Toxoplasma gondii, a parasite that can live on microplastics
BSIP SA/Alamy Stock Photo
Three disease-causing parasites found in cat and human faeces may be able to hitch-hike on microplastics to reach the sea, where they could potentially cause illness in marine life.
We already know that colonies of water-loving bacteria, viruses and parasites can attach to microplastics – which are plastic particles less than 5 millimetres in diameter – but this is the first time land parasites have been shown to hitch-hike on the fragments in seawater.
“What we’ve shown for the first time is that parasites that come from animal poop or human poop on land – which are known to infect humans and animals – can actually stick to these plastics, and the concentration of parasites on the surfaces of the plastic is really quite substantial,” says Karen Shapiro at the University of California, Davis.
To investigate, Shapiro and her colleagues first placed microplastics, including both spherical microbeads and long microfibres, in seawater for 14 days to enable a sticky layer of bacteria to grow on their surfaces, forming a community known as a “plastisphere”.
“The plastisphere forms a blanket of gooey, sticky material where things can physically get trapped, like how fly traps work,” says Shapiro.
They then placed the microplastics into beakers containing either seawater alone, or seawater contaminated with one of three land parasites – Toxoplasma gondii, Cryptosporidium parvum or Giardia enterica.
In people, T. gondii can cause flu-like symptoms such as fatigue and muscle aches. The parasite has also been linked to the death of marine mammals such as sea otters, dolphins and seals. Meanwhile, C. parvum and G. enterica are known to cause gut problems that can be deadly in young children and those with a suppressed immune system.
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By taking pictures of the samples over seven days, the researchers found that the parasites moved from the seawater onto the microplastics. From day one, more than 10 times as many parasites were found on 1 gram of microbeads as in 1 millilitre of seawater. For microfibres, there were at least 1000 times more parasites per gram of plastic than in 1 millilitre of seawater.
“We don’t know why microfibres attract more parasites [compared with microbeads], but they make up the majority of microplastics, which means this could have really important consequences,” says Shapiro.
The findings highlight the need to reduce plastic usage and adapt our water treatment systems to capture the plastic pollutants, she says.
“This paper provides further proof that microplastics can associate with harmful pathogens, including those that cause severe illness in humans,” says Alice Horton at the National Oceanography Centre in the UK.
“The intersection of microplastics and three serious [parasites] in seawater should raise red flags for everyone from beachgoers to wildlife conservationists,” says Mary Donohue at the University of Hawaii, who wasn’t involved in the work.
The researchers now hope to explore whether parasites stuck to microplastics in our waterways really can hitch-hike all the way to the ocean to traverse seas and infect marine life.
“Laboratory work is critical to understanding processes in a controlled environment, but ultimately investigation of the relationship of ocean plastics – of all sizes – and these diseases in real-world conditions will be needed,” says Donohue.
Journal reference: Scientific Reports, DOI: 10.1038/s41598-022-10485-5
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