Bird Flu: How afraid should we be of the next human pandemic?

When a single dead swan was found washed up in the harbour at Cellardyke, Fife in 2006, it made headlines across the world. Britain had been on high alert amid concern the highly pathogenic H5N1 strain of bird flu would jump species and cause a human flu pandemic which scientists had long thought overdue.

While ministers sought to publicly allay fears, behind the scenes contingencies – such as the widespread closure of schools – were being drawn up by the then Scottish Executive under the leadership of First Minister Jack McConnell.

It would be fourteen years before a pandemic did arrive, although it would be caused by a novel coronavirus, not flu. Yet while the world’s attention has been focused on Covid-19, a much-changed version of H5N1 has been decimating wild bird populations both in the UK and overseas.

No longer confined to a single sickly swan, the virus has killed tens of thousands of birds across the Scotland, dramatically thinning out the famous gannet colony on Bass Rock in the Firth of Forth and leading to serious concerns about the extinction of the great skua, a bird which migrates to islands including the St Kilda archipelago during the summer.

While none of this has so far caused undue alarm for those concerned with human health, there are now some indications of growing anxiety.

Last month the World Health Organization (WHO), which many accused of being slow to act on Covid, warned that while the risk to humans from bird flu is currently low, it may not necessarily stay that way. Director general Dr Tedros Adhanom Ghebreyesus said that the recent reports of infections in mink, otters and sea lions “need to be monitored closely”.

And Professor Ian Brown, director of scientific services at the Animal and Plant Health Agency, said work on creating a vaccine for humans should start. “This virus is absolutely on the march,” he told the BBC. 

There are good reasons to be vigilant. While the fatality rate from Covid is around one per cent, more than 50 per cent of the nearly 900 people who have become infected with H5N1 since 1997 later died, although the virus currently circulating in birds has changed significantly since it was first detected. And while there’s no indication at this stage that the virus will become a human one or that it would be particularly deadly if it did, that overdue flu pandemic – the one scientists have long feared – is still to materialise.

While avian flu is an annual threat, the number of cases recorded in the UK and the rest of Europe last year were unprecedented. According to the European Centre for Disease Prevention and Control, there were 2,467 outbreaks recorded in poultry between October 2021 and September 2022, leading to the cull of 48 million birds. While deaths in wild birds are harder to count, tens of thousands have perished in the UK alone. 

“Even though this virus is called H5N1, it’s a very different virus to the one that was around in the late 90s,” says Sheila Voas, the Scottish Government’s chief vet. “The reason this one is causing problems is because it has changed significantly enough that it’s infectious and there’s probably very little natural immunity out there to it. It’s a strain that seems to need quite a small amount of virus to cause serious clinical signs, infection and death.” 

Rural affairs minister Ross Finnie visits Cellardyke after the discovery of a dead swan in 2006 | Credit: Alamy

Last month, it was confirmed that four dead seals had tested positive for the virus after their carcasses were sent for screening last year. The animals were found in Aberdeenshire, the Highlands, Fife and Orkney. The virus has also been found in other mammals in the UK, including otters and foxes, raising concerns that it could mutate and become a risk to humans. But the numbers remain low; just nine dead mammals in Scotland and 14 in the whole of Great Britain have tested positive for H5N1 and all would have fed on dead birds.

Voas says she’s “not particularly worried” about the prospect of the virus becoming a risk to humans, given there’s no evidence of it spreading between mammals yet. 

“Flu viruses change and adapt the whole time. But this particular version of H5N1 is very adapted to birds and has very few mammalian adaptations. In order to get into the mammal population and to spread between mammals, you would need to see a whole number of changes to the virus… We’re monitoring for those changes but as yet, nothing much of significance has appeared.”

While there’s no evidence of avian influenza spreading between mammals in the UK, outbreaks elsewhere have given scientists cause for alarm. More than 50,000 minks had to be culled after H5N1 was detected at a farm in Spain last autumn. Genetic sequencing showed the animals had been infected with a new variant of the virus which includes genetic material from a strain found in gulls. The outbreak, in which a number of animals became sick and died before the cull took place, was seen as the strongest evidence yet that the virus can pass between mammals. 

