The H5N1 strain of bird flu has the "potential to become a pandemic", according to an expert at the UK Health Security Agency.
Richard Pebody, director of epidemic and emerging infections at UKHSA, explained that while the virus is "not new", and the risk to the public remains low, its features have changed in recent years.
It follows a confirmed case of bird flu in a single sheep in England for the first time on Monday.
At the agency's annual conference in Manchester on Tuesday, Mr Pebody described that news as "a new twist to the story" but stressed the risk "hasn't necessarily changed".
"H5N1 has the potential to become a pandemic, a future pandemic. And so that's why we're watching that so closely and managing it so closely.
"We know that the virus can infect other animal species. We need to clearly look into this a little bit more, understand better what's happened here and understand what the risk is to human health," he said.
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The infection was detected in a single sheep in Yorkshire after routine surveillance of livestock where avian influenza H5N1 had been confirmed in captive birds.
At the time, Dr Meera Chand, emerging infection lead at the UKHSA, said that "current evidence suggests that the avian influenza viruses we're seeing circulating around the world do not spread easily to people" and "the risk of avian flu to the general public remains very low".
The virus was also detected in dairy cattle in a number of US states last spring and Sky News revealed in December that horses can harbour the virus too.
Human case in West Midlands
Mr Pebody said: "H5N1 is not new. It's been around for quite a long while. It first popped up in the back in the 1990s, mid-1990s, but it's waxed and waned over the years. Over the past three or four years, its features have changed".
Bird flu in people is rare, although in January, a person in the West Midlands region tested positive after being infected while on a farm after "close and prolonged contact" with infected birds.
What are the symptoms?
Symptoms can vary depending on the strain but can include a flu-like illness with fever, body aches, cough, sore throat, sore eyes and runny nose.
Read more from Sky News:
What you should know about bird flu
Recovery from bird flu 'painfully slow'
Bird flu infects horses without symptoms
On Tuesday, the UKHSA published its first annual report detailing the infectious diseases impacting England, which included influenza A (H5N1) as an "emerging infection", along with mpox and swine influenza.
It said its bird flu surveillance programmes, which involve taking samples from people exposed to infected birds, detected a "small number" of workers who were asymptomatic [infected but not showing symptoms], with "one clinical infection in January 2025".
"There has yet to be demonstrated human-to-human transmission of avian flu despite extensive recent surveillance," the report added.
A list of "priority pathogens" published by UKHSA also includes the viral family orthomyxoviridae, which influenza A viruses belong to.
Be the first to get Breaking News Install the Sky News app for freeIn December, the government agreed a deal to buy more than five million doses of a vaccine for a strain of bird flu to bolster pandemic preparedness.
Mr Pebody said: "We do need to be prepared for the next emerging infection and indeed, the next pandemic. So that's very much the core business of the agency.!
