Plastic litter in rivers might allow dangerous diseases to travel downstream – and even drive antibiotic resistance.
A study focused on one river, the River Sowe in Warwickshire, and found that dumped plastic formed a "breeding ground" for microorganisms.
The researchers warned that plastic waste in rivers could form a reservoir of bacteria and viruses that could spread to humans or increase antibiotic resistance.
Lead author Vinko Zadjelovic, of the University of Antofagasta in Chile, told AFP: "Our findings indicate that plastics in freshwater bodies may contribute to the transport of potential pathogens and antibiotic resistance genes.
"This could have indirect but significant implications for human health."
Researchers submerged plastics in the River Sowe and found that its surface was overrun with microbes within minutes.
The river was downstream from a wastewater facility, and the water harboured human pathogens including salmonella, E.Coli, and streptococcus, which causes strep throat.
Traces of antibiotic resistant bacteria were also found.
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What are the risks of antibiotic resistance?
A University of Oxford study this year found that antibiotic resistance caused 1.27 million deaths in 2019.
That is more deaths than were caused by HIV/AIDS or malaria.
The research also found that antimicrobial-resistant infections played a role in 4.95 million other deaths.
Is this related to water companies pumping sewage into water?
Lead researcher Zadjelovic said there was a "pressing need" for more monitoring of wastewater treatment plants after the researchers found human bacteria in the water.
Water firms in the UK have recently come under fire for pumping raw sewage into rivers.
Rivers are the main way plastic enters the world’s oceans, with up to 2.41 million metric tons travelling through rivers into the sea every year.
How much plastic is in Britain's rivers?
Tiny particles of microplastic are already polluting every lake and river in Britain, posing as-yet-unknown risks to wildlife, scientists have warned.
Even remote bodies of water such as Loch Lomond are polluted with plastic particles.
Microplastics are tiny pieces of plastic less than 5mm in size, which scientists believe can harm wildlife and may pose health risks to humans in the food chain.
Researchers from Bangor University and Friends of the Earth collected water samples from rivers, reservoirs and lochs in England, Scotland and Wales.
Every sample contained plastic, with the River Tame in Greater Manchester having more than 1,000 particles per litre, and Loch Lomond having 2.4 particles per litre.
Ullswater in the Lake District had 29.5 shards of microplastic per litre.
How long can plastics stay in rivers?
Tiny fragments of plastic pollution known as microplastics could linger in riverbeds for years, scientists have warned.
Researchers had previously assumed that the tiny fragments flowed rapidly through rivers to the sea.
The scientists warned that the lingering plastic pollution could damage life in rivers.
But their research found that plastic fragments can linger in riverbeds for up to seven years, according to researchers from Northwestern University in the US and the UK's University of Birmingham.