Now that bird flu is spreading among cows, scientists worry where H5N1 will jump next

On March 25, American of‐ ficials published an urgent announcement: Dairy cows in Texas, Kansas, and New Mexico were falling sick.

The cows had low ap‐ petites, and produced less milk than normal. Some far‐ ms also discovered wild bird carcasses on their grounds. Tests on a cow throat swab and raw milk samples all con‐ firmed an unusual finding: for the first time, cattle were catching a dangerous form of bird flu.

Within days, highly patho‐ genic avian flu - a type of in‐ fluenza 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 bor‐ der.

Louise Moncla, an avian influenza researcher and as‐ sistant professor at the Uni‐ versity of Pennsylvania, was stunned.

"The overwhelming feel‐ ing 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 in‐ fluenza A viruses."

But the curveball wasn't entirely unexpected. And it may be a harbinger of more species-jumps to come, in‐ cluding the rising possibility of H5N1 appearing in pigs which could offer it a new route to better adapt to in‐ fect humans, inching the world closer to a bird flu pan‐ demic.

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 do‐ mestic 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 rumi‐ nant. (Cows are also rumi‐ nants, a group of herbivores known for their four-cham‐ bered stomachs.)

Sporadic human cases and deaths - are also occur‐ ring around the world. The second-ever human infection in the U.S. was reported just days ago in Texas, in an indi‐ vidual with mild symptoms who'd had direct exposure to cattle.

A somewhat reassuring genomic sequencing analysis from the U.S. Centers for Dis‐ ease Control and Prevention (CDC) found only "minor changes" between viral se‐ quences from cattle and the virus sequence from the hu‐ man patient. And in both cases, the sequences largely lacked any changes sug‐ gesting 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 real‐ ly efficiently in cows," Moncla said.

The genome for the hu‐ man case did feature one genetic tweak that signals adaptation to mammals - but the CDC stressed there was‐ n't evidence the virus had transmitted onward to other people.

Still, such rapid spread among dairy cattle herds, alongside other recent infec‐ tions reported in U.S. farm cats, poultry, and the coun‐ try's latest human case, all has scientists and health offi‐ cials 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 evi‐ dence of mammal-to-mam‐ mal transmission," said Caitlin Rivers, an epidemiolo‐ gist 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 con‐ cerning, several researchers agreed, is the growing poten‐ tial for bird flu to spread to another species of livestock: Pigs.

WATCH | Human bird flu case linked to U.S. dairy cattle outbreaks:

Pigs considered viral mixing vessels

While cattle aren't known for being an ideal host for many flu viruses, pigs are potent vi‐ ral mixing vessels. That's be‐ cause swine have both hu‐ man-adapted receptors and avian-adapted receptors in their respiratory tracts, Mon‐ cla said, meaning they can be infected with either type of pathogen.

If a pig catches both a hu‐ man influenza A virus and an avian influenza A virus at the same time, it can spark a pro‐ cess known as viral reassort‐ ment - a genetic exchange in which flu viruses swap gene segments.

Those swaps can intro‐ duce dramatic changes, pro‐ ducing a new virus with cer‐ tain properties of a non-hu‐ man 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 be‐ gan circulating alongside oth‐ er seasonal flu viruses and is now included in annual flu shots.

Moncla said her "worst fear" is something similar happening with highly patho‐ genic avian flu, given its health impacts.

The death rate in humans may be upwards of 50 per cent, World Health Organiza‐ tion data suggests, though it's possible that milder infec‐ tions 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 pan‐ demic - and it would be terri‐ ble," 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 cat‐ tle, according to the Canadi‐ an 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 sur‐ veillance.

"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 updat‐ ing emergency flu vaccine stockpiles, she said, since the time between the first hu‐ man cluster of cases and widespread transmission could be a "very short win‐ dow."

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 veteri‐ narian 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 En‐ glish as their first language, and may be hesitant to seek medical help, he added.

"So I think there's proba‐

bly underreporting on both sides," Armstrong said. "If [H5N1] gets into a pop‐ ulation where there's con‐ stantly animals going in and out … it might not ever leave."