Plastics are widely recognized as a pervasive marine pollutant. Microplastics have been garnering increasing attention due to reports documenting their ingestion by animals, including those intended for human consumption. Their accumulation in the marine food chain may also pose a threat to wildlife that consume species that can accumulate microplastic particles. Microplastic contamination in marine ecosystems has thus raised concerns for both human and wildlife health. Our study addresses an unexplored area of research targeting the interaction between plastic and pathogen pollution of coastal waters. We investigated the association of the zoonotic protozoan parasites Toxoplasma gondii, Cryptosporidium parvum, and Giardia enterica with polyethylene microbeads and polyester microfibers. These pathogens were chosen because they have been recognized by the World Health Organization as underestimated causes of illness from shellfish consumption, and due to their persistence in the marine environment. We show that pathogens are capable of associating with microplastics in contaminated seawater, with more parasites adhering to microfiber surfaces as compared with microbeads. Given the global presence of microplastics in fish and shellfish, this study demonstrates a novel pathway by which anthropogenic pollutants may be mediating pathogen transmission in the marine environment, with important ramifications for wildlife and human health.

Concerns about microplastic pollution have risen as numerous studies have reported detection of microplastics in foods, including seafood. One emerging concern is the ability of microplastics to vector pathogens that can adhere to biofilms on microplastic surfaces. Here, we investigated whether microplastics can facilitate zoonotic protozoan parasite contamination in shellfish. Oysters were selected for this study because they are commonly eaten raw and can harbor zoonotic protozoan pathogens. Acclimated live oysters were exposed in closed aquaria to Cryptosporidium, Giardia, and Toxoplasma (oo)cysts that had been incubated in seawater either as protozoa alone (P treatment) or with preconditioned polyester microfibers (P + M treatment). After overnight exposure, oysters were transferred to clean seawater flow-through aquaria for depuration. Over the experimental period, oysters exposed to both protozoa and microfibers had significantly higher numbers of protozoan pathogens than oysters exposed to protozoa alone. Our study provides experimental evidence that microplastics may facilitate protozoan pathogen contamination in shellfish. These results demonstrate how anthropogenic pollution may have unintended consequences on infectious disease transmission in coastal ecosystems, with potential risk to wildlife populations and human public health.