Summary
A global study assessing the occurrence and types of antibiotic-resistant genes (ARGs) in livestock waste, as well as their pathways for human pathogens, finds that animal husbandry accounts for over one half of all antibiotic use. It clearly links rates of antibiotic use within animal agriculture to ARG-caused human pathogens and health disturbances. Found at levels nearly 28,000 times higher than those of background soil or upstream water, ARG levels within livestock waste are found to be a likely result of the consistent use of antibiotics for animal growth promotion and disease prevention. A strength of the study is the assessment of ARG levels over several different types of farmed animals, as well as several countries and types of farms. A limitation of the study is a lack of an available quantitative measurement of the likelihood of ARG transfer from antibiotic resistant bacteria in livestock waste to human pathogens, crucial to prove a causal relationship. These findings have important implications for human health given the higher levels of ARG’s coming from animal husbandry for agricultural purposes end up in the food system for human consumption. Indeed, this study finds that the human gut is the main reservoir for ARGs, linking to the growing issue of antibiotic resistance within humans.
Antibiotics are widely used in animal husbandry, and various types of antibiotic resistance genes (ARGs) are frequently detected in livestock waste around the world. Conventional livestock waste treatment processes do not completely remove ARGs, resulting in their release to soil and water environments. Various exposure routes of these ARGs to humans, including inhalation and ingestion of antibiotic-resistant bacteria (ARB) that harbor them, may be contributing to the rise in resistant clinical infections that are increasingly difficult to treat with antibiotics. In this review, we assess the occurrence and variability of ARGs in livestock wastes and their potential propagation pathways to human pathogens. We also review the mechanisms and environmental factors that influence the dissemination of ARGs through these pathways, and evaluate the ARG removal efficiency of common livestock waste management approaches. Challenges and research needs for assessing and mitigating the risk of antibiotic resistance dissemination from livestock waste are also presented.