Death on the wing

The growing threat of bird flu (9 minute read)

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Summer has well and truly departed from Wellington, New Zealand. Daylight saving has ended and the nights are closing in. I’m wearing winter shoes and pulling out my polar fleece. I’m making soups instead of salads.

By this time, the ruddy turnstone, the bird whose name I co-opted for my newsletter, has flown from our shores and is probably halfway to the Arctic. These determined little birds, named for their habit of turning over stones while seeking their food, breed in the Arctic summer then travel south to avoid the Arctic winter. The global population is estimated to be a few hundred thousand, of which a few thousand come to New Zealand.

The turnstone’s epic journey puts it in the path of many dangers, but there’s one we don’t often think of. Migratory birds, such as the turnstone, are involved in the spread of avian influenza, or bird flu. I’ve been hearing occasional, worrying reports about bird flu over the last decade or so, but I recently learned from news items about bird flu killing sea lions in Peru. This was something I hadn’t heard of before, and it made me want to take a closer look at the disease. Was it becoming more of a threat to mammals? What did that mean for humans? And, I admit, I wondered whether it was a threat to turnstones.

We often talk about influenza as a single disease, but there are actually four different types of influenza virus, usually known as A, B, C and D. Although closely related, they are quite different in terms of impact. Influenza D is primarily a disease of cattle, and is not known to cause human disease. Influenza C infects people, but only causes mild illness. Influenza B also infects people and contributes to “seasonal flu” outbreaks during winter.

Then there’s influenza A. Although classed as a single species, it’s extremely diverse. It is primarily a disease of aquatic birds, including turnstones, but it is also known to infect humans, cats, chickens, whales, pigs, horses, bats and many other species. But it doesn’t mean that the same type of influenza A infects all those different species. Influenza A has many subtypes, each of them subtly different in the animals they infect and how sick they make their host.

A seagull I saw at Chesapeake Bay many years ago. If anyone happens to know the species, I’d love to know (image credit: Melanie Newfield)

It is influenza A which is responsible for flu pandemics in humans, most notably the 1918 pandemic, which was more terrible than I can possibly imagine. Although estimates vary widely, the USA Centres for Disease Control estimates that 500 million were infected and 50 million died. There have been other human pandemics since, but none has come close to 1918.

Why does influenza A cause these periodic pandemics? Part of the reason is because it mutates, that is, errors are made when its genetic code is being copied. All viruses mutate and form new variants, as we’ve seen with Covid-19, but influenza A has a particularly high rate of mutation. But it isn’t mutation alone which makes influenza A change so much. The other reason is a process called reassortment.

We’re used to the idea that children have genes which are a mix of genes from both parents. But the same thing can happen with viruses like influenza A. When two different subtypes of influenza A infect the same host, they can swap fragments of their genetic code, resulting in new subtypes of the virus.

In wild birds, influenza A mostly causes few signs of disease. Apart from one outbreak, which killed thousands of terns in South Africa in the early 1960s, it wasn’t considered to be a problem for wild birds until very recently. But chickens were another matter. The first records of bird flu killing chickens date back to the 1870s, when it was known as “fowl plague”.

Subtypes of bird flu which killed chickens earned the name “highly pathogenic” avian influenza. For most of last century, outbreaks were sporadic and usually confined to one or a few farms and small numbers of birds. But, during the late 1970s and 1980s, the trend to larger and larger poultry farms meant that an outbreak affecting a single farm could infect hundreds of thousands of birds.

By the mid-1990s, outbreaks were becoming more frequent and larger. In Pakistan in 1995, more than three million chickens died in one outbreak. Then came the outbreak in Hong Kong in 1997. First, it was confined to poultry, but then people began catching it from infected birds. In November and December 1997, eighteen people were infected and six died. The Hong Kong government acted fast, killing all poultry on the island in the space of a few months. But the virus came back, in 2001 and again in 2002, the same subtype as the 1997 outbreak. Then, in late 2003, it was reported in eight countries in south-east Asia, including China and Indonesia. This time, there would be no stopping it.

