Fish in the ocean

Pathogens hitch ride on microplastics to reach oceans

Image credit: Dejan/alenka Sarman/hren | Dreamstime

A new study has found that microplastics are a pathway for pathogens on land to reach the ocean, affecting human and wildlife health.

The study by researchers at the University of California, Davis, is the first to connect microplastics in the ocean with land-based pathogens. It found that microplastics can make it easier for disease-causing pathogens to concentrate in plastic-contaminated areas of the ocean.

According to the team, pathogens such as toxoplasma (T) gondii, cryptosporidium (Crypto), and giardia can infect both humans and animals. The World Health Organisation recognises these pathogens as underestimated causes of illness from shellfish consumption and are found throughout the ocean.

“It’s easy for people to dismiss plastic problems as something that doesn’t matter to them, like, ‘I’m not a turtle in the ocean; I won’t choke on this thing,’” said Karen Shapiro, an associate professor in the UC Davis School of Veterinary Medicine.

“But once you talk 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,” Shapiro, who is also an infectious disease expert, added.

The study’s findings indicate that, by hitchhiking on microplastics, pathogens can disperse throughout the ocean, reaching places a land parasite would normally never be found.

T gondii, a parasite found only in cat poop, has infected many ocean species with the disease toxoplasmosis - UC Davis and its partners have a long history of research connecting the parasite to the death of sea otters. It has 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. Meanwhile, pathogens such as crypto and giardia cause gastrointestinal disease and can be deadly in young children and people who are immunocompromised.

Microplastic fiber under microscope with pathogens

A piece of microplastic fibre and pathogens with biofilm (fuzzy blue). The pathogens shown are T. gondii (blue dot) and giardia (green dot).

Image credit: Karen Shapiro, UC Davis

“This is very much a problem that affects both humans and animals,” said Emma Zhang, a fourth-year veterinary student at the UC Davis 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.”

In the study, the team conducted lab experiments to test whether the selected pathogens can link with plastics in seawater. They used two different microplastics: polyethylene microbeads, found in cosmetic products, and polyester microfibres, found in clothing and fishing nets.

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

Emma Zhang in the lab at UC Davis

Emma Zhang works in the lab at the University of California, Davis.

Image credit: Courtesy Emma Zhang, UC Davis

According to the researchers, plastic makes it easier for pathogens to reach sea life in several ways, but this depends on whether these 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. Filter-feeding animals such as clams, mussels, and oysters live on the seabed, increasing the likelihood of them ingesting both plastic and pathogens.

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

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 effects of microplastics in the ocean. She noted microfibres are commonly shed in washing machines and can reach waterways via wastewater systems.

“This work shows 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 stormwater, and best management practices to prevent microplastic release from plastic industries and construction sites.”

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