A person in Texas has been infected with bird flu after exposure to dairy cows which had the virus, in the first confirmed case of a subtype of the virus, named H5N1, transmitting between a human and another mammal.
CNN reports that this is the first case of H5N1 flu in a person linked to dairy cattle and the second case in a person in the US. A Colorado man who had direct exposure to poultry and the culling of birds had bird flu in 2022.
Now, as scientists now probe how the virus is spreading, there is concern that if it can spread easily between cows, that could lead to larger, more sustained outbreaks, which would also give the virus more opportunities to adapt to its new mammalian hosts, increasing the risk of mutations, making it more dangerous to people.
The US Centres for Disease Control and Prevention (CDC) confirmed the positive test results over the weekend, saying the affected individual, whose only symptom was eye inflammation, is being treated with oseltamivir, the generic version of the antiviral medication Tamiflu.
The patient worked directly with sick dairy cows, said Lara Anton, a spokeswoman for the Texas Department of State Health Services. “We have tested around a dozen symptomatic people and only the one person has tested positive for the virus.”
The virus has decimated global bird populations – but appears to have now spread to dairy herds. Last week, cows across five US states tested positive for H5N1, reports New Scientist.
Previously, only mammals had been confirmed to contract the virus from sick birds.
The particular virus in these new cases was first identified in 1996 in geese in China, and in people in Hong Kong in 1997.
In 2020, a new, highly pathogenic form of H5N1 emerged in Europe and spread quickly around the world. In the US, it has affected more than 82m farmed birds, the worst bird flu outbreak in US history.
Since the virus was first identified, sporadic cases have been found in people in other countries. But most resulted from prolonged, direct contact with birds, reports The New York Times.
H5N1 does not yet seem to have adapted to spread efficiently among people, experts say, and cows were not thought to be a species at high risk.
“The fact that they are susceptible – the virus can replicate, can make them sick – that is something I wouldn’t have predicted,” said Richard Webby, an influenza virologist at St Jude Children’s Research Hospital in Tennessee.
He pointed out that there had been a couple of outbreaks that didn’t include humans, where it is possible there was mammal-to-mammal transmission.
For instance, 17 000 baby elephant seals died from bird flu in Argentina late last year. There was also an outbreak among farmed mink in Spain in 2022. But it was hard to rule out the possibility of other sources of the virus, such as contaminated food in these situations, he said.
There are various ways the virus might have found its way into cattle. The likely route, several experts said, wis that infected wild birds, which shed the virus in their faeces, saliva and other secretions, contaminated the cows’ food or water.
How is it spreading?
It is possible the infected cows are picking up the virus independently, especially if shared food or water sources have been contaminated.
But several scientists said they would be surprised if there were not some degree of cow-to-cow transmission. “How else could it move so rapidly?” said Dr Gregory Gray, an infectious disease epidemiologist at the University of Texas Medical Branch.
Other free-ranging animals known to be susceptible to the virus, like cats and raccoons, could also have brought the virus on to dairy farms.
Officials say if the virus can spread easily between cows, that could lead to larger, more sustained outbreaks. It would also give the virus more opportunities to adapt to its new mammalian hosts, increasing the risk that it acquires mutations, which make it more dangerous to people.
How will officials know if bird flu adapts to spread between people?
Analysing the genetic sequence of the virus from infected birds, cows and people can reveal whether H5N1 has acquired mutations that help it spread among people.
Scientists have been closely tracking infections in birds and sea mammals – and now cows – but so far, the virus does not seem to have the ability to spread efficiently between people.
In 2012, scientists showed that H5N1 was able to spread through air between ferrets – a popular model for studying transmission of respiratory viruses among people – after acquiring five mutations.
A sample of bird flu isolated from a Chilean man last year had two mutations that indicate adaptation to infecting mammals. But those mutations have previously been seen without the virus evolving further to spread between people, experts said.
The CDC said the risk of contracting bird flu remains low for most people. Those in close contact with infected birds or other animals, including livestock, have the greatest risk, the agency said, and while pasteurised milk remains safe, people should avoid consuming or handling raw milk products.
Pasteurisation “has continually proved to inactivate bacteria and viruses, like influenza, in milk”, the agency said.
Dr Gail Hansen, a veterinary public health expert and independent consultant, agreed that the risk of being infected by pasteurised milk was probably “very low”.
But the possibility could not be entirely ruled out, she said, expressing some concern that federal officials had been “overconfident in the face of so many unknowns”.
If cows are shedding virus into their milk before they show signs of illness, that milk could potentially find its way into the commercial milk supply, she said. And different pathogens may require different pasteurisation temperatures and durations; the specific conditions required to inactivate this particular virus remained unclear, she added.
“This is a rapidly evolving situation,” the US Department of Agriculture said.
“Because influenza viruses constantly change, continued surveillance and preparedness efforts are critical, and the CDC is taking measures in case the public health risk assessment changes. This is a developing situation, and additional updates will be shared as new relevant information becomes available.”
CNN article – CDC confirms second reported case of bird flu in a person in the US (Open access)
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