Microplastics—tiny particles produced as plastics weather and fragment—pose an increasing threat to the environment and human health. A recent laboratory study reveals that the presence of microplastics worsens the intensity of an important viral fish disease, going beyond direct physical or chemical impacts.
Virus and Microplastics. Mortality increased significantly when fish were co-exposed to a virus and microplastics, particularly microfibers, compared to the virus alone. Image Credit: Meredith Evans Seeley.
Dr. Meredith Evans Seeley, who carried out the research as part of her Ph.D. program at William & Mary’s Virginia Institute of Marine Science, is the lead author of the study published in Science of the Total Environment. VIMS professors Rob Hale, Andrew Wargo, and Wolfgang Vogelbein joined her as co-authors, as did W&M Professor Patty Zwollo and VIMS laboratory technician Gaelan Verry.
Microplastics and pathogens are everywhere, but they’re often present at highest concentrations in densely populated aquatic environments such as fish farms. We wanted to explore if microplastics could affect the severity of IHNV infections in aquaculture.
Dr. Meredith Evans Seeley, Virginia Institute of Marine Science
IHNV is a virulent pathogen in salmonid aquaculture, infecting members of the salmon family such as steelhead trout, rainbow trout, chinook salmon, and sockeye salmon.
The researchers intended to see if there was a “cause-and-effect” relationship between viruses, microplastics, and fish mortality. Seeley and co-workers subjected rainbow trout kept in aquariums to low, medium, and high levels of three different microparticles before adding the IHN virus to half of the tanks.
Investigators selected plastics that are both widely used in aquaculture and commonly found in nature as breakdown products: polystyrene foam (typically used in buoys, floats, home insulation, and food containers); and nylon fibers (lost from fishing lines, fishing nets, and clothing).
Infected and healthy fish were also exposed to the common saltmarsh cordgrass Spartina alterniflora. There were no viruses or microparticles in the control tanks. Trout were hatched and maintained in accordance with Institutional Animal Care and Use Committee.
According to Seeley, “We found that co-exposure to microplastics and virus increased disease severity, with nylon fibers having the greatest impact. This is the first time this interaction has been documented, and emphasizes the importance of testing multiple stressors, which is more environmentally realistic.”
“Our results show we must consider toxicity of microplastics not just alone but in combination with other environmental stressors,” states Dr. Rob Hale, an Environmental Chemist and Seeley’s doctoral advisor at VIMS.
“It originated in the Pacific Northwest, where it continues to cause major problems for both salmonid aquaculture and conservation. Our study shows there is an interaction between microplastics and IHNV. What we don't know yet is how this interaction is playing out in aquaculture or wild environments, which will ultimately depend on the amount of plastic pollution and IHNV in any given area,” explains Dr. Andrew Wargo, an expert in the ecology of infectious diseases.
Dr. Andrew Wargo also indicates that IHNV is a global concern.
Not All Microparticles are Created Equal
Depending on their experimental findings, the scientists believe that microparticle exposure worsens disease severity by physically harming the fragile tissues of the gills and gut lining, making it easier for the virus to infiltrate its host.
Exposure to synthetic microplastics (nylon and polystyrene) had a larger impact than exposure to natural microparticles produced from Spartina. The exposure to nylon-derived microfibers had the greatest effect. The researchers believe this is due to their increased size, prolonged length, or the greater hardness of the plastic compared to plant matter.
Nylon microfibers are larger and may be more likely to become trapped in and damage the delicate tissues of the gills and gut lining. That could make it easier for the virus to enter and stress the host, ultimately increasing disease virulence.
Dr. Meredith Evans Seeley, Virginia Institute of Marine Science
Broader Implications
The research has significant repercussions beyond fish farming.
Our research question is very relevant in aquaculture, but it’s applicable to natural environments as well. Microplastics are distributed worldwide, so at any given time they may be co-occurring with a variety of natural pathogens.
Dr. Meredith Evans Seeley, Virginia Institute of Marine Science
Hale notes, “Disease and microplastics may interact to produce worse outcomes across a range of aquatic and terrestrial systems, including in wild fishes, corals, and birds. If you just test microplastics alone you might not see any impacts and call it a day, but in the real world those microplastics may interact with pathogens, rising temperatures, decreasing pH, increasing water turbidity, and other variables.”
According to Seeley, the team's findings may be useful to human health as well. “Indoor environments are dense with microplastics—in household dust for example. This makes us wonder how indoor microplastic contaminants may affect the progression of airborne diseases such as COVID-19,” concluded Dr. Seeley.
Journal Reference:
Seeley, M. E., et al. (2023) Microplastics exacerbate virus-mediated mortality in fish. Science of The Total Environment. doi.org/10.1016/j.scitotenv.2022.161191.