Animal waste antibiotic residues and resistance genes: A review

Antibiotic resistance is an emerging risk for human and animal health, and mitigating the risk requires an improved understanding of various sources of risks and identifying the level of threats for each source. Many antibiotics are currently used against pathogens for treating infections in animals and humans, and it is considered that antibiotic resistance genes (ARGs) acquired by pathogens may have an environmental origin. Because of contamination in ambient waterways, it is likely that ARGs may affect both organic and non-organic farming. While health risk as a consequence of ARGs is difficult to determine because of lack of understanding of dose-response, the presence of ARGs in human waste and animal manure, and the subsequent application of these organic wastes as fertilizers has a potential of spreading ARGs in the environment. Additional research is needed to understand the presence, growth, and transport of ARGs through animal wastes such as dairy manure. In this review, we focus on synthesizing information regarding the occurrence of ARGs in dairy manure, potential transport pathways, and factors responsible for the spread of ARGs in the environment. Besides, we also explore potential treatment methods that may contribute to the ARG removal in dairy manure and help alleviate ARG contamination.