For the past few years, up until the pandemic hit, Bill Ristenpart, a chemical engineer at UC Davis (and next-level coffee geek), had been bringing a team of researchers and crates of expensive instruments across the country each summer to New York City and into the lab of Nicole Bouvier. An infectious disease physician and researcher at Mount Sinai Hospital, Bouvier studies respiratory viruses, influenza A in particular. Ristenpart’s specialty is fluid dynamics. In the case of flu, that means measuring how physical properties like temperature, humidity, and wind speed change the flight of the respiratory bloblets that fly out of human and rodent noses and mouths. Together, with dozens of guinea pigs and nearly $2 million from the National Institutes of Health, they hoped to figure out a century-old mystery: Why is there a flu season?
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That, they still don’t know. Instead, their work has turned up compelling evidence that some respiratory viruses, at least in lab animals, don’t always travel through liquid droplets, the way scientists have long assumed. Infected guinea pigs don’t just breathe or sneeze out bits of influenza. They can actually launch infectious particles into the air from their fur, paws, and cages.
Remember “fomites,” those germ deposits on surfaces that led to so much hand-washing and hand-wringing over face-touching during the early days of the pandemic? Well, sometimes, rather than settling on large objects like tables and cell phones, germs stick to the surfaces of solids that are so tiny you can’t even see them, like microscopic fibers, dead skin cells, and dust. Those minuscule solids can later get kicked up into the air. When they do, Bouvier and Ristenpart call them “aerosolized fomites.” And according to their research, these germy particles can make other animals sick. In fact, in their latest study, aerosolized fomites appeared to be the primary way their guinea pigs passed around the flu.
“Our experiments very clearly show that when guinea pigs move around they stir up dust. And if that dust is contaminated with virus, then it can transmit that virus through the air to another animal in a separate cage,” says Ristenpart. Their work also raises the possibility that this fourth route of transmission—aerosolized fomites—could potentially matter for human health as well, he says. Especially during a global outbreak of a new respiratory virus. “When you rub your face or brush your shirt or crumple a piece of tissue paper, you’re aerosolizing micron-scale particulates,” says Ristenpart. “And if that surface had been previously contacted by virus-containing mucus, then you’re also aerosolizing virus that other people can inhale.”
The UC Davis/Mount Sinai team published these assumption-shaking findings Tuesday in the journal Nature Communications. Though the experiments were conducted pre-pandemic, and with an influenza virus, their results now land in the middle of a heated dispute about how the novel coronavirus SARS-CoV-2 is transmitted. At the heart of the controversy is disagreement over the size, behavior, and relative importance of the droplets that infected people emit from their respiratory tracts—specifically, whether those expiratory particles can travel long distances and stay airborne for long periods of time. Now, this study adds a new wrinkle. What about viral particles released into the air through other routes—kicked up from the ground, shaken out from a bedspread, crinkled off a dirty tissue? How much do people need to care about that?
The answer—for now at least—is probably more than a little, says Richard Corsi, dean of engineering and computer science at Portland State University, who was not involved in the study. Now an administrator, Corsi spent decades studying the quality of indoor air. He observed that people are constantly modifying their environments with their movements, both by shedding skin and fiber from clothing and unsettling clouds of particles from the floor. Some scientists have even been able to measure the unique microbes that live in these personal aerosol clouds. So he’s not surprised that viruses might be able to hitch a ride in the same way that other microbes do. “I think this paper highly suggests that we shouldn’t assume away the pathway of resuspension of fomites from surfaces,” says Corsi. “It doesn’t mean that it’s the most important transmission pathway. But it’s a pathway.”