In March, the US Department of Agriculture discovered cows that were infected with H5N1, and that they could pass the virus to people - possibly through droplets splashed from milking machinery. Photo / Tim Gruber, The New York Times
In March, the US Department of Agriculture discovered cows that were infected with H5N1, and that they could pass the virus to people - possibly through droplets splashed from milking machinery. Photo / Tim Gruber, The New York Times
Scientists were slow to recognise that Covid spreads through the air. Some are now trying to get ahead of the bird flu.
In early February 2020, China locked down more than 50 million people, hoping to hinder the spread of a new coronavirus. No one knew atthe time exactly how it was spreading, but Lidia Morawska, an expert on air quality at Queensland University of Technology in Australia, did not like the clues she managed to find.
It looked to her as if the coronavirus was spreading through the air, ferried by wafting droplets exhaled by the infected. If that were true, then standard measures such as disinfecting surfaces and staying a few feet away from people with symptoms would not be enough to avoid infection.
Morawska and her colleague, Junji Cao at the Chinese Academy of Sciences in Beijing, drafted a dire warning. Ignoring the airborne spread of the virus, they wrote, would lead to many more infections. But when the scientists sent their commentary to medical journals, they were rejected over and over again.
“No one would listen,” Morawska said.
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It took more than two years for the World Health Organisation to officially acknowledge that Covid spread through the air. Now, five years after Morawska started sounding the alarm, scientists are paying more attention to how other diseases may also spread through the air. At the top of their list is bird flu.
Last year, the US Centers for Disease Control and Prevention recorded 66 people in the United States who were infected by a strain of bird flu called H5N1. Some of them most likely got sick by handling virus-laden birds. In March, the Department of Agriculture discovered cows that were also infected with H5N1, and that the animals could pass the virus to people – possibly through droplets splashed from milking machinery.
If bird flu gains the ability to spread from person to person, it could produce the next pandemic. So some flu experts are anxiously tracking changes that could make the virus airborne, drifting in tiny droplets through hospitals, restaurants and other shared spaces, where its next victims could inhale it.
“Having that evidence is really important ahead of time, so that we don’t wind up in the same situation when Covid emerged, where everyone was scrambling to figure out how the virus was transmitted,” said Kristen K. Coleman, an infectious-disease expert at the University of Maryland.
Scientists have been arguing over how influenza viruses spread for more than a century. In 1918, a strain of influenza called H1N1 swept the world and killed more than 50 million people. Some US cities treated it as an airborne disease, requiring masks in public and opening windows in schools. But many public health experts remained convinced that influenza was spread largely by direct contact, such as touching a contaminated doorknob, or getting sneezed or coughed on.
H5N1 first came to light in 1996, when it was detected in wild birds in China. The virus infected their digestive tracts and spread through their faeces. Over the years, the virus spread to millions of chickens and other farmed birds. Hundreds of people also became sick, mostly from handling sick animals. Those victims developed H5N1 infections in their lungs that often proved fatal. But the virus could not move readily from one person to another.
The threat of an H5N1 spillover into human populations prompted scientists to look closely at how influenza viruses spread. In one experiment, Sander Herfst, a virus expert at Erasmus University Rotterdam in the Netherlands, and his colleagues tested whether H5N1 could spread between ferrets in cages placed four inches apart.
“The animals can’t touch each other, they can’t lick each other,” Herfst said. “So the only way for transmission to happen is via the air.”
When Herfst and his colleagues squirted H5N1 viruses in the nostrils of ferrets, they developed lung infections. They did not spread the viruses to healthy ferrets in other cages.
But Herfst and his colleagues discovered that a few mutations allowed H5N1 to become airborne. Genetically modified viruses that carried those mutations spread from one cage to another in three out of four trials, making healthy ferrets sick.
An electron microscopic image shows an Influenza A (H5N1) virion, a type of bird flu virus. After scientists recognized that Covid is airborne, they are now investigating how influenza may spread through the air as well. Photo / CDC via The New York Times
When the scientists shared these results in 2012, an intense debate broke out about whether scientists should intentionally try to produce viruses that might start a new pandemic. Nevertheless, other scientists followed up on the research to figure out how those mutations allowed influenza to spread through the air.
Some research has suggested that the viruses become more stable, so they can endure a trip through the air inside a droplet. When another mammal inhales the droplet, certain mutations allow the viruses to latch on to the cells in the animal’s upper airway. And still other mutations may allow the virus to thrive in the airway’s cool temperature, making lots of new viruses that can then be exhaled.
Tracking the flu among humans proved harder, despite the fact that roughly one billion people get seasonal influenza every year. But some studies have pointed to airborne transmission. In 2018, researchers recruited college students sick with the flu and had them breathe into a horn-shaped air sampler. Thirty-nine per cent of the small droplets they exhaled carried viable influenza viruses.
Despite these findings, exactly how influenza spreads through the air is still unclear. Scientists cannot offer a precise figure for the percentage of flu cases caused by airborne spread versus a contaminated surface like a doorknob.
“Very basic knowledge is indeed missing,” Herfst said.
During last year’s flu season, Coleman and her colleagues brought people sick with the flu to a hotel in Baltimore. The sick volunteers spent time in a room with healthy people, playing games and talking together.
Coleman and her colleagues collected influenza viruses floating around the room. But none of the uninfected volunteers got sick, so the scientists couldn’t compare how often influenza infects people through the air as opposed to in short-range coughs or on virus-smeared surfaces.
“It’s hard to mimic real life,” Coleman said.
While Coleman and her colleagues keep trying to pin down the spread of influenza, bird flu is infecting more and more animals across the United States. Even cats are getting infected, possibly by drinking raw milk or eating raw pet food.
Some influenza experts are concerned that H5N1 is gaining some of the mutations required to go airborne. A virus isolated from a dairy worker in Texas had a mutation that may speed up its replication in airways, for example. When Herfst and his colleagues sprayed ferrets with airborne droplets carrying the Texas virus, 30% of the animals developed infections.
“Labs in the United States and all over the world are on the lookout to see if those viruses are getting closer to something that could be very dangerous for humans,” Herfst said.
It would be impossible to predict when – or even if – the bird flu viruses will gain the additional mutations necessary to spread swiftly from person to person, said Seema Lakdawala, a virus expert at Emory University. But with the virus running rampant on farms and so many people getting infected, the odds of airborne evolution are growing.
“What’s shocking to me is we’re letting nature do this experiment,” Lakdawala said.
