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Microplastics – tiny particles generated by weathering and plastic fragments – pose a growing threat to the ecosystem and human health. A new lab study shows that these threats go beyond direct physical or chemical impacts, revealing that the presence of microplastics increases the severity of an important viral fish disease.
Lead author of the study, published in Total Environmental Science, is Dr. Meredith Evans Seeley, who conducted the research as part of her Ph.D. program at William & Mary’s Virginia Institute of Marine Science. Joining her as co-authors were VIMS professors Rob Hale, Andrew Wargo and Wolfgang Vogelbein; Patty Zwollo, W&M teacher; and VIMS lab technician Gaelan Verry.
“Microplastics and pathogens are everywhere,” says Seeley, “but they are often present in the highest concentrations in densely populated aquatic environments such as fish farms. We wanted to explore whether microplastics could affect the severity of NHI infections in aquaculture.” IHNV is a virulent pathogen in salmonid aquaculture, affecting members of the salmon family including rainbow trout, rainbow trout, chinook salmon, and sockeye salmon.
The team wanted to determine if a “cause and effect” might be occurring between microplastics, viruses and fish kills. Seeley and his colleagues exposed rainbow trout kept in aquariums to low, medium and high concentrations of three different types of microparticles, then added the IHN virus to half of the tanks. They chose plastics that are both widely used in aquaculture and commonly found as degradation products in nature: polystyrene foam (often in floats, buoys, house insulation and containers food); and nylon fibers (lost from fishing nets, fishing lines and clothing). They also exposed infected and healthy fish to tiny fragments of the common salt marsh cordgrass. Spartine alterniflore. Control reservoirs contained no virus or microparticles.
Their results? “We found that co-exposure to microplastics and viruses increased disease severity,” Seeley says, “with nylon fibers having the greatest impact. This is the first time this interaction has been documented and highlights the importance of testing multiple stressors, which is more environmentally realistic.”
Dr. Rob Hale, environmental chemist and Seeley’s doctoral advisor at VIMS, agrees. “Our results,” he says, “show that we need to consider the toxicity of microplastics not just alone, but in combination with other environmental stressors.”
Infectious disease ecology expert Dr. Andrew Wargo notes that HIVN is a global problem. “It is native to the Pacific Northwest, where it continues to cause major problems for both aquaculture and salmonid conservation. Our study shows that there is an interaction between microplastics and NHI. What we don’t yet know is how this interaction takes place in aquaculture. or wild environments, which will ultimately depend on the amount of plastic pollution and IHNVs in a given area.”
Not all microparticles are created equal
Based on their lab results, the researchers suspect that exposure to the microparticles increases disease severity by physically damaging the delicate tissues of the gills and intestinal lining, thereby facilitating colonization of the virus by its host.
Exposure to synthetic microplastics – nylon and polystyrene – had a greater impact than natural microparticles derived from Accelerate. Exposure to nylon-derived microfibers had the most impact. Researchers suspect this may be due to their larger size, extended length, or the greater hardness of plastic compared to plant matter.
“Nylon microfibers are larger and may be more susceptible to catching and damaging delicate tissues in the gills and intestinal lining,” Seeley says. “This could facilitate virus entry and stress the host, thereby increasing the virulence of the disease.”
Wider implications
The team’s work has major implications beyond fish farming. “Our research question is very relevant to aquaculture,” says Seeley, “but it also applies to natural environments. Microplastics are distributed worldwide, so at any time they can co-exist with a variety of natural pathogens.”
“Disease and microplastics can interact to produce worse outcomes in a range of aquatic and terrestrial systems,” says Hale, “including in wild fish, corals and birds. If you only test microplastics you might not see any impact and call it a day, but in the real world these microplastics can interact with pathogens, rising temperatures, dropping pH, increasing water turbidity and other variables.”
Seeley says the team’s findings may also be relevant to human health. “Indoor environments are dense with microplastics – in household dust for example,” she says. “This leads us to wonder how indoor microplastic contaminants may affect the progression of airborne diseases such as COVID-19.”
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