May 28, 2024


Serious gaps in testing animals and people could be obscuring the true rate of avian influenza cases in the US and make it difficult to understand how the H5N1 virus is spreading – and how to stop it, experts say.

Facing reluctance from farms to test workers and animals, scientists are now turning to experimental studies to understand how H5N1, a highly pathogenic bird flu, is spreading through cows and on to other farms.

The bird flu count among dairy herds in the US continues rising, but infections are more widespread than previously realized, as testing in commercially available milk reveals.

While the risk to people is still low, that could change as the virus mutates, so its continued circulation remains a big concern.

“This epizootic has caught people tremendously by surprise,” said Gregory Gray, a professor of infectious diseases at the University of Texas Medical Branch. Scientists knew cows could be infected with all four different influenza types, “but we’ve never seen this amount of infection, nor have we seen it move so fast.”

Understanding how the virus moves is essential to stopping it – but testing, which can reveal such transmission patterns, has been slow and inadequate.

A dairy worker in Texas, the only person confirmed to have H5N1 in this outbreak and the first documented case of mammal-to-human transmission of the virus, sought out a test at a local health department, a recent study shows. The worker reported a form of conjunctivitis that caused their eyes to hemorrhage and turn red.

Yet after the positive test, officials were not able to test any other workers 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 has affected other people.

“The people that we need to get at most now are the other folks on these farms that are getting exposed to huge amounts of virus in these environments,” said Richard Webby, a virologist at St Jude children’s research hospital’s department of infectious diseases. “That’s not easy, and it’s not happening at a scope that we probably need.”

Only about two dozen people have been tested for H5N1 in this entire outbreak. 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 be obscuring the true rate of transmission to people, if workers and their close contacts are not experiencing symptoms severe enough, or if they are unable or unwilling, to seek medical care.

Barb Petersen, the veterinarian who discovered the first case of H5N1 in Texas cows, said dairy workers were also sick – some of them sick enough to miss work, which was very unusual, she said – but they were not tested for the highly pathogenic avian influenza.

Other types of cows, including beef cattle and calves, seem to be going untested, despite evidence that the virus can be asymptomatic in cattle.

“We don’t know when this thing moves in the beef cattle, and no one’s really talking about that,” Gray said.

And it seems that pigs, which play a role in sparking human influenza epidemics, are not being monitored any more than usual, despite evidence that avian flu has spread from cows to chicken farms nearby and could spread in a similar fashion to swine farms.

Pigs are a concern because they can mix animal and human flu viruses, which could result in variants that are more transmissible or virulent among people.

Cows may have similar abilities, according to early research co-authored by Webby. Like pigs, cows have receptors for avian and human influenza, and might potentially make a “hybrid virus” that could affect humans more, Webby said.

But, he cautioned, the animals would have to be infected with both types of influenza at the same time, which is relatively rare – especially at this time of year, when human influenza rates are low. “It’s theoretically possible, but perhaps unlikely – but at the same time, if we have this virus continuing to circulate, it does increase the chances, even if those chances are really small.”

Another challenge for scientists: the genomic sequences released thus far by US agencies were stripped of key data – like when and where they were collected – making it very difficult to track what’s happening and how the virus is evolving, scientists say. This has global implications for understanding and tracking outbreaks among livestock.

The animal agriculture industry has largely resisted any attempts to test, with one Texas agriculture official telling the Biden administration to “back off”, in part because of mistrust of federal agencies among farmers.

Another problem is that cow farmers don’t receive compensation for financial losses from lower milk yields or not being able to export cows to other markets, Gray said.

“They’re really concerned that if they wave the flag ‘we’ve got the virus here,’ they’ll be penalized either economically or through disruptions to their operating procedures,” he said. “We have to find a way to overcome that and protect the farms.”

He points to poultry farms, which do have a federal compensation scheme for culling infected birds – and which also monitor the poultry much more closely for infectious diseases, which allows them to take fast action to address outbreaks like these.

Scientists like Gray are also collaborating with farm veterinarians to test animals under nondisclosure agreements to avoid identifying farms.

And some of those veterinarians are conducting their own studies on the farms to understand transmission, Gray said. “For instance, is the virus being moved through the milking procedure from cow to cow, is the virus aerosolized, is the virus moving from cow to cow through other means?”

There are also questions about the extent to which people may be unwittingly spreading the virus, he said.

Some scientists, unable to track the transmission currently happening on farms, are turning to the experimental infection of healthy cows. The results of these experiments should come in the next few weeks, Webby said.

“Is there anything actually different about this particular virus itself? Does it have properties that the other H5 circulating in wild birds doesn’t?” he asked. He is hoping that outbreaks like these are rare among cows, but understanding how they happen and then how they spread is critical to responding now and in the future.

“Let’s say we do eradicate this. What’s the chance of it happening again?” Webby asked. “If we can figure out how it’s moving, then I think we can absolutely think about eradicating this virus from cows.”



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