The bird flu outbreak has affected more than 650 dairy cows. Dozens of people have been infected from contact with infected cattle and poultry.
Charlie Neibergall/AP
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Charlie Neibergall/AP
Scientists believe that about a year ago an influenza virus that sickened and killed birds occurred in a new and surprising host in the Texas Panhandle – dairy cattle.
That encounter was enough to trigger the current cattle outbreak, which scientists who study influenza warn has the potential to become another pandemic.
The virus has been among hundreds of animals and has repeatedly jumped to humans. And, in a disturbing twist, several cases have emerged in North America without a known source of infection, most recently in a child living in the San Francisco Bay Area and a teenager in British Columbia, who remains hospitalized in critical condition.
Genetic sequencing of the case in Canada suggests the culprit may be a wild bird — and points to changes in the virus that could help it more efficiently attach to human cells and clones.
“This is exactly what we don’t want to see,” he said Louise Monclavirologist at the University of Pennsylvania, “The case in British Columbia shows that the flu will always catch us by surprise.”
Fortunately, Canadian health authorities found no evidence that the teenager caught it from someone or spread it to others. And these isolated cases are unheard of in many areas where bird flu has been prevalent for a long time.
But scientists are clear about the risks ahead.
With reservoirs of the virus persisting in dairy cattle, poultry and wild birds, there is ample opportunity for spillover into humans. Meanwhile, the virus appeared in raw milk on store shelves. And flu season raises the troubling prospect that bird flu could mix with seasonal influenza.
“This virus is not easy to eliminate,” said Dr. Jürgen Richt, a veterinary microbiologist at Kansas State University. “We have to live with the last few years.“
The Canadian case is causing fear
There have been two exciting constants since the first human infection linked to dairy cows was detected in the spring.
There is still no solid evidence that people spread the virus, and infections generally cause mild illness.
However, on that second point, the case in Canada shows a departure.
What started with conjunctivitis in early November developed into a fever and eventually a full-blown acute respiratory distress syndrome, according to Canadian health officials.
The teenager had no underlying medical conditions.
A full investigation failed to determine how the teenager, who was too ill to be interviewed, contracted the virus. Repeated tests of the family dog ​​showed no signs of bird flu.
Based on the genetic evidence, the best bet is that some wild birds, or intermediate species, gave rise to the infection, he said. Dr. Bonnie Henry with the British Columbia Ministry of Health.
“We may not know exactly where he was seen,” he told reporters Tuesday, during an update on the case.
While the virus sampled from teenagers is still the same”clade“The H5N1 that is circulating in cattle, Moncla said is from a “rare, genetically distinct cluster” of viruses that arrived from Asia a few years ago. It is very similar to the virus that infected poultry workers in nearby Washington state.
However, it is especially important that the signs of the virus develop during replication in young children.
Moncla said some mutations affecting proteins on the surface of the virus — which it uses to bind to receptors on cells — could help it infect humans more efficiently.
The change may have allowed the virus to more easily infect cells inside the lungs and could explain why the teenager eventually developed severe disease, Canadian health officials said.
While more work needs to be done to understand these implications, this is an unsettling finding. Scientists are only monitoring these types of changes in the virus because they appear to be a key step on the path to pandemic bird flu.
So far, the mutation has not appeared in the version of the virus that moves through cattle.
Udders already have as many receptors that viruses use as birds do — meaning, at least in these animals, there may be less “pressure” to adapt in ways that are more dangerous to humans, Richt said.
But, he added, “there are a lot of unknowns here.”
An unchecked epidemic
For a while, Richt was optimistic that the country could eliminate the plague of dairy cows.
Experiments involving infected cows suggest bird flu spreads mainly through virus-laden milk, not respiratory disease, which would be more difficult to control.
“That’s the good news, I thought, you’re controlling the milk contamination and maybe you can control the outbreak,” he recalled. “Not happening, apparently.”
However, the virus ended up infecting more than 670 dairy animals in fifteen states, and California is currently experiencing infections.
“I think it’s fair to say that control efforts have largely failed,” he said Michael Osterholmwho runs the Center for Infectious Disease Research and Policy at the University of Minnesota.
Without a new strategy and closer work with the industry, there is no indication that will change, because of all the ways the virus “could move to a vulnerable dairy farm and explode,” he said. Gregory Grayepidemiologist at the University of Texas Medical Branch.
Milk with a high concentration of viruses can easily spread in the milking parlor; rodents and other animals can ferry infectious material; humans can do it in clothes or through farm equipment.
“Short of a big vaccine campaign, I just don’t know how we’re going to control it,” he said.
To date, there are more than fifty known human infections in the US, but the true number may be higher.
“Quite clearly, we’re probably missing a lot of cases,” Gray said.
For example, learn conducted by the Centers for Disease Control and Prevention testing workers at farms in Michigan and Colorado after bird flu appeared in cattle there. About 7% of people have evidence of a past infection and about half don’t remember having these symptoms.
Now, the country is repeating “the mistake of COVID,” he said Dr. Deborah Birxwho helped oversee the pandemic response during the first Trump administration.
“The most important thing is to track the location,” said Birx, now a fellow at the George W. Bush Institute, “And what have we learned in the last five years? Yes, many viruses spread asymptomatically.”
Unless there is more cattle screening and testing for human-linked infections, he said the scale of the outbreak will remain grim. It will be difficult to stay ahead of what may initially be a relatively quiet human-to-human spread.
The few isolated human cases in North America unrelated to infected animals are not encouraging, but Osterholm said, historically, this has occurred in parts of the world where the virus has long been circulating in wild birds.
“I’m not surprised,” he said, noting that some kind of contact with migratory birds could “definitely explain” the infection. “Are there more cases happening there? Absolutely. Are there many additional cases of severe disease? Not.”
Reassortment can change viruses
Scientists worry that, under the right conditions, a process known as reassortment – the genetic mixing of two viruses – could lead to a new version of bird flu that is better suited to humans.
The prospect of this occurring in pigs, which appear to be highly dangerous “mixed vessels,” has long been a concern for influenza researchers. To date, there has only been one documented case of bird flu infection in animals during the US outbreak.
But it is possible for humans to incubate the virus this way.
And flu season can set this in motion, said Kansas State Richt.
The thought: Some unlucky souls may be infected with seasonal influenza and bird flu.
“We think every past pandemic virus we’ve had for human influenza has been a reassortment event between a virus circulating in humans and a virus circulating in a different species,” Moncla said. “Translating into the possibility that we are close to a pandemic or that a pandemic will happen – I would say it is unlikely.”
