It’s now official: the highly pathogenic bird flu A(H5N1) that’s been spreading across the globe since 2020 has now been passed from a cow to a dairy farmer in the US, the first confirmed cow-to-human transmission of this virus on record.
The good news is the case was caught quickly – and the virus manifested as inflammation in the eye, rather than any type of upper respiratory infection. So the chances of it having been passed on to anyone else, if human-to-human transmission is even yet possible, are lower.
What’s more, after nervously watching it spread through poultry and wild animals, we’ve now got some solid data on how the bird flu presents in humans, which should help experts in assessing the threat to public health – and in identifying more cases if and when they appear.
“It’s a huge thing that the virus has jumped from birds to mammals, dairy cows in this case, and then to humans,” says environmental toxicologist Steve Presley, the director of the Biological Threat Research Laboratory at Texas Tech University.
Presley and his colleagues are behind the newly published paper on this one case of cow-to-human bird flu transmission, confirmed in tests carried out in highly biosafe laboratory conditions, and shared with the US Centers for Disease Control and Prevention (CDC).
The farm worker reported redness and discomfort in their right eye towards the end of March 2024. Though they hadn’t been in contact with birds or poultry, they were working with cows – some of whom had been showing signs of sickness.
It’s only recently that this bird flu passed from poultry to livestock in the US, which was something of a surprise for experts because it was the first time ever that highly pathogenic avian influenza (HPAI) had been detected in dairy cattle. Closer monitoring of dairy cows and those who come into contact with them will now be required.
This is only the second human case of this bird flu in the US, and human-to-human transmission hasn’t been observed anywhere yet. But each time the pathogen finds a human host, it has more chances to adapt and mutate to be more infectious to our species – which seems to have happened in this case.
“The virus identified in the worker’s specimen had a change (PB2 E627K) that has been associated with viral adaptation to mammalian hosts and detected previously in humans and other mammals infected with HPAI A(H5N1) viruses and other avian influenza A virus subtypes,” Presley and his CDC and Texas state health authority colleagues write in their paper.
The current bird flu outbreak started in 2020, and although human infection is rare, there’s a high mortality rate. That means it’s vital that we understand how disease is being passed between animals, and where this is happening.
We know it’s now in a host of mammals, including foxes, seals, sea lions, bears, and domestic cats. With the stakes so high, and the pandemic fresh in people’s minds, scientists are working overtime to try and minimize the ongoing spread of the influenza.
“[This study is] going to lay the foundation, I believe, for a lot of research in the future of how the virus is evolving,” says Presley.
The research has been published in The New England Journal of Medicine.
