If you think of bird flu as a stew, it appears that the latest batch has been simmering on low heat for a few years — at least for humans. Could it boil over into a pandemic?
Although there has been a slight uptick in human cases since the avian flu subtype named H5N1 appeared in North America about four years ago, science suggests that a full-fledged human pandemic is unlikely.
“There’s no human-to-human transmission,” said Demetre Daskalakis, a U.S. Centers for Disease Control and Prevention (CDC) official, at a join news conference with the U.S. Department of Agriculture (USDA) and the CDC. “We’re nowhere close to calling this an endemic infection.”
At the time of the news conference in December 2024, there have been 61 reported cases in humans. Over 65,000 people have been tested nationally for multiple kinds of flu.
It is a serious issue in chickens and cows. But could it spread as quickly in humans? Although nothing is certain with viruses — they have a notorious ability to mutate and develop new strategies to enter and infect a host — there are several reasons why this isn’t likely.
What We Know About H5N1
The strain now circulating is a subtype of influenza A. There are 16 known strains (labeled H1 through H16) of influenza A that originated in wild birds, and two more recently discovered ones that developed in bats.
“Only three of those have been able to actually get into humans and become established,” says Lauren Byrd-Leotis, a virologist and program officer at the U.S. National Institute of Allergies and Infectious Diseases (NIAID).
Those three are H1, H2, and H3, which now circulate in humans as seasonal flus. The current strain, H5N1, that is hitting flocks of chickens and herds of cows is not one of those three.
So, what’s the reason?
Read More: What is Bird Flu, And Why is it So Severe This Year?
Barriers To Entry
All viruses use two mechanisms to invade a cell. First, they must bind to a cellular receptor. Think of this as a “lock and key” situation. The receptors, akin to keyholes, on human cells aren’t great matches to any of the subtypes keys higher than the H3. So, although an occasional virus might slip into a human, widespread infection appears unlikely.
The other mechanism is the means by which the virus, once it gains entry into cell, dumps its genetic material into the host, thus taking it over. Viruses are tricky and use multiple methods to do so.
H5N1 needs specific enzymes to cleave a protein that then releases the genetic material into the cell. When lots of these enzymes are available, that protein splits apart more easily and is more likely to release an abundance of genetic material into the cell. That is what makes a particular virus “highly pathogenic,” which means it spreads more easily throughout the body of the host, and is, therefore, more deadly. This is not the case in humans.
Reasons Why Human Transmission Is Low
There’s also some epidemiological reasons that back up these mechanistic barriers to infection in humans. First, the instances of human infection have been sporadic, literally all over the map, and much lower than in cows or chickens.
Second, people who’ve come into contact with infected people have not themselves been infected. This anecdotal evidence backs up the notion that H5N1 both has a hard time getting into a human cell as well as, once there, dumping a ton of genetic info into it.
“What we are seeing is that in the people who are exposed and become symptomatic, their direct contacts — the people that live with them and the medical personnel that treat them — they are not becoming infected,” says Byrd-Leotis. “So we are not seeing sustained human-to-human transmission.”
The Unknowns
However, there are some worrying trends. First, the H5N1 subtype has given rise to two different genotypes. Each genotype is genetically similar enough to the original that it can’t be classified as an entirely new subtype, but there are enough genetic differences between them that they give rise to separate symptoms.
Scientists have so far not fully characterized either genotype, but there are some significant differences. One tends to occur primarily in birds and, when transmitted to humans, causes symptoms akin to respiratory flu. The other, which tends to manifest primarily in cattle, when passed on to humans produces symptoms resembling conjunctivitis.
The emergence of this virus in cattle is perhaps most concerning. “Nobody knew that influenza A virus could infect cows prior to about March of this year,” says Byrd-Leotis.
The spread within a herd and from herd to herd has been shocking to the farm community. Some news reports are referring to the dominant strain of bovine H5N1 as “COVID for cows.”
That leads to the next somewhat concerning unknown: the specific route by which the virus is transmitted in cattle. Does it spread through droplets? By contact with an infected surface? Knowing the specific mechanism could help slow the spread. Meanwhile scientists are developing vaccine candidates for cattle.
There are also a few human cases where epidemiologists have yet to trace infection to a particular source.
Read More: Bird Flu Has Been Found in Raw Milk − A Reminder of How Pasteurization Protects Health
What We Can Do To Stay Safe
However, the CDC has identified those most at risk: people who handle raw poultry meat, backyard chicken farmers, dairy farmers, and people who drink raw (unpasteurized) milk.
“People with exposure to infected animals, including birds and dairy cows, are at higher risk of infection,” said a CDC spokesperson at the conference.
Among other recommendations, the agency discourages people from drinking unpasteurized milk or products made from it.
While the recent genotypes and the rapid spread among cows are concerning, the virus would have to mutate both quickly and in many places — in what Byrd-Leotis calls a “constellation of changes” to become a virus that spreads rapidly from human to human. Although much remains unknown about the new genotypes, such a scenario is highly unlikely.
Article Sources
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Before joining Discover Magazine, Paul Smaglik spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.
