As dairy cows contract bird flu, researchers are trying to determine what mutations could make the virus a threat to humans.
Charlie Neibergall/AP
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Charlie Neibergall/AP
As dairy cows contract bird flu, researchers are trying to determine what mutations could make the virus a threat to humans.
Charlie Neibergall/AP
The avian flu epidemic in dairy cattle continues.
On Wednesday, North Carolina became the seventh state to detect the virus in a dairy herd.
The unlikely spread among cattle and a dairy worker is prompting scientists to examine the data to better understand these spillovers. They say the risk to humans depends on the virus’s ability to evolve in key ways to better infect mammals.
For now, the news is reassuring: At a recent meeting, U.S. Department of Agriculture scientists said the virus does not present as a respiratory disease in cattle, meaning the animals do not do not seem to excrete large amounts of virus through the nose. or mouths.
Instead, federal health officials investigating the outbreak suspect that some form of “mechanical transmission” is responsible for spreading the virus within the herd. This may occur during the process of milking cows, a theory supported by the fact that high concentrations of the virus are found in milk.
“I would like to emphasize how unusual this is,” says Thijs Kuiken, professor of comparative pathology at the Erasmus University Medical Center. “In other mammal species infected with the influenza virus, it is mainly a respiratory illness, which does not appear to be the case. in these cattle. »
Samples collected from infected animals and shared publicly do not suggest that the virus has undergone drastic changes that would be alarming.
But there are some signs of trouble in the virus’s genome that scientists are looking for as it spreads to more mammals.
“We really need to stay on top of this, because I think we’re on the edge of a precipice where something interesting or unfortunate could happen,” says Michelle Wille, a senior researcher at the Center for Pathogen Genomics at the University of Melbourne. .
With bird flu spreading among cows, scientists are concerned about potential future risks to humans.
Photos VW/VWPics/Universal Images Group vi
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Photos VW/VWPics/Universal Images Group vi
With bird flu spreading among cows, scientists are concerned about potential future risks to humans.
Photos VW/VWPics/Universal Images Group vi
What to watch for: A virus evolving to infect humans
Genetic sequencing of the virus in a Texas dairy worker showed that he had a mutation in a gene, PB2, that is typically affected when the virus infects mammals.
That’s a clue that the virus is evolving to replicate better inside a mammal, but it’s not enough to make the virus more easily transmitted between humans, says Nichola Hill, a disease ecologist at the University of Texas. Massachusetts to Boston.
“It takes a handful (of markers) coordinated across several different genetic segments for it to really be this breakthrough and the next pandemic,” she says.
And it should be transmitted better through the air, like the seasonal flu viruses that humans tend to catch. Currently, most cases of avian flu in humans are linked to direct contact with an infected animal, often when a chicken is slaughtered, says David Swayne, an avian veterinarian who worked for the USDA.
“It requires a very, very high dose,” he says. “It’s likely not just exposure to infected poultry, but also exposure to processes that aerosolized the virus.”
But the fear is that this could change as the virus spends more time in mammals:
Specifically, the protein that the virus uses to bind to cells could evolve to latch onto receptors in humans’ upper respiratory tract. This would allow it to easily access cells so it can produce copies of itself.
“This is considered a primary barrier that prevents this virus from spreading effectively between people,” says Darwyn Kobasa, head of high-containment respiratory viruses at Canada’s National Microbiology Laboratory.
When the virus spread among mammals, scientists didn’t see much change in this function in recent years, says Anice Lowen, a professor of microbiology and immunology at Emory University.
Lowen says previous research has shown that the virus’s protein should not only recognize human receptors in our upper respiratory tract, but also become more stable, likely so it doesn’t break apart during airborne transmission.
These two changes – along with mutations in the PB2 gene to promote replication – should all come together to promote efficient spread in mammals, she says. Of course, she adds, “there are potentially other factors that we don’t understand yet.”
Big questions remain about exactly how avian flu manifests itself in cattle, as it is only now being closely monitored. “Many mutations have certainly occurred in this shift from wild birds to cattle and we don’t necessarily understand what they mean,” says Hill.
How mammals can spread it among themselves: clues from ferrets
With millions of infected birds around the world, mammals become infected through eating dead birds or exposure to droppings.
Wille says the virus may have been introduced to dairy cattle in the same way.
“It’s not that hard to imagine that we are facing some sort of contaminated food situation,” she says.
But this type of reasoning may not fully explain cases of mass infection in some mammals, including “unprecedented” deaths of seals and sea lions in South America and an outbreak at a mink farm in Spain .
It’s still unclear exactly what’s driving the transmission in these cases, perhaps the animals were passing the transmission to each other, says Wille.
Laboratory experiments offer some clues. They showed that mammals can transmit this version of H5N1 and even offer preliminary evidence suggesting limited airborne transmission.
In a recent study, scientists at the Centers for Disease Control and Prevention analyzed how an isolate of the virus from a severe human case in Chile spread among ferrets.
They found that this variant had a “high capacity to cause fatal disease” in animals and showed an increased ability to replicate in laboratory-grown human cells, but “did not exhibit productive transmission in droplets.” respiratory” or via surfaces contaminated during testing. with the animals.
A separate study by scientists at Canada’s National Microbiology Laboratory infected laboratory ferrets with samples of the virus taken from wild animals.
These experiments revealed that a particular version of the virus, taken from a hawk, could spread very quickly from one ferret to another through direct contact and cause a fatal infection in initially uninfected animals, says lead author Kobasa. of the study which has not yet been published. .
They also found evidence that the virus had spread through the air between ferrets in different cages, but they did not see serious illness in animals infected this way. It’s possible that there wasn’t enough virus transmitted to “overcome the immune barriers that would prevent infection,” he says.
The results are “very preliminary” and what happens under controlled laboratory conditions is not necessarily indicative of what can happen in nature, he says. “We certainly don’t see any changes that would suggest that there is a way to support effective airborne transmission.”
Although useful, Lowen says ferret experiments should be interpreted with caution, particularly in the context of humans.
She says that overall there is still very little evidence of airborne transmission: “The fact that ferrets transmit fairly consistently through contact exposure is a bit concerning, but these results do not don’t raise a lot of red flags for me.”
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