Seasonal flu outbreaks are responsible for the deaths of hundreds of thousands of people worldwide every year. In a pandemic, such as the 1918 Spanish flu pandemic, millions can lose their lives.
In order to reduce transmission, scientists need to understand exactly how influenza viruses spread from person to person.
Experts have assumed that the droplets produced when a person with the virus breathes, talks, coughs, or sneezes are solely responsible for the airborne transmission of viruses.
But a new study suggests that dust, fibers, and other microscopic particles can also transmit influenza viruses through the air, with far-reaching implications for preventing and controlling outbreaks.
“It’s really shocking to most virologists and epidemiologists that airborne dust, rather than expiratory droplets, can carry influenza virus capable of infecting animals,” says Professor William Ristenpart of the Department of Chemical Engineering at the University of California Davis (UC Davis).
Prof. Ristenpart is one of the authors of the new study, along with scientists at UC Davis and the Icahn School of Medicine at Mount Sinai, NY. The findings appear in the journal Nature Communications.
“The implicit assumption is always that airborne transmission occurs because of respiratory droplets emitted by coughing, sneezing, or talking,” he adds.
“Transmission via dust opens up whole new areas of investigation and has profound implications for how we interpret laboratory experiments, as well as epidemiological investigations of outbreaks.”
People can contract viruses by touching contaminated objects, such as doorknobs, toys, towels, and used tissues. Scientists call these contaminated objects fomites. The researchers believe that aerosolized fomites, or contaminated dust particles, can also carry viruses.
Experiments found that the influenza virus remained viable on materials, such as paper tissues and on guinea pigs’ bodies, for long enough to become airborne on dust particles. They showed that these particles could then transmit the infection to new hosts.
In their experiments, they found that the influenza virus remained viable on materials such as paper tissues and the bodies of guinea pigs for long enough to become airborne on dust particles. They showed that these particles could then transmit the infection to new hosts.
First, the scientists used a device called an aerodynamic particle sizer to sample the air from a cage containing a guinea pig.
The device revealed that the animal generated airborne particles ranging in size from 0.3 to 20 micrometers (or thousandths of a millimeter) in bursts of about 1,000 particles per second whenever it moved.
Healthy anesthetized animals exhaled only 0.10 to 0.18 particles per second, and anesthetized animals with influenza generated 0.5 particles per second.
This suggested that dust, rather than respiratory droplets, accounted for the vast majority of particulate matter released into the air while the animals were active.
To test whether these particles were likely to become contaminated with the virus, the researchers infected guinea pigs with a strain of influenza. Two days later, swabs of their fur, ears, paws, and cages all yielded viable virus.
Next, the researchers investigated whether aerosolized fomites from one animal could infect another. To do this, they applied a solution of flu virus particles to guinea pigs’ bodies using a paintbrush.
Crucially, scientists had previously infected these animals with this strain of flu, so they were immune to reinfection. This meant they would not breathe out virus-laden droplets.
When the researchers placed these cages near those containing guinea pigs still susceptible to the virus, 3 out of 12 of these animals developed the infection.
“Thus, we conclude that airborne particulate matter from a non-respiratory source can transmit influenza virus through the air to a susceptible host,” the researchers write.
In their final experiment, the researchers investigated whether the dust from an inanimate source, namely a contaminated paper tissue, could carry viable virus particles.
The scientists applied a solution of the virus to the tissues and allowed them to dry out for 30-45 minutes. They then crumpled, folded, and rubbed the tissues next to the aerodynamic particle sizer, which recorded the release of around 900 particles a second.
They found that the particles, which were small enough for inhalation, carried a virus that was still capable of infecting cell cultures in the lab.
“These results show that dried influenza virus remains viable in the environment, on materials such as paper tissues and on the bodies of living animals, long enough to be aerosolized on non-respiratory dust particles that can transmit infection through the air to new mammalian hosts.”
– Sima Asadi, et al
The researchers emphasize that scientists will need to carry out further research in people and other animal models to confirm their results.
If confirmed, scientists may be able to apply the discovery to other viral respiratory infections, including SARS-CoV-2, the virus that causes COVID-19.
In April, Medical News Today reported on a study that took place in hospitals during the COVID-19 outbreak in China. It found that the highest levels of airborne viral RNA were in rooms where healthcare workers removed personal protective equipment.
This hints that removing contaminated clothing might aerosolize the virus, the authors of the new study write.
“In light of our experiments, we conclude that the contribution of aerosolized fomites to respiratory virus transmission in both humans and animal models requires further scientific consideration and rigorous investigation.”
