Residual veterinary pharmaceuticals in animal manures and their environmental behaviors in soils

The worldwide heavy use of veterinary pharmaceuticals in confined animal-feeding operations has resulted in annual discharge of 3,000-27,000 tons of drug chemicals via livestock manure into the environment. More than 50 major antibiotics have been detected in poultry, swine, cattle, and horse manures at 0.01-765 mg kg dry manure mass. In animal manures, most veterinary pharmaceuticals degrade rapidly via biochemical reactions, demonstrating a half-life time 2-30 days. In soils, veterinary pharmaceuticals interact with soil minerals, organic matter, and organisms and are subject to sorption, photohydrolysis, oxidation, and biodegradation. The soil distribution coefficient (K) values of animal pharmaceuticals range from 0.3 to 6,300 L kg, varying with the chemical species and soil properties. The persistence of veterinary pharmaceuticals in soils is influenced by soil type, organic matter content, pH, moisture content, and temperature. Though certain antibiotics such as roxithromycin, sarafloxacin, and virginiamycin are persistent, the vast majority of veterinary pharmaceuticals are degradable (half-life <30 days) in soils. The sorption, rapid degradation, and physical attenuation limit residual pharmaceuticals in the top 30-cm soil of agricultural land at generally less than 1 μg kg, posing little impacts on soil microorganisms, fauna, and plants. Nevertheless, veterinary pharmaceuticals could migrate from manured fields to water bodies via surface runoff and leaching. In North American drainage ditches and streams, up to 290 ng L of animal antibiotics had been detected, although the concentrations were far below the no-observed-effect concentration levels of veterinary pharmaceuticals to aquatic organisms. Antibiotic-resistant bacteria have been identified in animal manures and livestock-handling workers, indicating the risk of antibiotic-resistant genes spread in association with veterinary pharmaceutical overuse and manure disposal. Future research should focus on developing standard composting protocols to eliminate residual veterinary pharmaceuticals and antibiotic-resistant pathogens from animal manures and on cultivating animal-feeding methods alternative to drug administration.