This article was originally featured on Hakai Magazine, an online publication about science and society in coastal ecosystems. Read more stories like this at

Microplastics wreak havoc on fish in myriad ways, disrupting everything from eating behavior to brain development. While it’s clear these pesky particles can cause animals a world of trouble, scientists have found it much harder to pin down exactly how they cause so many problems.

“We know that if you expose animals to plastics, then oftentimes we’ll see pathology,” says Andrew Wargo, a disease ecologist at the Virginia Institute of Marine Science (VIMS). “But what we don’t really know are the secondary effects.”

That, however, is starting to change.

In controlled laboratory experiments, Wargo and his VIMS colleagues have shown how microplastics leave rainbow trout more vulnerable to a common salmonid disease, infectious hematopoietic necrosis virus (IHNV). The effect can be dramatic: by exposing trout to a high concentration of either polystyrene beads or nylon microfibers for one month and then subjecting them to IHNV, the scientists found that fish were three to six times more likely to die, respectively, than IHNV-infected fish that hadn’t been exposed to plastics.

As well as increasing the lethality of IHNV, the microplastics also caused the exposed fish to have higher viral loads and shed more virus.

Taking tissue samples from the fish at different points in the experiment, the scientists found that the plastics were damaging the fish’s gills and provoking an inflammatory response. This likely makes it easier for the virus to invade the fish’s body, leading to more severe disease.

“There’s this kind of priming happening with some plastics,” says Meredith Evans Seeley, an environmental chemist at the National Institute of Standards and Technology and the study’s lead author. “That allows the pathogens to be more successful at colonizing the host.”

“Understanding the mechanism of how microplastics can increase the virulence of a virus? That’s pretty new,” says Bettie Cormier, an aquatic ecotoxicologist at the Norwegian University of Science and Technology who was not involved in the work.

The deadly synergy between microplastics and viruses could be especially troubling in aquaculture operations, Wargo says. Infections spread easily on fish farms, and farmed fish frequently encounter plastics such as nylon and polystyrene, which are used for buoys and nets.

Wild fish encounter microplastics and viruses, too, Cormier adds, so similar interactions between microplastics and pathogens could be having ecosystem-level effects.

“Plastics and pathogens are everywhere,” Wargo says. “I think if we want to understand the effects of both, we probably need to consider them together.”

This article first appeared in Hakai Magazine and is republished here with permission.