Data on how and why medically important antibiotics are used in agricultural settings—and the implications of that—are central to ensuring the judicious use of these drugs. Yet this information is limited in availability. The only national estimates of total antibiotic use in food animal production come from sales data. Important progress has been made in improving this information—notably, the Food and Drug Administration’s most recent data broke down sales estimates by major food animal species (chickens, cows, pigs, and turkeys) for the first time—but much more detailed information is needed.
A recent FDA proposal to establish a method for estimating the size of the animal populations potentially being treated with antibiotics could help address this challenge and provide context for sales numbers—a welcome improvement. Ultimately, however, data on actual antibiotic use are needed.
Q: What is animal biomass, and how is it applied to antibiotic sales data?
A: Biomass refers to the total mass of an animal species’ population that may require antibiotics. For the purpose of putting antibiotic sales data in context, biomass is generally calculated from the number of food-producing animals in a country during a given year and their average weights.
This information can provide useful insights and help explain trends and fluctuations in antibiotic sales, which may be affected by differences or changes in animal populations.
For example, adjusting sales data by a country’s biomass numbers could help account for variations in antibiotic use driven by differences in animal weight and population size due to such factors as:
- Composition of the animal population (species and breeds). For example, chickens weigh significantly less than pigs or cows, and beef cattle tend to weigh more than dairy cattle.
- Average animal life spans. For example, the average broiler chicken lives for just over a month, whereas cattle tend to live for at least two years, leading to significantly different periods when antibiotics might be required.
- Animal management practices. Some animals, such as cattle, can be slaughtered at varying ages and weight ranges, and keeping animals longer can increase the biomass of the species. Such practices can differ considerably by country.
Q: How can biomass be calculated?
A: All approaches to calculating biomass follow the same general method, but differences arise from available data and assumptions about animal populations. One such difference is how the animal weight is estimated. For example, the World Organisation for Animal Health (OIE) uses the weight of animals at slaughter as a proxy for how much the animal weighed when treated with antibiotics, while the European Surveillance of Veterinary Antimicrobial Consumption (ESVAC) and Canada estimate animal weight based on when it is most likely to be treated with antibiotics. The types of animals included in different biomass calculations also vary. For example, OIE and ESVAC include goats and sheep in their biomass calculations, whereas FDA would look only at what the U.S. considers major food animal species: chicken, cows, pigs, and turkeys.
Q: What method is FDA proposing for U.S. biomass calculations?
A: In August 2017, FDA requested public comments on a proposed methodology for determining animal biomass in the United States. The proposal is similar to the OIE’s but with a notable difference: FDA would use detailed information on animal populations available from livestock inventories maintained by the U.S. Department of Agriculture (USDA). This approach would provide more granular estimates of animal populations that could require antibiotics and therefore more precise adjustments of sales data than methodologies that calculate only a combined biomass for all animal species.
Q: What are the next steps on biomass calculations in the U.S.?
A: The public comment period closed in November. In July, FDA Commissioner Scott Gottlieb announced the agency’s continued commitment to implement a biomass methodology in the U.S., and Pew urges FDA to do so as soon as possible.
Regardless of the method ultimately chosen, it should be as clear and transparent to stakeholders as possible, and thoroughly documented so anyone can reproduce the calculations. Additionally, while the methodology should use available data on the U.S. animal agriculture industry, it should also be as consistent as possible with other international methodologies to help track global trends.
Q: Beyond the application of biomass to sales data, what information is needed to better understand how antibiotics are used on the farm?
A: Biomass calculations are essential to make antibiotic sales data more helpful but cannot replace actual use data. Biomass information can provide valuable context to generate broad insights about antibiotic use patterns, but such estimates have limitations. For example, they do not indicate how or why the drugs are administered and don’t necessarily correspond to the amount of antibiotics used. This information is essential to effectively target and evaluate stewardship efforts and better understand how resistance emerges.
Ultimately, the combination of sales data adjusted by the animal’s biomass and more granular on-farm use data will be pivotal to the broader effort of combating resistant bacteria. FDA’s continued collaboration with the USDA and the Centers for Disease Control and Prevention to collect and evaluate such data is critical.