February 3, 2023
Potential for microplastics to raise severity of viral fish disease, study shows
Microplastics — tiny particles generated as plastics weather and fragment — pose a growing threat to the ecosystem, with a new study showing that these threats extend beyond direct physical or chemical impacts, and revealing that microplastics can increases the severity of an important, viral fish disease.
"Microplastics and pathogens are everywhere," said Dr. Meredith Evans Seeley, lead author of the study. "…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."
IHNV is a virulent pathogen in salmonid aquaculture, affecting members of the salmon family including rainbow trout, steelhead trout, chinook salmon and sockeye salmon.
Dr. Seeley's team, which is based at the Virginia Institute of Marine Science (Virginia, the United States), wanted to determine if a "cause-and-effect" may occur between microplastics, virus and fish mortality. The team exposed aquarium-kept rainbow trout to low, medium and high concentrations of three different types of microparticles and later added the IHN virus to half the tanks. They chose plastics that are both widely used in aquaculture and commonly found as breakdown products in nature: polystyrene foam (often in floats, buoys, home insulation and food containers) and nylon fibers (lost from fishing nets, fishing lines and clothing).
The team also exposed infected and healthy fish to tiny fragments of the common saltmarsh cordgrass Spartina alterniflora. Control tanks held no virus or microparticles. Trout were hatched and reared according to guidelines from the Institutional Animal Care and Use Committee.
"We found that co-exposure to microplastics and virus increased disease severity with nylon fibers having the greatest impact," Dr. Seeley said. "This is the first time this interaction has been documented and emphasises the importance of testing multiple stressors, which is more environmentally realistic."
Dr. Rob Hale, an environmental chemist and Dr. Seeley's doctoral advisor at VIMS, agreed. "Our results show we must consider toxicity of microplastics, not just alone but in combination with other environmental stressors," he said.
"(IHNV) originated in the Pacific Northwest, where it continues to cause major problems for both salmonid aquaculture and conservation," said Dr. Andrew Wargo, a member of the research team and an expert in the ecology of infectious diseases. "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."
Based on laboratory results, the researchers suspected that exposure to microparticles increases disease severity by physically damaging the delicate tissues of the gills and gut lining, thus making it easier for the virus to colonise its host.
Exposure to synthetic microplastics — nylon and polystyrene — had greater impact than natural microparticles derived from Spartina. Most impactful was exposure to the nylon-derived microfibers. The researchers thought this may be due to their larger size, extended 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," said Dr. Seeley. "That could make it easier for the virus to enter and stress the host, ultimately increasing disease virulence."
The team's work has major implications beyond fish farming. "Our research question is very relevant in aquaculture, but it's applicable to natural environments as well," said Dr. Seeley. "Microplastics are distributed worldwide, so at any given time, they may be co-occurring with a variety of natural pathogens."
"Disease and microplastics may interact to produce worse outcomes across a range of aquatic and terrestrial systems, including in wild fishes, corals and birds," said Dr. Hale. "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."
- Technology Networks