Although this subtype of influenza could spread to humans if they had contact with infected birds, and was often lethal if it did so, it was mainly a threat to other bird species. It didn’t just affect domestic poultry either. It spread among wild birds, infecting many different species and sometimes causing significant numbers of deaths. Through a combination of poultry being moved and migratory birds, the virus flew from south-east Asia, through central Asia and on to Europe and Africa. In 2015 it was detected in North America, and it continued its habit of killing wild birds as well as domestic poultry.

Among humans, bird flu remained a rare but lethal disease. But other mammals, too, were affected – mainly carnivores which fed on dead birds. Then, the picture began to get worse. Up until 2020, lethal outbreaks in wild bird had been sporadic. But then, while we had our attention on Covid-19, increasingly large numbers of birds began dying in Europe and North America, spread by migratory birds. As well as killing larger numbers of wild birds than usual, the disease spread further, moving into South America by the end of 2022.

Last month, as well as killing tens of thousands of seabirds, the virus was reported killing marine mammals in Peru, particularly sea lions. Sea lions do eat birds, so they could have caught the virus from infected birds. However, the large numbers found dead or dying suggested the possibility that the virus was not just moving from infected birds to mammals, but from mammal to mammal. There was also an outbreak among mink in Spain, where the specific subtype found had mutations which made it more likely to transmit from mammal to mammal.

More gulls at Chesapeake Bay (image credit: Melanie Newfield)

At this point, I have so many questions I’m not sure where to go next. I want to know which bird species are affected. I want to know whether New Zealand’s birds are at risk. I want to know what this means for humans. But, most of all, I want to know why this is happening. Why is bird flu an increasing problem, especially for wild aquatic birds, such as seabirds and waterfowl, the natural hosts of the virus?

Unfortunately, the answer to the question of why bird flu is an increasing problem seems to be “scientists don’t know”. Most answers are suggestions that relate to changes in the virus, such as increased transmission or an ability to survive for longer in the environment. But that doesn’t answer the question of why the virus has changed in this way, to be more harmful to its hosts.

At this stage, the picture is far from clear. There are suggestions that climate change may affect influenza by changing migratory patterns in birds. But I haven’t yet found anything saying the outbreak over the last two years is linked to climate change. Another possibility is our intensive agriculture, specifically the way that we rear poultry. Huge flocks of chickens which are genetically very similar to each other are kept under stressful conditions. This combination may be contributing to the development of more virulent bird flu, which may then spread to wild birds. It may be a numbers game too – more people eating more meat means more poultry to come into contact with wild birds, giving the virus more chances to mingle and swap genes.

So far, bird flu has been largely unsuccessful as a human disease. People do become infected from contact with birds, and can become very sick or die as a result, but it doesn’t happen often. The virus would have to change a lot in order to spread from person to person. The problem is, though, that there is a lot of the virus about. As we have seen with Covid-19, the more prevalent a virus, the more opportunity it has to form new variants.

Whether or not it evolves to cause a human pandemic, this virus is a real threat to both domestic poultry and wild birds. Dozens of wild bird species have been infected with highly pathogenic forms of bird flu. Although not all have suffered large numbers of deaths (so far there are only a couple of records of the turnstone being affected, for example), reported figures are likely to be an underestimate. It’s not clear yet whether the disease is a threat to the survival of species, but experts are worried about rare species and those which are restricted to small areas.

New Zealand, though, has been spared the ravages of the disease, so far. Although mild forms of bird flu have been reported here, the highly pathogenic forms have never made it this far. One reason is that waterfowl, such as ducks and geese, are among the most important migratory species involved in the spread of the virus, but they don’t migrate here. Nonetheless, experts aren’t ruling out the possibility of other migratory species bringing the virus here.

I wish that I could say something more hopeful about bird flu – something positive about what’s being done, or what we could do. But I didn’t find any good news here, I’m sorry. Humans might be mostly safe for now, but millions of beautiful birds are under threat, and there seems little that we can do apart from watch and hope that the virus abates soon.

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