Measuring the amount of antibiotics given to feed animals is key to understanding how antibiotic resistance can be slowed down as dangerous microbes become so used to antibiotics that they develop stronger defenses against them. However, measuring actual antimicrobial use in animals on a large scale remains a logistical challenge. Because it is easier to obtain data on the sale of antibiotics for use in farm animals, it is widely used as a proxy for antibiotic use at the national level. In a first study of its kind, recently published in the Journal of Antimicrobial Chemotherapy, scientists at Cornell University College of Veterinary Medicine analyzed four different measurement methods used around the world in hopes of guiding government groups towards a more unified system. The study was supported by The Pew Charitable Trusts.
Each government group used similar equations to calculate how many veterinary antimicrobial drugs were sold each year for use in feed animals – but with some major differences and neither method was a panacea, said Renata Ivanek, Ph.D. ’08, Associate Professor of Population Medicine and Diagnostic Sciences. “Our study will support the global action against antimicrobial resistance,” said Ivanek.
Ivanek and Dr. Ece Bulut, postdoc in Ivanek’s laboratory, examined methods of the FDA, the European Surveillance of Veterinary Antimicrobial Consumption (ESVAC), the Public Health Agency of Canada (PHAC) and the World Organization for Animal Health (OIE); using each one with US specific antimicrobial sales and livestock herd data. “We are grateful to the experts at FDA, ESVAC, PHAC, and OIE for their help with details on their methods,” said Ivanek.
Overall, the scientists found that the FDA’s method of estimating total animal weights in one country had a higher level of detail, while the OIE’s method was easier to use and applicable to many countries around the world, with the other two methods roughly in between the high degree of resolution of the FDA and the easy comparability of the OIE.
Each method uses a similar formula: the total kilograms of antimicrobial sales in a year for a food producing animal species in a country is divided by the total weight of all animals of that species (i.e. biomass) present in a year. The resulting number is the total amount of antimicrobial sales per kilogram of animal weight in one year.
All four methods use national antimicrobial sales, animal population data, and the average weight of animals in a country in their calculations to estimate weight-adjusted antimicrobial sales per animal category.
“For example, the European Union does not include beef cattle that have calved in its weight-adjusted antimicrobial sales estimate, based on the characteristics of its agriculture, but this is an important category of cattle production in the US,” said Bulut. “That is why one would underestimate the cattle weight in the USA if one applied the methodology of the European Union.”
“It was surprising that the four methods resulted in significantly different estimates,” said Bulut, noting that the FDA and OIE had made higher biomass estimates than the others. The reason for this, Bulut says, is because the FDA and OIE use the animals ‘weight at the time of their slaughter, while the Canadian and European methods use the animals’ weight at the time of treatment.
Every method has errors. Using an animal’s slaughter weight usually overestimates the true biomass count as most animals are typically heavier when slaughtered than when given antimicrobial treatment. On the other hand, it is difficult to get accurate annual data on the weight of animals being treated, so the Canadian and European methods use the same standardized weight values for several years, ignoring the potential weight changes for a category of animal in a country, such as because animals be raised differently or for a different period of time.
“Understanding the nuances of the weight parameters used in the four methods and their impact on weight-adjusted antimicrobial sales not only helps interpret estimates but can guide future research efforts to monitor antimicrobial sales,” said Ivanek.
The study also shows that none of these tools are perfect for monitoring the sales of veterinary antimicrobial products. “All methods are limited by the quality of the databases on the actual animal population and weight of the animals,” said Bulut. “In addition, the weight parameters used by all methods are incorrect.”
By exposing these issues, the scientists hope that the systems used to monitor antimicrobial use will become more stringent. “We hope that our results will pave the way for a better, and hopefully more consistent, methodology for tracking antimicrobial use around the world by endeavoring to address the identified limitations,” said Ivanek. “More importantly, once we have a good understanding of when, why, and how antibiotics are actually used in farm animals, we will be able to assess whether the regulations are working, and future guidelines and studies on the relationship between support the use of antibiotics in animals “and the one health burden of antimicrobial resistance.”
Lauren Cahoon Roberts is the director of communications at the College of Veterinary Medicine.