What is bird flu?

A new strain of this H5N1 virus - which infects birds but has thankfully so far only rarely infected humans, albeit with very high mortality rates of around 70% - has been spreading widely since late 2021, leading to the most severe bird flu outbreak yet documented. Globally, more than a billion birds are said to have been killed or culled over the last 2 years. So why is this happening, and, as cases of H5N1 are now also being found in mammals, is the virus edging closer to us and possibly a pandemic? Human influenza that you're probably more familiar with is a virus that attacks our body by infecting the cells in our respiratory system. The proteins on the surface of the virus act as a key that unlocks access to cells in our nose and throat. The influenza virus hijacks these cells and uses them to make copies of itself. In most cases, the inflammation caused by this process alerts the body’s immune response and the virus is eventually destroyed and removed, but often not before we cough it out onto someone else. Most people, because they've been vaccinated, caught flu in the past, or both, are at least partially immune to the infection, which limits its intensity and how fast it can spread through the population. It is still, nevertheless, a serious infection that kills over 650,000 people per year, usually the frail elderly. That though is a tiny number compared with how many could potentially become infected during a pandemic. This happens when a "new flu", with a totally different appearance to the viruses that we catch normally, begins to circulate. The usual source is a bird form of the virus that may have jumped directly into humans, or come via an intermediate, like farm livestock. Because no one has seen the virus before, no one is immune and everyone can catch it, producing very high rates of infection very quickly, just like with the Covid-19 coronavirus pandemic. And because these viral newcomers are not well adapted to humans, they tend to produce very severe infections with high mortality rates, even among young, healthy people, which is why H5N1 human infections have been lethal in 70% of cases. But how does avian flu transfer from bird to bird, and how do potential pandemics come about? I spoke to Glasgow University flu virologist, Ed Hutchinson.

Chris - A new strain of this H5N1 virus, which infects birds, but has thankfully so far, only rarely infected humans, albeit with very high mortality rates of around 70%, has been spreading widely since late 2021 and produced the most severe bird flu outbreak that we've documented yet globally. More than a billion birds have said to have been killed or culled in the last couple of years. So why is this happening? And as cases of H5N1 are also being found in mammals, is the virus edging closer to us and possibly a pandemic? The human influenza that you are probably more familiar with is a virus that attacks our body by infecting the cells of our respiratory systems, the proteins on the surface of the virus act as a key that unlocks access to cells in our noses and throats. The influenza virus hijacks these cells and uses them to make copies of itself. In most cases, the inflammation caused by this process alerts the body's immune response and the virus is eventually destroyed and removed, but often not before. It's had a chance to be coughed or sneezed onto somebody else. Most people, because they've been vaccinated, caught flu in the past or both are at least partially immune to the infection, which limits its intensity and how fast it can spread through the population. It is still nevertheless a serious infection though, and one that kills over 650,000 people a year. Usually the frail elderly. That though is a tiny number compared to how many could potentially become infected during a pandemic. This happens when a new flu with a totally different appearance to the viruses that we catch normally begins to circulate. The usual source is a bird form of the virus that may have jumped directly into humans or come via an intermediate like farm livestock. And because no one has seen this virus before, no one is immune and everyone can catch it, producing very high rates of infection very quickly, just like with the Covid 19 Coronavirus pandemic. And because these viral newcomers are not well adapted to us humans, they tend to produce very severe infections with very high mortality rates, even among the young and healthy people, which is why H five N one human infections have been lethal so far in up to 70% of cases. But how does avian flu transfer from bird to bird and other reports of H5N1 getting into mammals? Anything we should worry about? I spoke to Glasgow University flu virologist, Ed Hutchinson.

Ed - Influenza viruses cause seasonal influenza for us every winter, but they can also grow in many animals. In fact, they're not primarily a human virus at all. They are a virus of birds and particularly of wild waterfowl where they mainly spread through what we call the fecal oral route, which means it comes out the back end of a bird and then goes back in through the front. And most of the time it doesn't make them particularly sick as far as we can tell. Some of those strains do make them more sick. And there's been a strain of bird flu, which has been on the march recently, which has been making wild birds extremely unwell and wiping out huge numbers around the world.

Chris - And that's H5N1.

Ed - Yes.

