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Pathogens can stick to microplastics and hitchhike to the sea

Microplastics are everywhere – in our blood, our food and our water. These tiny fragments of plastic, no bigger than 5 millimeters, have been introduced into many different environments by human activities. The effects they will have on humans and wildlife are currently not known.

And now a study, published today in the journal Scientific Reports, has shown that microplastics in the ocean are linked with pathogens that come originally from the land. The authors show that pathogens that cause disease in both humans and other animals can adhere to the surfaces of microplastics and hitch a ride out to the plastic-contaminated areas of the oceans. 

“It’s easy for people to dismiss plastic problems as something that doesn’t matter for them, like, ‘I’m not a turtle in the ocean; I won’t choke on this thing,’” said study co-author Karen Shapiro, an infectious disease expert and associate professor in the UC Davis School of Veterinary Medicine. “But once you start talking about disease and health, there’s more power to implement change. Microplastics can actually move germs around, and these germs end up in our water and our food.”

The pathogens studied – Toxoplasma gondiiCryptosporidium (Crypto) and Giardia – can cause disease in both humans and animals. They are recognized by the World Health Organization as causes of illness from shellfish consumption and are found throughout the ocean.

Toxoplasma gondii, a parasite found only in cat feces, has infected many ocean species with the disease toxoplasmosis. Previous UC Davis research has linked this parasite to the deaths of sea otters, while it has also been implicated in the deaths of endangered Hector’s dolphins and Hawaiian monk seals. In people, toxoplasmosis can cause life-long illnesses, as well as developmental and reproductive disorders.

Crypto and Giardia cause diarrhea and gastrointestinal disease. They are parasites that can infect any mammal, and can be deadly in young children and people who are immunocompromised.

“This is very much a problem that affects both humans and animals,” said first author Emma Zhang. “It highlights the importance of a One Health approach that requires collaboration across human, wildlife and environmental disciplines. We all depend on the ocean environment.” 

In order to test whether these pathogens adhere to the surfaces of microplastics, the researchers conducted laboratory experiments using polyethylene microbeads and polyester microfibers. Microbeads are a common constituent in cosmetics, such as exfoliants and skin cleansers, whereas microfibers derive mostly from clothing made of synthetic materials. 

The results showed that the microbes adhered more readily to microfibers than to microbeads, although both types of plastic particles could carry the pathogens to the sea and disperse them there. Microplastic particles make it easier for the pathogens to reach places that a land parasite would otherwise never be found, such as in the flesh of coastal shellfish. 

Since the plastic particles may sink or float in the ocean, the hitchhiking pathogens can end up in two different habitats. Microplastics that float can travel long distances on the surface of the ocean, spreading pathogens far from their sources on land. In contrast, microplastics that sink may deliver pathogens to the benthic environment where filter-feeding animals such as zooplankton, clams, mussels and oysters remove the particles, and their associated pathogens, from the water and ingest them. 

“When plastics are thrown in, it fools invertebrates,” Shapiro said. “We’re altering natural food webs by introducing this human-made material that can also introduce deadly parasites.”

Microfibers are most commonly shed from synthetic clothing when it is washed in a washing machine or dried in a tumble drier. The fibers are released in the waste water or hot air, and can reach waterways or the atmosphere in this way. Study co-author Chelsea Rochman, a plastic-pollution expert and assistant professor of Ecology at the University of Toronto, said there are several ways humans can help reduce the impacts of microplastics in the ocean. 

“This work demonstrates the importance of preventing sources of microplastics to our oceans,” said Rochman. “Mitigation strategies include filters on washing machines, filters on dryers, bioretention cells or other technologies to treat storm water, and best management practices to prevent microplastic release from plastic industries and construction sites.”

The study was funded by the Ocean Protection Council and California Sea Grant program, with student financial support provided by the UC Davis School of Veterinary Medicine Students Training in Advanced Research (STAR) program.

By Alison Bosman, Staff Writer

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