With at least 280 million birds dead since October 2021, the highly infectious H5N1 strain of avian flu has devastated poultry and caused the biggest sudden drop of the world’s wild bird population in decades. The millions of wild birds killed includes tens of thousands of endangered and endemic species – and tens of thousands of mammals have died too. Today, new data, published in Nature Communications, documents the disease’s spread to the southernmost tip of the planet – the Antarctic region – where it has inflicted significant die-offs in elephant seals and fur seals. This outbreak has affected every continent except Oceania, and yet there has been little coverage of the impact on global biodiversity and farming systems – or of potential risks to human health. “I’m not sure the public is aware of the significance of the avian influenza,” said lead author Prof Ashley Banyard, a virologist at the UK’s Animal and Plant Health Agency (APHA) lab in Weybridge, Surrey. “It’s not been front-page news.” Experts are racing to find out more about the disease and understand where it might go next. “We can’t rest on our laurels … we have to make sure we are continually monitoring and tracking the threat from avian influenza viruses,” said Banyard. “It’s almost impossible for me to predict what’s going to flare up where.” Dr Connor Bamford, an assistant professor in virology from Queen’s University Belfast, said there could be further surprises. “One major open space for the virus remains the human population.” How did it start? The bird flu variant that caused these deaths is more than a quarter of a century old. Highly pathogenic H5N1 was first reported in China in 1996, when it was found at a goose farm. The origins of all highly pathogenic avian influenzas – the ones that cause severe disease and death – have been tracked back to poultry farms. “The history of [bird flu strain] 2.3.4.4b goes back before we noticed these mass die-offs of wild birds,” said Bamford. “But something seemed to happen in 2021.” That is when the strain became the dominant bird flu virus globally, and is now highly pathogenic and highly contagious. What does an outbreak look like? The ornithologist Peter Stronach was among the first people in the UK to witness the unfolding disaster in the breeding bird population. On 11 May 2022, he was walking along thebeach at Littleferry in Scotland, a walk he had done often, to watch birds flying overhead. But that day was different: “There were just corpses of birds scattered along the high tide line,” he said. In total, Stronach counted 72 dead birds, including puffin, guillemot and great skua. The virus had killed birds from 17 different species, the first time several species had been killed in summertime (rather than overwintering species). “That’s really unusual. It signalled something was wrong,” he said. As the season wore on “everything escalated”, said Stronach. Fishers were reporting carcasses of birds far out at sea. “Doing some primitive maths we were able to work out huge numbers were involved and no one was monitoring it.” In the coming months, tens of thousands of seabirds would die. The virus survived the summer in Europe and quickly went global, spreading along bird migration routes. What has the toll been? Bird flu has caused the most significant and sudden loss of birds in decades, with some of the long-lived seabird species likely to take several years to recover. It has led to the death and mass slaughter of more than 280 million poultry birds since October 2021, based on data from the World Animal Health Information System. There is no total figure for wild bird deaths, though it is estimated to be in the millions. It has been particularly destructive in species of birds with a small geographic spread. The first big outbreak in the winter of 2021 was among the Svalbard barnacle geese in the Solway Firth where at least 13,000 birds died: a third of the global population of that species. In March 2022, more than 2,200 dalmatian pelicans died in Greece, or 40% of the south-east European population. The UK, which is home to most nesting great skua pairs, has lost three-quarters of those on surveyed sites since bird flu struck, according to a study published in February. Across Europe, more than 20,000 sandwich terns died during the 2022 breeding season– or 17% of the population in north-west Europe – in an outbreak that killed almost all chicks within colonies. In total it has killed birds from 320 species, and mammals from dozens of species. How does it spread? One bird can infect as many as 100, with the virus present in faeces, mucus, blood and saliva. A teaspoon of faeces is enough to kill an entire house of chickens or turkeys, experts say, with mortality rates of up to 100%. Where has it been detected? Australia and New Zealand are the only places still free of outbreaks among wild birds. In November 2021, the first case in North America was recorded in Newfoundland, Canada, probably carried by a bird migrating from northwest Europe along the East Atlantic flyway. By mid-2022, more than 230 outbreaks had been recorded in wild animals, and it had spread throughout Canada and the US. In October 2022 it arrived in South America. Data suggests there were three separate introductions. It spread the length of the continent (6,000km) in less than six months, arriving in Tierra del Fuego in April 2023. In total more than 500,000 wild birds have died in South America. In October 2023, bird flu arrived on the sub-Antarctic island of South Georgia. In total it has killed animals from about 10 species in the Antarctic region (most of them on South Georgia), including fur seals, elephant seals and gentoo penguins. Researchers warned of large geographic “black boxes” where it is not known whether there is bird flu in wild populations, particularly in parts of Africa and Asia. “It’s pretty unclear what’s happened in Asia and Africa. If you do look, it is there, but it has not been well tracked,” said Bamford. There are endemic strains in Bangladesh and Cambodia. Japan has recorded recent cases in large-billed crows; South Korea has reported it in whooper swans. Studies from west and south Africa confirm cases there: since January 2022, Burkina Faso has recorded 133 outbreaks, including three in wild birds. In early 2023, thousands of seabird deaths were recorded in Senegal, Gambia and Guinea-Bissau, including caspian terns, royal terns and grey-headed gulls. What about other animals? In South America, bird flu caused mass die-offs of mammals. More than 20,000 South American sea lions died in Chile and Peru, which is 9% of the regional population. An estimated 17,000 southern elephant seal pups died in Argentina – equivalent to 96% of all pups born in the country in 2023. This amounts to the loss of nearly an entire generation of seals, which take at least three years to reach maturity. In March this year, bird flu started spreading among US cattle. It is the first time a bird flu virus has been found in cows, with more than 100 herds affected and probably more that have not yet been reported. Poultry operations near farms have also reported cases, suggesting the virus may be spilling back into birds. Barn cats and mice have also tested positive for H5N1 so they could spread the virus out of farms into the wider environment. What is the risk to humans? As the virus evolves, it could become more of a threat to people, according to a recent report by the US Centers for Disease Control and Prevention (CDC). There have been nine cases of H5N1 in humans in the US (although symptoms have been relatively mild) and officials said they expected to see more. “The more infections there are among cows, the more risk there is for infections to occur among humans,” said Dr Nirav Shah, principal deputy director of the CDC. The UN health agency’s chief scientist has also said the risk of bird flu spreading to humans is an “enormous concern”. So far, there is no evidence that H5N1 is spreading between humans. But in the hundreds of cases where humans have been infected through contact with animals over the past 20 years the mortality rate is above 50%. What’s next? In October 2023, bird flu arrived in the Antarctic region. Researchers on the ground have counted hundreds of carcasses of dead seals in South Georgia. But it is a big area with few eyes on the ground. “It could be that the number we detect is only a fraction of the number that have actually died,” said Norman Ratcliffe, a bird ecologist at the British Antarctic Survey (BAS), who has worked on penguins and seabirds on South Georgia. Although it has killed gentoo and king penguins, losses among penguins have been relatively small. “There is cause for optimism. It’s not as bad as we thought it could be when we saw the first cases,” said Ratcliffe. Generally, viruses evolve in a way that means they do not kill their hosts – they tend to become less dangerous and more transmissible over time. The virus has also travelled around the whole world, so it has little new territory to cover – except for Oceania. “Predicting the future of highly pathogenic avian influenza virus is one of the biggest challenges facing the field,” according to Bamford. “It is possible this season could be less affected due to a build up of immunity in wild birds who were infected and survived in the preceding seasons. However, we continue to detect the virus in European birds sporadically so we should expect to see further outbreaks.”