Chris - The other thing birds do of course is move. They don't need a passport to do it. And presumably they are therefore a really powerful vector for moving the virus from where they pick it up to where they may go and spend the winter, spend the summer or where they may just stop for a rest among other birds.

Ed - Absolutely, yes. So we can follow the spread of influenza along the bird migration route. We can detect it in ponds where migrating water birds settled. But the other thing birds do is that they won't respect the keep out signs on the edge of farms so that if you have poultry farms, the birds there are at risk of being infected by influenza as well.

Chris - And what's the difference between what's going on in these birds and pandemic flu?

Ed - A pandemic is when a new infection, in this case a new virus, gets into humans and spreads all around the world. So pandemic literally means all the people. There are two things which might stop a virus from doing this. One is that it's just not very good at growing in people. So viruses take over the machinery of ourselves and use them to make more copies of themselves. And what that means is that they get really used to growing in one particular type of host, one particular species and are a bit rubbish when it comes to most other species. So a bird virus wouldn't normally grow well in humans, but influenza has an unusual trick in the way it copies itself, which is it has broken its genes into eight physically separate segments. These are a bit like chromosomes. This basically allows the virus to have sex in the slightly boring genetic sense of having sex, which is that you have parents and they exchange genes to produce offspring. So imagine that you were infected by a human influenza virus and then an influenza virus virus from a bird. An infected cell might produce a virus where almost all of those genes are from a human influenza virus. And so it's good at growing in humans, but the genes for the proteins on the outside of a virus, the ones which the immune response can most easily recognize an attack from, are from a bird virus and human immune systems have never seen them before. You won't be vaccinated against them, and that's when a new pandemic virus can appear and spread rapidly around the world. That's what happened in 1957, 1968, 2009. So the recent influenza pandemics have come about that way.

Chris - And this is why various people including yourself are concerned that if we've got human flu circulating at the moment, which we have, and you've got this bird flu circulating, then it's possible that a farmer looking after a big flock of birds could be exposed to a bird flu. And then you get this, this sort of genetic pick and mix that you are talking about, and we could end up with a human adapted bird flu strain that will then spread as a pandemic.

Ed - Yeah. So that's the situation we want to avoid. Now although influenza is unusually good at doing this, it still finds it very difficult to do. There's a huge amount of influenza out there, a huge amount of humans, a huge amount of birds, and we still only tend to get pandemics every few decades. So this is still a hard thing to happen. But in a situation where you have a lot of avian influenza and then get into farms where there are people and other mammals, that increases the risk of those rare events happening.

Chris - So how does the point that was made or made public by the UK Health Security Agency last week, that they were detecting cases of this bird flu virus among wildlife mammals like foxes and badges, for example, in the UK countryside. How does that fit this formula that you've been setting out for us? Because if it's not going straight into a human, it's into one of those animals. What does that mean? What are the implications of that and how's that happening?

Ed - So in all likelihood that happened from scavenging. So we've got this very aggressive strain of bird flu. Birds will die and their bodies will be riddled with the virus. And then an animal like a badger or a dog will come along and will take a chunk out of that and get a mouthful of virus. That animal may or may not get sick, but in any case, it's quite unlikely to pass any virus onto another mammal so the virus will not have many chances to adapt to mammals. We do focus on farms though, because the whole setup of farming is to take mammals and house them together at unnaturally high densities and unnaturally close to people. That's probably where the human influenzas we have originally came into humans through exposure to farmed animals. And it's why if we look at, for example, the case in the mink farm in Spain recently, which didn't take off, but that was a case where mink got infected. But because it was living very, very close to other mink, there was a chance to infect others and others and others, and the virus had some extra chances to adapt to mammals. Now in that case it didn't, but that's the sort of situation where we would worry about how an avian virus could adapt to humans.

Chris - I'm sort of reading between the lines and forgive me for almost putting words in your mouth, but are you saying then that if we keep an eye on farms and minimize the chance of the virus, establishing that sort of chain of transmission under that sort of pressure, we reduce the risk of it becoming a human infection?

Ed - Yeah, so I think farms are the biggest place where we need to look for controlling the risk. Obviously, you know, from a conservation perspective, we should be concerned about what's happening with wild bird populations as well. But looking to where it's going to spread over into humans, biosecurity on farms is a really important issue right now.