Microplastics in manure: Sources, analytical methods, toxicodynamic, and toxicokinetic endpoints in livestock and poultry

Microplastics are pollutants of serious environmental and public health concern. Although predominantly known to occur in environmental and biological matrices, emerging scientific evidence has indicated that they can also occur in fecal matter. Animal manure, both raw and treated, is usually applied on farmlands as organic fertilizer, serving as an entry point of microplastic particles into agricultural soil. The present study analyzes the literature on the occurrence of microplastics (sources and analytical methods of detection in livestock and poultry manure), their toxicodynamic, and toxicokinetics endpoints in farm animals. Based on the studies examined herein, there is a lack of harmonization in the sampling, digestion, and extraction procedures for microplastics in animal manure. Microplastics abundance was 9.02 × 102 ± 1.29 × 103 particles/kg-1 in pig, 7.40 × 101 ± 1.29 × 102 particles/kg-1 in cow, 0 to 5000 particles/kg−1 in sheep, and 129.8 ± 82.3 particles/g−1 (0.1298 particles/kg) in chicken manure. Microplastics that have been found in farm animal manure were primarily ingested from microplastics-contaminated feed and plastic mulching film attached to crop residues. However, there are other likely sources (e.g., water, soil, and air) that need to be studied carefully. Despite the limited studies on the fate of microplastics in raw manure, the weight of the currently available scientific evidence shows that they exhibit different characteristics during the treatment of manure. The previous studies were all based on composting experiments. Therefore, research on different treatment methods is required to have a profound sense and a better understanding of the influence of physicochemical properties (shape, size, type, and composition) of microplastics on their fate during manure treatment. Moreover, exposure of animals to microplastics revealed several toxicological effects but more research is needed to clearly understand the dose-response relationship, and absorption, distribution, metabolism, and elimination mechanisms with respect to other factors (microplastic sizes, shapes, polymers, types of additive, and co-contaminants).