Microplastics are showing up all over the world, from inside human bodies to the waters around Antarctica.
Now, a new study warns that these dispersing particles of plastic waste may also help to concentrate some particularly unpleasant parasites in the ocean, with potential consequences for both public health and wildlife.
Previously, parasites have been found to accumulate in naturally occurring films that cover surfaces submerged in the ocean, Karen Shapiro, a veterinarian at the University of California, Davis who studies pathogens, told The Independent.
At a dock at a beach, for instance, below the water the structure will be covered in living things in a sticky layer called a biofilm. In 2014, a study co-authored by Dr Shapiro found that biofilms in kelp forests can host parasites such as Toxoplasma gondii, a microscopic creature that can cause a disease in both animals and humans. In humans, symptoms often incude fever, headaches and swollen lymph nodes.
Without the sticky biofilm to cling to, Dr Shapiro said, the parasites would be diluted in the enormity of the ocean. But with somewhere to gather, the parasites could concentrate — and potentially infect marine life.
With that in mind, she adds, she and her colleagues were interested in seeing if the same phenomenon would occur on more unnatural surfaces in the ocean, like microplastics.
This question has human health implications, too. If the parasites were to concentrate around beds of shellfish, for instance, they could end up in foods that people eat — especially troublesome for things that people often eat whole and raw, like oysters, Dr Shapiro notes. Oysters and other bivalves have been found to ingest microplastics in the past, according to the National Oceanic and Atmospheric Administration.
To learn more, the research team mixed seawater, microplastics and parasites together. In addition to Toxoplasma gondii, they looked at Giardia enterica and Cryptosporidium parvum, which are found in untreated water and when ingested, can cause days, if not weeks, of diarrhea, vomiting and dehydration.
They looked at how the parasites associated with both microbeads (circular) and microfibers (threadlike) over the course a week by counting the number of parasites found in the water and on the microplastics themselves.
On day one, there were significantly more of all three parasites in the water than on the microbeads or the microfibers. But by day seven, they found significantly more Giardia and Toxoplasma on the microbeads than in the water. There was no significant difference between the water and the microbeads for Cryptosporidium on day seven.
The study did not find significantly more of any parasites on the microfiber than in the water on day seven.
In addition, starting at day one and continuing at days three and seven, there were significantly more of all three parasites found per every one gram of both microbeads and microfibers than per every one milliliter of seawater (equal to approximately one gram).
Part of what makes this study unique, Dr Shapiro said, is that it looks at the interaction between two different types of human-caused pollution.
Microplastics are obviously a human-derived pollutant. But to some degree, so are the parasites.
Both Giardia and Cryptosporidium can spread via human, livestock and pet feces, Dr Shapiro said, while Toxoplasma is spread via cat feces, including that of domestic house cats.
“Because of the way we live our lives, and our expansion, and that of our domestic species, we have a lot more fecal pathogens,” Dr Shapiro said. “And concurrently, we have relied more and more on plastics.”
To follow up, Dr Shapiro and her colleagues will look at whether the presence of microplastics influences the actual concentration of parasites inside the shellfish.
And these questions are also important to more than just seafood lovers — the disease caused by Toxoplasma parasites is a danger to threatened marine mammals like Hawaiian Monk Seals and Maui Dolphins, Dr Shapiro says.
But she also stressed that people can help by taking steps to reduce the amount of plastic they use.
