Microplastic fibers can alter viral disease severity, contingent on particle shape and exposure timing.

Microplastics are globally distributed persistent pollutants. They are diverse in physical form and chemistry. As such, their effects on organisms and ecosystems vary widely. Research to date has demonstrated that microplastics can alter organismal functions in a variety of species and impact infectious disease severity. The work presented here compares the effects of nylon microparticles and microfibers on the virulence of a commercially important viral pathogen, infectious hematopoietic necrosis virus, using a well-studied host, rainbow trout. Results show that mortality was significantly higher in fish exposed to virus plus microfibers than virus plus spheroid microplastics, or virus alone. However, the effect was only significant if fish were chronically exposed to microplastics from 30 days before to 30 days after virus introduction, compared to only pre or post virus exposure. We hypothesize, based on this and previous work, that this pattern relates to damage to epithelial tissue integrity. This has meaningful implications for a variety of systems (including aquaculture and indoor environments) and should motivate research into the role of microplastic physical form and exposure timing/duration on infectious disease severity.