A man in Chile is infected with bird flu that has “concerning” mutations, US health officials have said.
The mutations may have occurred after the 53-year-old, who is in hospital, became sick.
But there is no evidence the mutated virus has spread to other people, mixed with other flu viruses, or developed resistance to current medicines or vaccines.
The threat to people from the virus is low, Vivien Dugan from the Centres for Disease Control and Prevention said.
“Nevertheless, it’s important to continue to look carefully at every instance of human infection,” she added.
“We need to remain vigilant for changes that would make these viruses more dangerous to people.”
Past animal studies suggest mutations could cause the bird flu virus to be more harmful or spread more easily.
But there is no evidence these mutations would make it easier for the virus to take root in a person’s upper lungs – a development that would cause concern about spread among people.
Similar mutations have been found in past bird flu infections.
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What is bird flu?
Bird flu, called Type A H5N1, was first identified during a 1997 outbreak in Hong Kong when visitors to live poultry markets caught it.
More than 450 people have died from bird flu in the past two decades, according to the World Health Organization.
The vast majority of people infected got it directly from birds.
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It comes as UK authorities announced bird flu protection measures will be lifted next week.
The rules, which were introduced on 7 November last year, made it a legal requirement to keep animals inside and follow stringent biosecurity measures to protect flocks.
From 18 April, bird keepers will be allowed to keep their flocks outside again, and eggs laid by poultry that is kept outdoors can now be labelled as “free range”.
However, bird keepers have been urged to remain vigilant to prevent further outbreaks.
Meanwhile, a 56-year-old woman in China died in recent days from a type of bird flu that is rare in humans.
She was the third person known to have been infected with the H3N8 subtype of bird flu.