Now that bird flu is spreading among cows, scientists worry where H5N1 will jump next
Possibility of H5N1 appearing in more livestock, including pigs, could help it adapt to better infect humans
On March 25, American officials published an urgent announcement: Dairy cows in Texas, Kansas, and New Mexico were falling sick.
The cows had low appetites, and produced less milk than normal. Some farms also discovered wild bird carcasses on their grounds. Tests on a cow throat swab and raw milk samples all confirmed an unusual finding: for the first time, cattle were catching a dangerous form of bird flu.
Within days, highly pathogenic avian flu — a type of influenza A known as H5N1 — was identified in at least a dozen herds across six states, from Texas in the south, up to Michigan and Idaho on the Canadian border.
Louise Moncla, an avian influenza researcher and assistant professor at the University of Pennsylvania, was stunned.
"The overwhelming feeling that all of us have is that this is mostly just incredibly strange," she said. "To our knowledge, I've never seen a cow be infected with any influenza A viruses."
But the curveball wasn't entirely unexpected. And it may be a harbinger of more species-jumps to come, including the rising possibility of H5N1 appearing in pigs — which could offer it a new route to better adapt to infect humans, inching the world closer to a bird flu pandemic.
Various species getting infected
Over the last two decades, this deadly form of bird flu began striking more and more wild and farmed bird species. The threat exploded in 2022 with tens of millions of global bird deaths. And a rising number of mammals are also getting infected, from mink to seals to domestic dogs and cats.
This March, prior to the discovery of cases among cattle, Minnesota reported an H5N1 infection in a young goat, marking the first known U.S. case of bird flu in a ruminant. (Cows are also ruminants, a group of herbivores known for their four-chambered stomachs.)
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Sporadic human cases — and deaths — are also occurring around the world. The second-ever human infection in the U.S. was reported just days ago in Texas, in an individual with mild symptoms who'd had direct exposure to cattle.
A somewhat reassuring genomic sequencing analysis from the U.S. Centers for Disease Control and Prevention (CDC) found only "minor changes" between viral sequences from cattle and the virus sequence from the human patient. And in both cases, the sequences largely lacked any changes suggesting the virus had better adapted to infect mammals.
"There is no evidence at this time that this virus is some sort of new, adapted strain that's transmitting really efficiently in cows," Moncla said.
The genome for the human case did feature one genetic tweak that signals adaptation to mammals — but the CDC stressed there wasn't evidence the virus had transmitted onward to other people.
Still, such rapid spread among dairy cattle herds, alongside other recent infections reported in U.S. farm cats, poultry, and the country's latest human case, all has scientists and health officials on high alert.
"Dairy cows have not been affected before in the United States, or anywhere else in the world to my knowledge, and we've never before seen such clear evidence of mammal-to-mammal transmission," said Caitlin Rivers, an epidemiologist at the Johns Hopkins Center for Health Security in Baltimore.
That possibility of spread between cows "does take us, maybe, a half-step closer to a scenario where the virus would be better adapted for humans," she told CBC News.
What's even more concerning, several researchers agreed, is the growing potential for bird flu to spread to another species of livestock: Pigs.
Pigs considered viral mixing vessels
While cattle aren't known for being an ideal host for many flu viruses, pigs are potent viral mixing vessels. That's because swine have both human-adapted receptors and avian-adapted receptors in their respiratory tracts, Moncla said, meaning they can be infected with either type of pathogen.
If a pig catches both a human influenza A virus and an avian influenza A virus at the same time, it can spark a process known as viral reassortment — a genetic exchange in which flu viruses swap gene segments.
Those swaps can introduce dramatic changes, producing a new virus with certain properties of a non-human strain coupled with the capacity to infect and spread between people.
That sort of shift hasn't been documented yet with H5N1. But it did happen with a new form of H1N1 — a virus resulting from a mashup of genes between various pig, bird, and human flu viruses — which began infecting people for the first time in 2009, sparking a pandemic.
Death rates from H1N1 were higher than typical flu seasons, but it eventually began circulating alongside other seasonal flu viruses and is now included in annual flu shots.
Moncla said her "worst fear" is something similar happening with highly pathogenic avian flu, given its health impacts.
The death rate in humans may be upwards of 50 per cent, World Health Organization data suggests, though it's possible that milder infections are getting missed, skewing the case fatality ratio. Still, in a population that's never been exposed, the global impacts could be dire.
"Absolutely nobody wants to go through another pandemic — and it would be terrible," Moncla said.
'It might not ever leave'
There are no signals that H5N1 has spread to pigs, at least for now. It also hasn't appeared yet in Canadian livestock, including dairy cattle, according to the Canadian Food Inspection Agency.
But scientists on both sides of the border worry it's just a matter of time if swift action isn't taken to contain outbreaks and heighten surveillance.
"The more that we can be doing to prepare now, just in case, the faster our reaction will be, and the more likely we will be to get ahead of the virus," said Rivers.
That includes dusting off pandemic plans and updating emergency flu vaccine stockpiles, she said, since the time between the first human cluster of cases and widespread transmission could be a "very short window."
Increased surveillance is also essential, along with serology studies to find out if other cattle herds have been unknowingly impacted, said Dr. Joe Armstrong, a veterinarian with the University of Minnesota who regularly travels the state to educate dairy and beef producers.
Armstrong warned the outbreaks may already be bigger — and tougher to track — than they initially appeared.
Dairy producers, scared for their livelihoods, can be wary of reporting sick cattle, given poultry producers are often forced to cull entire flocks experiencing H5N1 outbreaks, Armstrong said.
More human cases could also be happening under the radar among farm workers who've moved to the U.S. from abroad, don't speak English as their first language, and may be hesitant to seek medical help, he added.
"So I think there's probably underreporting on both sides," Armstrong said.
"If [H5N1] gets into a population where there's constantly animals going in and out … it might not ever leave."
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