Microplastics – small particles generated by weather and plastic fragments – pose a growing threat to ecosystems and human health. A new lab study shows that these threats go beyond direct physical or chemical effects, revealing that the presence of microplastics increases the severity of an important viral disease of fish.
Lead author of the study published in macroenvironmental science, is Dr. Meredith Evans Seeley, who conducted the research as part of her Ph.D. program at the William and Mary Virginia Institute of Marine Sciences. She is joined as co-authors by VIMS professors Rob Hale, Andrew Wargo and Wolfgang Vogelbein. Professor W&M Patty Zwollo; and VIMS lab technician Gaelan Verry.
“Microplastics and pathogens are everywhere, but they are often found in the highest concentrations in densely populated aquatic environments such as fish farms,” says Seely. “We wanted to explore whether microplastics could influence the severity of IHNV infection in aquaculture.” IHNV is a virulent pathogen in salmon aquaculture, affecting members of the salmon family including rainbow trout, steel trout, chinook salmon, and salmon.
The team wanted to determine whether “cause and effect” might occur between microplastics, viruses, and fish deaths. Thus Seely and colleagues exposed rainbow trout kept in aquariums to low, medium, and high concentrations of three different types of microparticles, and then added IHN virus to half of the tanks. They choose plastics that are widely used in aquaculture and commonly found as cracking products in nature: polystyrene foam (often in buoys, buoys, home insulation, and food containers); and nylon fibers (missing from fishing nets, fishing lines, and clothing). They also exposed infected and healthy fish to small fragments of ropegrass common in salt marshes Alternative Spartina. Control cabinets do not contain viruses or microparticles.
their results? “We found that combined exposure to microplastics and viruses increases disease severity,” Seeley says, with nylon fibers having the biggest effect. This is the first time this interaction has been documented, and it underscores the importance of testing for multiple stressors, which is more realistic from an environmental point of view.”
Dr. Rob Hill, an environmental chemist and Seeley’s doctoral advisor at VIMS, agrees. He says, “Our results show that we must consider not only the toxicity of microplastics but in combination with other environmental stressors.”
Dr. Andrew Wargo, an expert in the infectious disease ecology, notes that IHNV is a global issue. “It originated in the Pacific Northwest, where it continues to cause significant problems for both salmonid aquaculture and conservation. Our study shows that there is an interaction between microplastics and IHNV. What we don’t know yet is how this interaction occurs in aquaculture or wild environments.” , which will ultimately depend on the amount of plastic pollution and IHNV in any given area.”
Not all microparticles are the same
Based on their laboratory results, the researchers suspect that exposure to the microparticles increases the severity of the disease by physically damaging the soft tissues of the gills and the lining of the intestine, making it easier for the virus to colonize its host.
Exposure to synthetic microplastics – nylon and polystyrene – had a greater effect than natural microparticles derived from Spartina. Most impactful was exposure to microfibers derived from nylon. The researchers think this may be due to their larger size, longer length, or greater plastic hardness compared to plant materials.
“The fine nylon fibers are larger and may be more likely to snag and damage the delicate tissues of the gills and the lining of the intestine,” Seely says. “This can make it easier for the virus to enter and stress the host, ultimately increasing the virulence of the disease.”
The team’s work has big implications beyond fish farming. “Our research question is relevant to aquaculture, but it also applies to natural environments,” Seely says. “Microplastics are distributed all over the world, so at any given time they may co-infect a variety of natural pathogens.”
“Disease and microplastics may interact to produce worse outcomes across a range of aquatic and terrestrial systems, including wild fish, coral reefs, and birds,” Hill says. “If you tested microplastics alone, you might not see any effects and call them today, but in the real world, those might interact.” Microplastics with pathogens, higher temperatures, lower pH, increased water turbidity, and other variables.”
Seely says the team’s findings may be relevant to human health, too. “Indoor environments are dense with microplastics — in household dust for example,” she says. “This makes us wonder how indoor microplastic pollutants might affect the development of airborne diseases such as COVID-19.”