The sources and impact of microplastic intake on livestock and poultry performance and meat products: a review

Due to the large and growing quantity of microplastics being generated, their ubiquity in agricultural landscapes, their likelihood of being ingested by livestock and poultry, and their potential impacts on performance and meat products, microplastics are a potential risk to livestock and poultry production. Here, we reviewed the literature for microplastic effects on ruminant, pig and poultry health, productivity, and meat products. It was observed that controlled experimental studies show that microplastics have localised effects on livestock and poultry health, as indicated by oxidative stress, inflammation and apoptosis, following short-term exposure to concentrations higher than is environmentally typical. However, it is unclear if microplastics have gross effects on disease, productivity and welfare at natural exposure levels. Microplastics are present in livestock and poultry tissues at levels that make it a potential consumer safety issue (0–7700 mg per kg or 100–180,000 particles per kg). However, the detection methods used are prone to contamination, meaning that true concentrations remain unknown, as does the source of microplastics in terms of whether they originate from production or meat processing and packaging. Microplastics have been detected in the livestock and poultry environment, with 36–300 particles detected per kg livestock feed and 0.34–7900 particles detected per kg soil. Livestock ingest microplastics from their environments, as evidenced by microplastics being detected in chicken excreta at 667–129,800 particles per kg, in ruminant faeces at 74–50,583 particles per kg, and in pig faeces at 0–112,000 particles per kg. However, preliminary data have neither examined correlations to animal productivity, nor have they estimated the total amount and type of microplastics to which livestock and poultry are exposed. This information is needed to inform the doses used in controlled experiments aiming to understand the effect of natural exposure levels on health, productivity and meat quality. To accurately estimate microplastics in livestock supply chains, there is a need to optimise and standardise microplastic detection methods by including procedural blanks, and calculating limits of detection, recovery rate of sample digestion, sample size calculations, and reports of microplastic size, density, weight and number of particles detected. No study has investigated the sources of microplastics and effective mitigation measures in livestock supply chains. Preliminary data also show that microplastics are vectors for heavy metals, antibiotics, antibiotic resistance genes and microbes. Further research is strongly warranted to quantify the effects of microplastics as vectors. In conclusion, microplastics are present in livestock and poultry production systems, and this poses a threat to animal welfare, productivity and consumer perceptions of meat. This review has highlighted paucities in current knowledge that must be addressed to understand the scope of microplastic effects on the livestock and poultry industries, as well as the opportunities for risk mitigation.