Flu spreading on dandruff
Across the world, Covid-19 is making a comeback. Boris Johnson’s dubbed it a “second wave”, and the WHO have suggested that “people dropping their guard”, together with a relaxation of public health measures in many countries, is translating into a surge in cases. France is seeing nearly 4000 daily cases, and across Europe as a whole the total is close to 26,000 people testing positive each day. As a result people are trying to learn as much as possible about the manner in which coronaviruses - and other respiratory infections - spread. A lot of emphasis has been placed on respiratory droplets - blobs of moisture that come out of the airways when we breathe and talk, and can contain virus particles - and people are wearing face coverings and washing their hands to ward off the risk. But at Icahn School of Medicine at Mount Sinai, New York, Nicole Bouvier has been testing how flu spreads among guinea pigs. She’s found that respiratory droplets are indeed important, but that viruses can cling to other things, which may be even more numerous, as well…
Nicole - When humans breathe, or talk, or cough, or sneeze, there's all kinds of microscopic particles that come out of your respiratory tract; droplets of pure water, proteins, bits and pieces of dead cells, most of which is invisible. So we don't really think about it, but it's there. So presumably, guinea pigs were doing the same thing, and we wanted to know what exactly is coming out of them when they are infected with flu and putting flu out into the air.
Chris - How did you do the experiments?
Nicole - We put the guinea pigs in a special cage that had a fine air filter that prevents stuff from the environment coming in. And then we had the other side of the cage hooked up to an aerodynamic particle sizer, or an APS, and that's basically just a machine that can, not only count microscopic particles, but can tell you how big they are. And what we were finding is that there were just thousands of particles per second. Every time the guinea pig moved, there was this big poof of particles that were being detected. It suggested to us that actually the particles coming out of the cage were associated with movement. And that's when we started thinking, well, maybe this isn't just what's coming out of the respiratory tract, but it's just dust.
Chris - So these are guinea pigs with dandruff? Is that basically what you're saying?
Nicole - Yeah. You know, they are dandruffy animals, but so are we, I mean, humans slough millions of skin cells off every day.
Chris - But flu viruses don't grow in the skin. They grow in the nose and throat. So why is this dander and other particulate matter that's coming off the guinea pigs relevant?
Nicole - So what we did is, we took a guinea pig, and we infected it with influenza virus. And then we put the guinea pigs in a cage, and we sampled virus from the cage and from the animal's body, we just took a cotton swab and dipped it in some saline, and swabbed the animal's ears, and their paws, and their fur, and the sides of the cage. And we were able to actually grow a lot of viable flu virus from these swaps. And that indicated to us that the virus was actually being spread all over itself and its environment. And it kind of makes sense when you think about what guinea pigs are doing all day long, which is grooming themselves, and snuffling around, it makes sense that virus from their noses could be getting all over the place.
Chris - Does that mean then, that because there's virus on things other than droplets coming out of the nose and throat, that that could be infectious too?
Nicole - Exactly. We also did some measurements where we were trying to measure exactly what was coming out of just the guinea pigs respiratory tract. And what we found was that the amount of particles coming out of their respiratory tract, was just orders of magnitude smaller than what was coming out of the cage when it was awake and moving around. And so it seemed reasonable to think that maybe some of these particles from the environment, that if they were contaminated with flu virus, might actually be transporting the virus through the air, to the susceptible guinea pig next door.
Chris - Could you infect other animals? If you take those particles, can you demonstrate that there is viable virus, they're capable of infecting an uninfected individual?
Nicole - Yeah. So what we did is we took some virus and just painted it onto their fur. And then we put this animal into the cage next to a susceptible animal, and we were able to see transmission to the susceptible animal. And that suggests that particles that were conveying the virus, were actually not coming from the respiratory tract, because there was actually no virus replicating in the donor animal's respiratory tract at that point.
Chris - Do you think this is relevant to humans, then?
Nicole - It's entirely possible. You know, you could imagine a person who's sick in bed with the flu, if their bedsheets or their pillowcase gets contaminated. And then, you know, the nurse or their partner comes in the next morning, and flaps the sheet to straighten it out, that possibly viable flu virus could be aerosolised into the air in that way. And there was actually a really interesting experiment done in the 1940s where somebody intentionally contaminated a blanket with influenza virus, shook it in a closed container, and was able to sample a live virus from the air. So, you know, we know that this is possible to do. It's just something we haven't really thought about in many decades.
Chris - The obvious question is that the new coronavirus that we're all enthralled to at the moment is about the same size as flu. It's a respiratory infection. So do you think what you're finding for flu could be considered relevant to the coronavirus as well?
Nicole - Certainly, it's not out of the question to think it could be relevant. I think there are a couple of clues that we've seen in some of the COVID research so far. For instance, there was a study done in China where scientists did air sampling in various areas of a few hospitals. What they found is that the highest levels of airborne virus that they could detect, was in a room where healthcare workers were taking off their PPE. And that suggested to the authors that contaminated gowns, or bonnets, or gloves, in the process of being taken off could be shaken or rubbed in such a way that it was releasing coronavirus into the air. I think what we need to do is a little bit more research on what the mechanisms are by which the virus gets into the air. I think a lot of us just assume that it's coming out of the respiratory tract directly, with coughing, and sneezing, and breathing, and talking. But there may be other mechanisms at play that we need to consider and systematically study.