Microplastics are a pathway for pathogens on land to reach the ocean, with likely consequences for human and wildlife health, according to a new study.

The study is the first to connect microplastics in the ocean with land-based pathogens. It shows that microplastics can make it easier for disease-causing pathogens to concentrate in plastic-contaminated areas of the ocean.

The pathogens in the study, Toxoplasma gondii, Cryptosporidium (Crypto), and Giardia, are found throughout the ocean and can infect both humans and animals. The World Health Organization recognizes them as underestimated causes of illness from shellfish consumption.

“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,’” says corresponding author Karen Shapiro, an infectious disease expert and associate professor in the School of Veterinary Medicine at the University of California, Davis.

“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.”

Microplastics harm animals and people

Microplastics are tiny plastic particles smaller than 5 millimeters, no bigger than a grain of rice. They’ve contaminated waters as remote as Antarctica. The study’s findings indicate that hitchhiking on microplastics allows pathogens to disperse throughout the ocean, reaching places a land parasite would normally never be found.

T. gondii, a parasite found only in cat feces, has infected many ocean species with the disease toxoplasmosis. UC Davis and its partners have a long history of research connecting the parasite to sea otter deaths. It’s also killed critically endangered wildlife, including Hector’s dolphins and Hawaiian monk seals. In people, toxoplasmosis can cause lifelong illnesses, as well as developmental and reproductive disorders.

Crypto and Giardia cause gastrointestinal disease and can be deadly in young children and people who are immunocompromised.

“This is very much a problem that affects both humans and animals,” says first author Emma Zhang, a fourth-year veterinary student with the School of Veterinary Medicine. “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.”

Microfibers and microbeads

For the study, the researchers conducted laboratory experiments to test whether the selected pathogens can associate with plastics in seawater. They used two different types of microplastics: polyethylene microbeads and polyester microfibers. Microbeads are often found in cosmetics, such as exfoliants and cleansers, while microfibers are in clothing and fishing nets.

The scientists found that more parasites adhered to microfibers than to microbeads, though both types of plastic can carry land pathogens. The wispy particles of microfibers are common in California’s waters and have been found in shellfish.

The authors say plastic makes it easier for pathogens to reach sea life in several ways, depending on whether the plastic particles sink or float.

Microplastics that float along the surface can travel long distances, spreading pathogens far from their sources on land. Plastics that sink may concentrate pathogens in the benthos environment, near the bottom of the sea. That’s where filter-feeding animals like zooplankton, clams, mussels, oysters, abalone, and other shellfish live, increasing the likelihood of their ingesting both plastic and pathogens.

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

There are several ways humans can help reduce the impacts of microplastics in the ocean, says coauthor Chelsea Rochman, a plastic-pollution expert and assistant professor of ecology at the University of Toronto. She notes that microfibers are commonly shed in washing machines and can reach waterways via wastewater systems.

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

The study appears in Scientific Reports. Additional coauthors are from the University of Nebraska and UC Davis.

The Ocean Protection Council and California Sea Grant program funded the work. The UC Davis School of Veterinary Medicine Students Training in Advanced Research, or STAR, program provided student financial support.

Source: UC Davis