H5N1 Bird flu spreading through cow's milk

Governments and scientists internationally are becoming increasingly alarmed by the the threat posed by the H5N1 strain of bird flu. What’s got people worried is that, having produced the largest outbreak of bird infections yet documented over the last few years, the virus has now spread from birds into mammals, including in the US into farm animals like cows; and in them, it’s evolved the ability to maintain sustained infections over several months, spreading from animal to animal. Apart from bringing the infection a step closer to us, it’s also adopted a completely novel mode of spread, via milk and milking machines. And farm cats that have come into contact with the milk have died. Consequently many countries have quietly notched up their risk assessments of the threat of a human H5N1 pandemic. In the UK, disease experts say we’re sitting at position 4 on the 7 point scale. Virologist Wendy Barclay is based at Imperial College London…

Wendy - Since about 1997, this particular bird flu known as H5N1, which is what we call a highly pathogenic avian influenza virus. There was a lot of fuss about it in the early two thousands when it caused a lot of disease both in birds and in humans. This particular H5N1 has continued to evolve in two different ways. It mutates a little bit and it also reasserts shuffles up its genes with the genes of other bird flu viruses. And since around about 2020, a derivative of that bird flu H5N1 from back in 1997 and the early 2000s has reemerged as a very vigorous virus, and it has spread all the way across Asia, Europe, and now across into the Americas, which is pretty much unheard of for bird flus until now. They've tended to sort of at least stay in two groups, the Eurasian viruses and the American ones. But this one has caused what we call a panzootic, which basically means it's a disease of animals and it's just everywhere.

Chris - What about into non bird populations? Because that's the other thing that's beginning to surface. We are seeing reports of other animals that are also being diagnosed with this. Where are they getting it from?

Wendy - Well, absolutely. So, what we've seen is that there's been so much of this particular H5N1 in the wild birds and they have been dying, that we've started to see cases cropping up in some wild mammals, seals and sea lions that scavenge on dead sea birds. We've also seen it in foxes and otters. Also into farmed mammals, for example mink being farmed for their fur in Scandinavia, Denmark, Finland. And also certain breeds of arctic fox, for example, which are also farmed for their fur. And then finally we get to 2024 where I think we were all quite surprised to hear that now this same H5N1 virus has found its way into dairy cattle.

Chris - Is there any evidence when it gets into these non avian, in other words, non bird species, that it's actually transmitting? Or are they just being exposed to so much virus when they find a dead bird or come into contact with one in their environment that there's almost an inevitability they're going to pick it up like the humans you mentioned earlier?

Wendy - It's very difficult to say for sure that we're seeing mammalian to mammalian transmission, but the outbreaks that have occurred in seal colonies down in the South Americas, for example Georgia and the Falkland Islands, do look as if we're starting to see some transmission. Some of the outbreaks in farmed mink look as if there could be some transmission. What's really important when we start seeing mammalian to mammalian transmission is to ask whether or not the virus is mutating in a way that is enhancing the transmissibility of the virus, not only between those mammalian species, but also with the potential to enhance airborne transmission between humans. Thankfully, we haven't yet seen mutations associated with the mammalian transmission events, which lead us to think that that has happened.

Chris - Because we have seen some human cases. So how is it then that it can infect an animal including a human, but not transmit between those species?

Wendy - I'd certainly say that the human cases that have been reported so far look as if they've been directly acquired from birds or contact mammals to which the people have had significant exposure. Receiving either a large dose or a dose under a circumstance where that kind of transmission could happen. But no further mutation within the person to drive human to human airborne transmission. The cattle is quite an unusual story. There's this very unusual situation going on in the United States where most of the transmission is through the milking machinery. The virus has found a niche in the mammary glands of these cows. It's replicating to very high titres there. And then if traces of milk contaminated with virus are left behind on milking machinery and that's not adequately cleaned before the next cow is milked, it seems that there's some level of mechanical transmission of the virus from one cow to another.

Chris - If it did, heaven forbid, because it spent so long in mammals, it would adapt better to get into the farmer as well as the animals the farmer keeps. Could we end up with a disease that could transmit via human breast milk and you'll end up with mothers infecting their babies?

Wendy - That's a very interesting question. I mean, certainly some of the animal studies that are being published now on the H5N1 shows, this isn't unique to a cow's. Laboratory mice have been fed contaminated milk and they indeed become infected through that route and lactating mice can pass the virus onto their pups through the milk. So I wouldn't exclude that there could be a derivative of, of a virus that could be passed from mother to child through the milk.