Serious gaps in animal and human testing could obscure the true number of bird flu cases in the U.S. and make it difficult to understand how the H5N1 virus spreads — and how to stop it, experts say.
Faced with farms’ reluctance to test laboratory workers and animals, scientists are now turning to experimental studies to understand how H5N1, a highly pathogenic bird flu, spreads through cows and to other farms.
The number of bird flu cases among U.S. dairy herds continues to rise, but infections are more widespread than previously thought, as tests of commercially available milk show.
While the risk to humans is still low, this could change as the virus mutates. Its continued spread therefore remains a major concern.
“This epizootic has really taken people by surprise,” said Gregory Gray, professor of infectious diseases at the University of Texas Medical Branch. Scientists knew that cows could be infected with all four different types of influenza, “but we have never seen so many infections, nor so quickly.”
Understanding how the virus moves is key to stopping it – but testing, which can reveal such transmission patterns, has been slow and inadequate.
A Texas dairy worker, the only person confirmed to have H5N1 in this outbreak and the first documented case of mammal-to-human transmission of the virus, has requested testing from a local health department, a recent study shows. The employee reported a form of conjunctivitis that caused their eyes to bleed and become red.
But after the positive test, officials were unable to test other employees or animals at the farm where the person worked. That makes it difficult for scientists to understand how the virus spread to the worker and whether it affected other people.
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“The people we need at most right now are the other people on these farms who are exposed to enormous amounts of viruses in these environments,” said Richard Webby, a virologist in the infectious diseases division at St Jude Children’s Research Hospital. “That’s not easy, and it’s not happening at a scale that we probably need.”
During this entire outbreak, only about twenty people have been tested for H5N1. The Centers for Disease Control and Prevention does not recommend testing unless symptoms develop after close contact with animals — even if someone has milked a sick cow or lives with an infected person.
The lack of testing could obscure the true rate of transmission to humans, if workers and their close contacts do not experience symptoms severely enough, or if they are unable or unwilling to seek medical attention.
Barb Petersen, the veterinarian who discovered the first case of H5N1 in cows in Texas, said dairy workers were also sick — some of them sick enough to miss work, which was highly unusual, she said — but that they had not been tested for the highly pathogenic bird disease. influenza.
Other types of cows, including beef cattle and calves, appear not to be tested, despite evidence that the virus may be asymptomatic in cattle.
“We don’t know when this thing will move into the beef cattle, and no one is really talking about that,” Gray said.
And it appears that pigs, which play a role in causing human influenza epidemics, are not being monitored more than usual, despite evidence that bird flu has spread from cows to nearby chicken farms and could similarly spread to pig farms.
Pigs are of concern because they can mix animal and human flu viruses, which could result in variants that are more transmissible or virulent among humans.
Cows may have similar abilities, according to early research co-authored by Webby. Like pigs, cows have receptors for bird flu and human flu, and could potentially create a “hybrid virus” that could affect humans more, Webby said.
But, he cautioned, the animals would have to be infected with both types of flu at the same time, which is relatively rare – especially at this time of year when human flu rates are low. “It is theoretically possible, but perhaps unlikely – but at the same time it increases the chances if this virus continues to circulate, even if those chances are very small.”
Another challenge for scientists: the genomic sequences released so far by US agencies do not contain important data – such as when and where they were collected – making it very difficult to track what is happening and how the virus is developing, say scientists. This has global implications for understanding and monitoring outbreaks among livestock.
The livestock industry has largely resisted any testing efforts, with a Texas agriculture official telling the Biden administration to “back off,” in part because of distrust of federal agencies among farmers.
Another problem is that cow farmers do not receive compensation for financial losses due to lower milk yields or the inability to export cows to other markets, Gray said.
“They are really concerned that if they wave the flag ‘we have the virus here’ they will be punished economically or through disruption to their operating procedures,” he said. “We have to find a way to overcome that and protect the farms.”
He points to poultry companies, which do have a federal compensation scheme for culling infected birds – and which also monitor poultry much more closely for infectious diseases, allowing them to take quick action to address outbreaks like this.
Scientists like Gray also work with farm veterinarians to test animals under non-disclosure agreements to avoid identifying farms.
And some of those veterinarians are conducting their own studies on the farms to understand transmission, Gray said. “For example, is the virus moved from cow to cow during the milking process, is the virus aerosolized, does the virus move from cow to cow in other ways?”
There are also questions about the extent to which people unknowingly spread the virus, he said.
Some scientists, unable to track the transmission currently taking place on farms, are turning to the experimental infection of healthy cows. The results of these experiments should appear in the coming weeks, Webby said.
“Is there actually anything different about this particular virus itself? Does it have properties that the other H5 circulating in wild birds do not have?” he asked. He hopes outbreaks like this are rare among cows, but understanding how they happen and then spread is crucial to responding now and in the future.
“Let’s say we eradicate this. What are the chances of it happening again?” Webby asked. “If we can figure out how it moves, then I think we can absolutely think about eradicating this virus from cows.”