What’s killing our birds?

If you are a bird, any kind of bird, the current pandemic of avian influenza rampaging through your kind is far more terrifying than anything the hairless apes on the ground below experienced in 2020 and 2021. Britain’s seabirds – guillemots, gannets, gulls, kittiwakes and skuas – have been hardest hit because they breed in dense colonies, facilitating infection. The death toll this summer among 2,600 sandwich tern chicks on Coquet island, off the Northumberland coast, approached 100 per cent.

The worst may be only just beginning for the many thousands of geese, ducks and waders. They scatter across the Arctic tundra in summer and gather in dense flocks on estuaries and salt marshes around our shores in winter. Last winter barnacle geese died in their thousands on the Solway Firth – and many buzzards and crows perished from eating their corpses.

This vicious version of bird flu reached Europe from Asia last year, spread to North America this summer and will be carried to the Pacific coast and South America by migrating birds this winter. From pelicans to plovers, no species seems immune: the disease has already been diagnosed in 108 wild bird species in America.

What has changed to cause this unprecedented pandemic? The strains that are killing birds are collectively called H5Nx 2.3.4.4b. Piecing together the history is not easy, but it appears that the immediate ancestor H5N1 originated in poultry farms in China in 1996. Before that, episodic eruptions of avian influenza in domestic fowl were common, and the virus has always been endemic in wild birds. Indeed, it’s likely that human flu came originally from birds, perhaps via pigs.

The 1996 outbreak led to 18 human cases in Hong Kong the following year and six deaths, precipitating a series of scares about the threat to human beings and new precautions in the poultry industry. But the virus never seemed to make the jump into being an efficient infector of mammals: only people exposed to massive doses caught it, and they mostly failed to pass it on.

Ten years ago there was a fractious scientific controversy about two carefully controlled experiments in Wisconsin and Rotterdam, which altered an H5N1 virus’s genes in such a way as to give it the ability to infect ferrets by airborne transmission. This led to a temporary moratorium in America on ‘gain of function’ experiments in viruses, which in turn seems to have encouraged some US funders to pass the baton to a laboratory in Wuhan in China, which began doing similar experiments on bat coronaviruses – but that’s another story.

The new lethal strains of bird flu are occasionally infecting mammals, albeit without so far establishing chains of transmission. Two fox cubs have died in the Netherlands, and in America seals, dolphins, skunks, opossums, raccoons, bobcats and a bear have caught it, presumably after eating sick birds. The risk to people isn’t high, but nor is it zero. There was one case in a poultry worker in Spain this month and another in the US in April.

The 2.3.4.4b variants seem to have emerged in poultry facilities about three years ago, though it is not clear where. The sudden outbreak of such a virulent virus is hard to explain. Vague generalisations about climate change or human encroachment on ecosystems are unpersuasive – migrating wild birds have always experienced a vast range of climates, and human beings have been trespassing on natural habitats for centuries.

A more likely culprit is the increasingly intensive rearing of poultry, which provides a target-rich environment for ambitious but reckless strains of influenza virus. Whereas in the past, new strains began in the wild and spread to domestic flocks, the direction of travel with 2.3.4.4b appears to have been the other way, and that may explain why the virulence is so much worse. In a packed poultry facility, killing the host while releasing a massive pulse of new virus particles is quite a sensible strategy for the virus, while in more scattered and mobile wild populations a virus gets more chance to spread if it keeps the host fairly healthy. So killer strains may be more successful, at least initially.

Disturbingly, a paper published last year concluded that China’s official efforts to control the H5N1 virus may have made things worse. Mass vaccination and other controls led to a sharp fall in domestic poultry H5N1 outbreaks in 2005. This seems to have opened the door for new, rival strains: ‘We show that H5Nx viruses emerged during the successful suppression of H5N1 virus populations in poultry, providing an opportunity for antigenically distinct H5Nx viruses to propagate,’ write the authors from Duke-NUS Medical School in Singapore. The 2.3.4.4b variants are part of this family.

What I find baffling is the ability of the new strains to kill any species of bird. Few viruses are so indiscriminate, because specialising at infecting one species generally renders a pathogen less good at infecting another. After all, we don’t expect measles, smallpox or Covid to slay any species of mammal from an elephant to a shrew.

The experience has already been devastating for poultry farmers. Wardens of UK bird reserves complain of a distressing and drawn-out ordeal getting decisions from government agencies to enable them to do their utmost to shield the survivors by removing sick birds, let alone push for an action plan for future outbreaks.

Local extinctions of rare species cannot be ruled out. Most great skuas, for example, breed in Britain, mainly in the Northern Isles. Keen scavengers of sick and dead seabirds, they have readily picked up the infection and many have died. Coquet Island’s RSPB warden Paul Morrison had painstakingly built up Britain’s only colony of roseate terns from 34 pairs in 2000 to 154 in 2022. This summer 55 per cent of the adults died.

Vaccinating wild birds is impractical, so there is little conservationists can do except worry and hope that the virus becomes milder as resistance grows among the survivors.