Usually when people hear about plastic pollution, they can imagine seabirds with their bellies full of garbage or sea turtles with plastic straws in their noses. However, plastic pollution poses another threat invisible to the eye and has important consequences for human and animal health.
Microplastics, tiny plastic particles found in many cosmetics, can form when larger materials, such as clothing or fishing nets, break down in water. Microplastics are now widespread in the ocean and have been found in fish and shellfish, including those that people eat.
As researchers study how waterborne pathogens spread, we wanted to better understand what happens when microplastics and disease-causing pathogens end up in the same body of water. In our recent study published in the journal Scientific Reports, we found that pathogens from land can hitchhike ashore on microscopic pieces of plastic, providing a new way for germs to concentrate along coastlines and travel to the seafloor.
Investigating how plastics and pathogens interact
We focus on three parasites that are common contaminants in marine water and seafood: single-celled protozoa toxoplasma gondii (toxo), Cryptosporidium (Cryptography), and giardia. These parasites end up in waterways when the feces of infected animals, and sometimes people, contaminate the environment.
Cryptography and giardia cause gastrointestinal diseases that can be fatal in young children and immunocompromised individuals. toxo it can cause lifelong infections in people and can be fatal for those with weak immune systems. Infection in pregnant women can also cause miscarriage or blindness and neurological disease in the baby. toxo it also infects a wide range of marine wildlife and kills endangered species including southern sea otters, Hector’s dolphins and Hawaiian monk seals.
To test whether these parasites can adhere to plastic surfaces, we first placed microspheres and microplastic fibers in beakers of seawater in our lab for two weeks. This step was important in inducing the formation of a biofilm – a sticky layer of bacteria and gel-like substances that coats plastics when they enter fresh or marine waters. Researchers also call this sticky layer the echo-corona. We then added the parasites to the test bottles and counted how many were trapped in the microplastics or remained free-floating in seawater for a period of seven days.
Biofilms are vast communities of microbes that can form on almost any surface, including teeth.
We found that a significant number of parasites were clinging to microplastic, and those numbers were increasing over time. So many parasites were attaching to the sticky biofilms that, gram for gram, the plastic had two to three times as many parasites as seawater.
Surprisingly, we found that microfibers (commonly from clothing and fishing nets) harbored a greater number of parasites than microbeads (commonly found in cosmetics). This result is important because microfibers are the most common type of microplastic found in marine waters, on coastal beaches and even in seafood.
Plastics could change the transmission of ocean diseases
Unlike other pathogens that are commonly found in seawater, the pathogens we focus on are derived from terrestrial animals and human hosts. Its presence in marine environments is entirely due to the contamination of faecal waste that ends up in the sea. Our study shows that microplastics can also serve as transport systems for these parasites.
These pathogens cannot replicate in the sea. Hitching a ride on plastics in marine environments, however, can fundamentally alter how these pathogens move in marine waters. We believe that microplastics floating along the surface can potentially travel long distances, spreading pathogens far from their original sources on land and taking them to regions they would not otherwise be able to reach.
On the other hand, sinking plastics will concentrate pathogens on the seafloor, where filter-eating animals such as clams, mussels, oysters, abalones and other molluscs live. A layer of sticky biofilm can camouflage synthetic plastics in seawater, and animals that normally eat dead organic material can unwittingly ingest them. Future experiments will test whether live oysters placed in plastic and plastic-free tanks end up ingesting more pathogens.
A-A health problem
One Health is an approach to veterinary and human research, policy and medicine that emphasizes the close connection between animal, human and environmental health. While it may seem like plastic pollution only affects animals in the ocean, it can have consequences for human health.
Our project was led by a multidisciplinary team of experts, from microplastics researchers and parasitologists to crustacean biologists and epidemiologists. This study highlights the importance of collaboration across human, animal and environmental disciplines to address a challenging problem affecting our shared marine environment.
Our hope is that a better understanding of how microplastics can move disease-causing pathogens in new ways will encourage others to think twice about reaching for that plastic straw or polyester T-shirt.
This article was originally published on The conversation in Emma Zhang and Karen Shapiro at the University of California, Davis. Read the original article here.