A mass outbreak of bird flu among sea lions in Peru has already killed more than 600 animals since the start of the year, raising the possibility that the virus has learned how to pass from mammal to mammal. However, the death could still be explained by the animals having preyed on dead birds.

“The influenza virus has often gone back and forth between animals and humans – it’s nothing new,” says David Heymann, an epidemiologist at the London School of Hygiene and Tropical Medicine. “The 1917 pandemic was thought to have arisen from pigs that became infected and then the virus mutated over a period of time and spun off into a pandemic.”

Heymann says the next human pandemic needs a “Swiss cheese event”, a confluence of factors where all the holes align, allowing the worst-case scenario to come to pass. While we’d have to be particularly unlucky, there’s no room for complacency. 

“We’d have to be unlucky, but these events do take place,” Heymann says. “That’s why I don’t understand why the WHO says there’s a low risk of a pandemic from H5N1 because nobody knows – nobody can predict what’s going to happen.”

When an 11-year-old girl died from H5N1 in Cambodia at the end of last month, it gave an indication of just how jumpy the global scientific community is about the prospect of human-to-human transmission. The father of the girl, who had fallen ill with a cough and a fever, also tested positive for the virus, although he was asymptomatic.

While all the evidence is that H5N1 cannot easily spread between humans, the UK Health Security Agency (UKHSA) has begun modelling to establish how many people may become very sick if it does and whether lateral flow tests and blood tests would be helpful in tracking the spread. 

Much of the surveillance is being carried out by the Human Animal Infections and Risk Surveillance (HAIRS) group, which includes representatives from the Scottish Government and Public Health Scotland. Last year, it concluded that the risk to the human population from an avian influenza outbreak in grey seals was very low. 

Voas says that while there have been cases of the virus appearing in mammals, there’s no evidence of a “step change” in how H5N1 is behaving. 

“In order to get a strain that moves from person to person, there needs to be a whole number of changes to the genome and we are monitoring for them,” she says. “Even if it got into human populations and was able to spread, there’s nothing to say that the disease it causes would be severe. Some avian influenza viruses which have got into humans have caused nothing more than a mild conjunctivitis.”

When it comes to Scotland’s wild bird populations, however, there is plenty of evidence of the destructive power of H5N1. Among the birds which are particularly badly affected are gannets, great skuas and roseate terns, a species which lost around a quarter of its population in the UK last summer. Large numbers of deaths have also been reported in hen harriers and white-tailed eagles, while mute swans have been found dead across the UK, including recently in Holyrood park, just a short walk from the Scottish Parliament. 

“For things like gannets, the UK has around 55 per cent of the world’s population and most of them are in Scotland,” says Claire Smith, the RSPB’s senior policy officer on avian influenza.

“They are a really slow breeding bird…it could be really hard for them to recover – they’re not a species that’s going to bounce back. And great skuas, we possibly lost around five to ten per cent of that population last year. 

“A lot of the wild bird monitoring is really set up to protect poultry farming. It’s meant to be an early warning system. We have had lulls in the number of birds dying, but this winter it has gone up again.”

The depleted gannet colony on Bass Rock in the Firth of Forth | Credit: Alamy

Voas says that birds should eventually develop a level of immunity to the virus, although populations are likely to be badly affected before that point is reached.

 “It will be devastating, it might be 30 per cent, 50 per cent or 70 per cent but I think it’s unlikely that any species will be wiped out unless things change dramatically,” she says.

“We need to keep monitoring it and see what’s happening. But because the virus changes so regularly, the chances are it will evolve to be a less severe version in birds because for the virus itself, it’s not a good evolutionary strategy to kill its host. It wants a living host that can replicate the virus so that the virus can spread. A dead animal doesn’t spread virus.”

For Heymann, whose focus is on the human population, extreme vigilance is key, although he says it’s impossible to know, should H5N1 mutate to allow human-to-human transmission, just how deadly the virus would be. 

“Right now, it’s extremely virulent in humans but it doesn’t transmit easily and there have been very few humans that have died from H5N1,” he says. 

“There likely will be another flu pandemic. Hopefully our political leaders are really alert to this now and will be doing something. 

“The UK risk register has influenza at the very top along with antimicrobial resistance. Hopefully political leaders are paying attention to that and making sure pandemic planning is in place.”