Mass bird flu infections in wild birds pose a significant risk to vulnerable species and increase the opportunity for the virus to spill over into people.

A highly infectious and deadly strain of avian influenza virus has infected tens of millions of poultry birds across Europe, Asia, Africa and North America. But scientists are particularly concerned about the unprecedented spread in wild birds — outbreaks pose a significant risk to vulnerable species, are hard to contain and increase the opportunity for the virus to spill over into people.

Since October, the H5N1 strain has caused nearly 3,000 outbreaks in poultry in dozens of countries. More than 77 million birds have been culled to curb the spread of the virus, which almost always causes severe disease or death in chickens. Another 400,000 non-poultry birds, such as wild birds, have also died in 2,600 outbreaks — twice the number reported during the last major wave, in 2016–17.

Researchers say that the virus seems to be spreading in wild birds more easily than ever before, making outbreaks particularly hard to contain. Wild birds help to transport the virus around the world, with their migration patterns determining when and where it will spread next. Regions in Asia and Europe will probably continue to see large outbreaks, and infections could creep into currently unaffected continents such as South America and Australia.

Although people can catch the virus, infections are uncommon. Only two cases have been reported since October, one each in the United Kingdom and the United States. But scientists are concerned that the high levels of virus circulating in bird populations mean that there are more opportunities for spillover into people. Avian influenza viruses change slowly over time, but the right mutation could make them more transmissible in people and other species, says Ian Barr, deputy director of the World Health Organization (WHO)-collaborating influenza centre at the Doherty Institute in Melbourne, Australia. “These viruses are like ticking time bombs,” he says. “Occasional infections are not an issue — it’s the gradual gaining of function of these viruses” that is the real concern, he says.

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