Pathogens hitching a ride on microplastic to reach the sea

Microplastics can allow disease-causing land pathogens to hitchhike into and disperse throughout the ocean – reaching places a land parasite would normally never be found – according to the first study to connect microplastics in the ocean with land-based pathogens.

The pathogens studied – Toxoplasma gondii, Cryptosporidium Crypto” and Giardia – can infect both humans and animals and were found to associate with two different types of microplastics in contaminated sea water.

These parasites are recognised by the World Health Organization (WHO) as underestimated causes of illness from shellfish consumption in humans and are found throughout the ocean.

Though both types of plastic can carry land pathogens, more parasites adhered to microfibres than to microbeads, according to the new research published in the journal Scientific Reports.

Microplastics are tiny plastic particles smaller than 5 millimetres in size, and they’re so ubiquitous in the environment that they’ve been found contaminating waters as remote as Antarctica.

“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 senior author Karen Shapiro, an infectious disease expert and associate professor in the University of California Davis School of Veterinary Medicine, US.

“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,” she says.

Microplastics – a pathway for pathogens

The researchers wanted to test whether the three pathogens could associate with microplastics in contaminated sea water. They looked at two different types: polyethylene microbeads (often found in cosmetics) and polyester microfibres (from the disintegration of clothing and fishing nets).

They found that more parasites adhered to microfibres than to microbeads, although both types of plastic can carry them. And while the experiments were not designed to specifically test for the mechanism through which pathogens associate with microplastic surfaces, microscope observations suggest the sticky biofilms that form on plastics in seawater play an important role in mediating it.

Parasites associated with a microplastics fibre
A piece of microplastic fiber is shown under a microscope with biofilm (fuzzy blue) and T. gondii (blue dot) and giardia (green dot) pathogens.

The pathogens chosen in this study are particularly relevant for both human and wildlife health.

T. gondii is the infamous brain parasite found only in cat poo, that can cause lifelong illness, and developmental and reproductive disorders in humans. Ocean species have also been affected by the disease toxoplasmosis, that has killed critically endangered wildlife, including Hector’s dolphins and Hawaiian monk seals.

Crypto and giardia cause gastrointestinal disease and can be deadly to young children and people who are immunocompromised, and can infect virtually all mammals.

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

Unfortunately, microplastics may make it easier for these pathogens to reach parts of the ocean they couldn’t have accessed by themselves.

Microplastics that float along the surface can travel long distances and spread pathogens far from their sources on land, while those 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 ingesting both plastic and pathogens.

“This work demonstrates the importance of preventing sources of microplastics to our oceans,” says co-author Chelsea Rochman, a plastic-pollution expert and assistant professor of ecology at the University of Toronto, Canada.

“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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