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