have need of antibiotic treatments. To overcome this, separate metrics have been developed by some sectors to allow calculation of use on-farm for management and purposes.

Dairy

Advice on measuring antibiotic use in dairy herds was originally published in 2019 by the CHAWG Antimicrobial Use subgroup, and was updated in November 2020 in the Dairy Benchmarking Metrics Report, which contains new information on measuring antibiotic use in calves up to six months of age. Supplementary material for beef and dairy benchmarking is also available, detailing breeds and assumptions.

Beef

It has been a more complex task to establish the best metrics to measure antibiotic use in beef herds due to the range of systems in operation, from dairy calf rearing to suckler cows, store cattle growing to finishing. Following an extensive consultation, recommendations for measuring and comparing the use of antibiotics on beef farms were published by the CHAWG Antimicrobial Use subgroup in February 2020. An updated Beef Benchmarking Metrics Report was published in November 2020, with new information on measuring antibiotic use in calves up to six months of age. Supplementary material for beef and dairy benchmarking is also available, detailing breeds and assumptions.

Sheep

Advice on measuring antibiotic use in sheep production was published in 2019 by the Sheep Antibiotic Guardian Group and is detailed in its Calculation of metrics for benchmarking antibiotic use on sheep farms report.

Pigs

The method of measuring and recording antibiotic use in pigs is explained as part of the protocols for uploading data to the sector’s e-Medicine Book (eMB). Uploading usage data to this portal is a requirement of Red Tractor assurance and therefore antibiotic use data for over 90% of animals in the sector is captured in this way. The User Guide published in 2017 explains the metrics on p29, and follow-up advice added in 2018 adjusted the category names.

Laying hens

The laying hen sector uses bird-days as the standard denominator to calculate all proportions or percentages. For the year, it is the mean daily population of birds in the Lion scheme (including breeding birds and pullets in rear) multiplied by 365. A daily dose is a single chicken treated with an antibiotic for one day. Courses of treatment will vary with the clinical need, but are typically 3-5 days. So a flock of 5,000 chickens treated with an antibiotic for 3 days is 15,000 daily doses. The standard reporting metric is daily bird doses/100 bird days at risk (% doses).

Game birds

Game birds do not have a standard PCU figure, and so usage in the sector is reported in tonnes of antibiotic used by the sector as a whole in a year and change is the % by which that tonnage goes up or down. A game farmer wishing to monitor their annual use could use a simple metric of total antibiotic use in mg divided by the number of birds reared.

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