CHICAGO (Reuters) – A new strain of bird flu that spreads easily among wild birds has unleashed an explosive spread into new corners of the globe, infecting and killing a variety of mammalian species and raising fears of a pandemic more deadly than COVID – 19.
But the same changes that allowed the virus to infect wild birds so efficiently likely made it more difficult to infect human cells, disease experts told Reuters. Their views support assessments by global health authorities that the current H5N1 outbreak poses a low risk to people.
The new strain, called H5N1 clade 2.3.4.4b, emerged in 2020 and has spread to many parts of Africa, Asia and Europe, as well as North and South America, causing an unprecedented number of deaths among wild birds and domestic.
The virus has also infected mammals, from foxes and grizzly bears to seals and sea lions, likely by feeding on sick birds.
Unlike previous outbreaks, this H5N1 subtype is not causing significant illness in people. So far, only around half a dozen cases have been reported to the World Health Organization (WHO) in people who have had close contact with infected birds, and most of them have been mild.
“We think the risk to the public is low,” said Dr. Timothy Uyeki, medical director of the Division of Influenza at the US Centers for Disease Control and Prevention (CDC), in an interview. The WHO expressed a similar opinion in an assessment earlier this month.
The way this virus enters and infects cells is one reason for the unspoken concern, flu experts told Reuters. They say the attributes that made this virus thrive in wild birds likely make it less infectious to people.
“It’s clear that this is a very, very successful virus in birds, and that almost excludes it from being a very, very successful virus in mammals,” said Richard Webby, director of the WHO Collaborating Center for Studies on the Ecology of Influenza in Animals and Birds at St. Jude.
Experts view the spillover to mammals as an early warning sign to step up surveillance of the virus rather than a sign of a new pandemic.
“Everybody take a deep breath,” said Dr. Michael Osterholm, an infectious disease specialist at the University of Minnesota who has tracked H5N1 since it first emerged in 1997, about these alarms.
And the minks?
What raised concern among virologists was a study published in January in the medical journal Eurosurveillance, showing the potential transmission of the virus between mammals on a mink farm in Spain.
“It is highly plausible that a virus capable of mink-to-mink transmission is capable of human-to-human transmission,” Michelle Wille, an expert in wild bird virus dynamics at the University of Sydney, said in an email.
This is a scenario that disease experts have warned about for decades. Minks share many attributes with ferrets, an animal often used in flu experiments because of its resemblance to humans.
While the exact changes required for an avian flu virus to become easily transmissible in people are not known, a pair of landmark studies done a decade ago offer some clues.
Using so-called gain-of-function experiments, the scientists intentionally altered the H5N1 virus to make it transmissible in ferrets and found that only five highly specific mutations were needed.
Most mammalian cases so far have only had one of these mutations – in a gene called PB2 – which was present in the mink. Webby said the virus can easily make that change.
What hasn’t changed, even in mink, is that the virus still prefers to attach itself to avian-type receptors to enter and infect cells. Minks have both avian and human receptors, but avian receptors are scarce in humans and located deep in the lungs.
Human flu viruses normally bind to receptors found in the upper respiratory tract.
“We know that avian viruses can occasionally affect people, but it takes what appears to be lots and lots of contact with birds,” said James Lowe, professor of clinical veterinary medicine at the University of Illinois.
According to the CDC’s Uyeki, studies of the genetic sequences of H5N1 in the mink outbreak “do not indicate any changes that suggest an increased ability to infect the upper respiratory tract in humans.”
This change is necessary for the avian flu virus to spread easily between people.
“The saving grace for humans right now is that it seems very, very difficult for this virus to change receptor preference,” Webby said.
None of the experts ruled out the possibility that H5N1 or another avian flu virus could mutate and trigger a pandemic, and many believe the world has not seen its last flu pandemic.
“Should we keep an eye on that? Yeah,” Lowe said. “Are we supposed to lose our minds over this? Probably not.”
Reporting by Julie Steenhuysen Editing by Bill Berkrot