A strain of bird flu known as H5N1 or highly pathogenic avian influenza has made a worrying leap to cattle herds across the US over the past month. This development has sparked "enormous concern" among health experts, including the World Health Organization's (WHO) chief scientist, who warned of the virus' "extremely high" mortality rate in humans.
As things stand, the virus has infected more than 20 herds across eight US states since it was first reported to have made the jump last month.
The states affected so far are Idaho, Kansas, Michigan, New Mexico, North Carolina, Ohio, Texas and South Dakota.
What's more, on 1 April the Centers for Disease Control and Prevention (CDC) confirmed that in Texas the virus had made a further leap, infecting a human who had been exposed to dairy cattle.
“On an individual level, there’s no immediate cause for concern. From a public health standpoint, however, there is an extremely high level of concern,” Dr Jeremy Rossman, an honorary senior lecturer in virology at the University of Kent, tells BBC Science Focus.
“Extreme vigilance, research and assessment of the situation are needed because that risk scenario for an individual person could change very rapidly.”
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The spread to cows is unprecedented
This virus is nothing new – H5N1 was first detected back in 1996 when it was found in domestic waterfowl in Southern China.
A new version of the H5N1 virus reared its head in 2021 and has since gone on to infect wild and domestic bird flocks just about everywhere on the planet. In the ensuing years, millions of birds have tragically died or been culled as the disease has spread.
As the virus ripped through bird populations, the occasional spillover to other animals has also occurred. According to the CDC, the list of mammals affected includes foxes, seals, sea lions and us, humans.
Spillover to mammals has always been concerning, and fatal – according to the WHO, of the 868 human cases detected between 2003 and 2022, over 50 per cent resulted in death – but, as Rossman explains, it’s usually short-lived.
“What's happened before is that normally the bird flu spreads very well in wild birds or farmed birds but once it spills over to mammals, that sort of stops the outbreak. It's not really adapted very well to mammalian populations, especially humans.”
That all changed in October 2022, however, when infections on a mink farm in Spain were reportedly spreading from mink-to-mink, as opposed to typical transmission from birds directly to the mammals.
Now, the US Department of Agriculture (USDA) has confirmed that not only have cows been infected for the first time, but that cow-to-cow transmission is a factor in the recent bird flu spread on dairy farms.
Experts aren’t sure exactly how the disease is spreading among cattle populations. Typically influenza viruses are respiratory illnesses, meaning they are spread through the air like COVID. But, according to a recent USDA report, this doesn’t appear to be what’s happening here.
So, how might it be spreading? The USDA instead points to milk as a possible vector. In the same report, they claimed: “We know that the virus is shed in milk at high concentrations; therefore, anything that comes in contact with unpasteurized milk, spilled milk, etc. may spread the virus.”
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Is it still safe to have milk?
In short, yes, so don’t go chucking out your milk cartons just yet.
“Almost all dairy nowadays is pasteurised and that will kill the virus,” Rossman explains. “So for the vast majority of people drinking milk, there’s absolutely no reason to be concerned.”
“The only potential concern at all would be people that are drinking unpasteurised milk. But of course, if you're drinking unpasteurized milk, you also have a risk of a lot of other infections that could occur.”
Rossman also points out that we have no idea how long the virus can survive in milk, so even unpasteurised milk could be safe if it's not fresh.
“Even if you did get unpasteurised milk from an infected cow that was fresh enough to have the virus in, we still don’t know if it would cause any severe disease.”
But the presence of a viral load in the milk does present an interesting problem: how did it get there?
Since flu viruses are respiratory diseases, we typically don’t find much of the virus outside of the respiratory tract in infected individuals. Where the virus does spread to the rest of the body, you’d expect to see a more severe case of the disease.
“The cattle situation is strange because we’re seeing viruses in the milk but the cows aren’t very sick. It’s a really confusing area,” Rossman says.
How worried should we be?
Thankfully, the authorities are hot on this and the situation is being monitored closely.
Several states in the US have now begun restricting imports of dairy cattle from states where infections have been detected and exposed farmworkers are being kept under close observation.
Meanwhile, scientists are constantly sequencing the virus to identify any genetic changes that could help it spread more easily to and between people, or for changes that could render antiviral drugs less effective.
Farms are implementing biosecurity measures, although concerns have been raised over how effective these are at containing the spread.
Matthew Hayek, assistant professor at New York University’s Department of Environmental Sciences, tells BBC Science Focus, “Biosecurity is the first word that will come out of the mouths of anyone who works closely with the animal agriculture industry, but we already know from peer-reviewed work that where animals are confined, viruses spread very rapidly.”
Hayek also points out that farm workers often work across multiple farms which could easily contribute to spread across different herds. “A lot of safety protocols like spraying down boots, wearing hair nets and gloves, and spraying down farm vehicles are often skipped. There’s no regular monitoring.
“Where dairy farms are outdoors, biosecurity measures can’t stop birds from simply flying over the top.”
Could H5N1 cause the next pandemic?
Right now, there’s a lot of uncertainty surrounding what could happen next, but the chances of a full-blown pandemic remain small.
“We just don’t really understand why this is happening like this and that’s clearly concerning,” Rossman says. “We need to get more information because our level of concern might change rapidly.”
As things stand, the spread among cattle could peter out, but even if it doesn’t there are several hoops the virus would need to jump through to begin spreading among humans. The infected individual in Texas, for example, was not severely ill and didn’t pass on the virus to anyone else.
Even if H5N1 is able to spread effectively among people, it might not retain its usual pathogenicity in the process, as appears to be happening with cattle.
Moreover, flu viruses have been identified as a major pandemic threat for years, and authorities are prepared as a result.
Governments around the world maintain stockpiles of vaccines and antivirals that they think should be effective against threatening flu strains. Two existing candidate vaccines for H5N1 are already available to manufacturers and at least four FDA-approved antivirals should work to combat bird flu.
“Whether or not the vaccines and antivirals would be enough to ensure that there’s not robust spread if a pandemic did stem from this is hard to say, but it’s a very strong starting point,” Rossman concludes.
About our experts
Jeremy Rossman is the honorary senior lecturer in virology at the University of Kent. His research focuses on the process of infectious disease outbreaks, and he has contributed to studies published in journals including PLoS Pathogens, Bioinformatics and Cell.
Matthew Hayek is an assistant professor at New York University in the Department of Environmental Studies and was a Farmed Animal Law & Policy Fellow at the Harvard Animal Law & Policy Program from 2017-2019. His research quantifies the impacts of food production on climate change, ecological processes and the spread of zoonotic diseases.
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