Holstein dairy cows at a dairy farm outside of Muleshoe, Texas, on Jan. 4, 2016.ALLISON TERRY/The New York Times News Service
In the weeks leading up to the discovery of bird flu in cows in the United States, a crucial clue was the missing cats.
Veterinarians in the Texas Panhandle noticed fewer cats slinking around barns as they drove from farm to farm, trying to figure out what was causing dairy cattle to lose their appetites and produce less milk.
The cats, it turned out, had been going blind and losing their balance as they died of a highly pathogenic version of avian influenza known to infect felines. Barb Petersen, an Amarillo field veterinarian, wondered whether there was a connection between the dead cats and the sick cows, even though H5N1 had never been definitively diagnosed in cattle before.
Dr. Petersen sent samples of the Texas cows’ raw milk to a lab at the Iowa State College of Veterinary Medicine, where Drew Magstadt, a veterinary diagnostician, tested them expecting to rule out avian flu.
“I honestly thought that it was going to come back negative, and we were going to say that the cats got sick from eating infected birds,” he said. “And then we were going to move on to something else.”
Instead, he found himself “incredibly surprised,” to learn the Texas cows’ milk was teeming with H5N1.
Those positive tests, which the U.S. Department of Agriculture announced on March 25, thrust bird flu back into the consciousness of a pandemic-weary public, setting off a race to determine how the virus is infecting cattle and what threat it might eventually pose to humans.
Public-health leaders on both sides of the border say the risk of bird flu to humans is very low unless they have sustained direct contact with farmed or wild birds. The sole confirmed human case linked to the cattle outbreaks in the U.S. was in a Texas farmworker whose only symptom was pinkeye.
“Right now, the risk is probably close to zero,” said Brian Ward, a professor of infectious diseases at McGill University.
But all that could change if the virus mutates significantly while circulating in livestock. “The closer the potential vector is to humans, the more we need to pay attention to it,” Dr. Ward said. “And cows are obviously closer to humans than birds.”
On Friday, the Canadian Food Inspection Agency announced that it would begin actively searching for undetected H5N1 in cows. The agency said it would test retail milk for harmless traces of the virus and offer voluntary testing of cows without obvious symptoms of bird flu.
Earlier in the week, the agency mandated negative flu tests for American cows transported across the border.
No cases have been confirmed in Canadian cattle, but some experts suspect that’s because officials have been waiting for farmers to report cows with symptoms, rather than pro-actively seeking the virus, as the agency is now promising to do.
“I’ve been disappointed,” said Lawrence Goodridge, a professor with the Canadian Research Institute for Food Safety at the University of Guelph. “The standard talking point is that it’s not here in Canada. So how do we know that?”
Dr. Goodridge, who spoke to The Globe and Mail before the Canadian Food Inspection Agency’s Friday announcement, was part of a growing scientific chorus calling on the federal government to launch a comprehensive surveillance program, including testing wastewater for H5N1, something his own lab recently started doing.
He and others have good reason to suspect the virus could be flying under the radar on Canadian dairy farms. Bird flu probably circulated in American cattle for four months before Dr. Magstadt and his colleagues ran their tests, according to a USDA analysis of viral genomic sequences published Thursday that concluded the virus likely jumped from a wild bird to a cow in Texas last December.
The USDA also found fragments of H5N1 in one in five retail milk samples last month. Further tests confirmed the milk was safe to drink because it was pasteurized, but the viral traces suggested bird flu spread widely in cows before it was picked up. Tests of Canadian milk could find – or rule out – the same phenomenon here.
The situation on U.S. dairy farms has scientists particularly worried because the version of bird flu found in cows has already decimated wildlife around the world.
A member of the influenza A H5N1 family known as 2.3.4.4b has wiped out poultry flocks, annihilated wild birds and spilled into an ever-expanding list of mammals that includes foxes, skunks, bears, cats and dogs. It caused a mass mortality event in South American sea lions, killing at least 24,000 in Peru, Chile, Argentina, Uruguay, and Brazil last year.
In Canada, more than 11 million farmed birds have died or been culled since H5N1 2.3.4.4b was first detected here in late 2021.
Fortunately, no cows in the U.S. are known to have died of the virus, which seems to cause mild illness in bovines. It’s still unclear if or how the virus is transmitting among cows, although milking paraphernalia is a prime suspect because of the high viral load in the cows’ milk and mammary tissues, USDA officials have said.
The Texas dairy worker with pinkeye may be the first detected example of avian flu transmitting from a mammal to a human, according to a new report published Friday in the New England Journal of Medicine. The worker was not in contact with sick birds, unlike people who’ve caught avian flu in the past.
Dr. Ward said that humans have receptors for H5N1 flu deep in their lungs, not in their noses or upper airways, which explains why the viruses rarely infect people. When they do, the outcome can be frightening. Of nearly 900 confirmed human cases of bird flu identified worldwide since 2003, 52 per cent were fatal.
The true case fatality rate is almost certainly lower because mild cases were missed by authorities, skewing the ratio, and because the deaths happened in low-income countries where poor farmers have scant access to health care.
Still, Dr. Ward said it would be cavalier to assume first-rate medical care would substantially reduce mortality if bird flu adapted to transmit between humans. “These are bad viruses when they get into your lower respiratory tract,” he said, “whether you’re a poor person living in Bangladesh or a rich person living in Westmount.”
If avian influenza evolved to transmit easily between humans, the race would be on to produce and distribute a vaccine.
Health Canada has authorized three pandemic flu vaccines as a precautionary measure, two of which were developed using an older H5N1 strain. Like seasonal flu shots, the pandemic jabs are produced in eggs and take three to six months to make.
Canada generally doesn’t stockpile them because they only have a shelf life of two years, said André Gagnon, a spokesman for the Public Health Agency of Canada. Instead, under deals with several domestic and international pharmaceutical companies, the manufacturing process would kickstart as soon as the World Health Organization declared an influenza pandemic or advised vaccine makers to switch from producing seasonal to pandemic shots.
For now, scientists like Fiona Brinkman, a professor of genomics and bioinformatics at Simon Fraser University, are keeping a close eye on viral genomic sequences from cows and other creatures as they are uploaded to public databases.
The sequences form an evolutionary trail that can be followed in reverse to see how the avian flu virus has mutated over time. One red flag for Dr. Brinkman is that a mammalian-adaptive mutation was seen in the cattle cases. “It looks like what’s happened is it jumped over once, from birds into cows, and now it’s sort of jumping back into birds,” she said.
That’s a concern because wild birds could carry a version of the virus that more readily infects mammals north as they migrate, she added.
The prospect of H5N1 2.3.4.4b jumping to pigs is another worry, said Samira Mubareka, a clinician-scientist and virologist at Sunnybrook Health Sciences Centre in Toronto. Swine are notorious mixing vessels for influenza viruses because they have mammalian and avian receptors for flu viruses. The version of H1N1 that caused a mild pandemic in 2009 spilled over from pigs.
Dr. Mubareka said now is the moment for everyone committed to human health to pay extra attention to animal health, including by supporting veterinary scientists like those who solved the mystery of the sick barn cats and cows in Texas.
“We know the next pandemic pathogen is highly, highly likely to circulate in animals before it spills over into humans,” she said. “By the time it’s impacting human health, we’ve lost a major opportunity to protect human health.”