BIRD flu has been detected at "very high concentrations" in milk, health officials have revealed.
The World Health Organisation (WHO) said on Friday that bird flu, also known as H5N1, has been found in raw cow milk in the US.
Pasteurised milk found in supermarkets - which has been processed to kill harmful bacteria - is still safe to drink, health officials have said.
H5N1 first emerged in 1996, but since 2020, the number of bird outbreaks has grown exponentially, leading to the deaths of millions of wild birds and poultry across the world.
It has also started to jump to other mammals, including humans, cats, bears, foxes, mink and penguins.
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Cows were added to that ever-growing list this earlier month,
The US reported H5N1 infections in dairy herds and since then, cases have been confirmed or are under investigation in eight states - Texas, Kansas, Michigan, New Mexico, Idaho, Ohio, North Carolina and South Dakota.
US authorities earlier this month said a person working on a dairy farm in Texas was recovering from bird flu after being exposed to cattle.
Dr Wenqing Zhang, head of the WHO's global influenza program, said: "The case in Texas is the first case of a human infected by avian influenza by a cow.
"Bird-to-cow, cow-to-cow and cow-to-bird transmission have also been registered during these current outbreaks, which suggest that the virus may have found other routes of transition than we previously understood.
"Now we see multiple herds of cows affected in an increasing number of US states, which shows a further step of the virus spillover to mammals.
Dr Zhang said there was a "very high virus concentration in raw milk" from infected cows, but experts were still investigating exactly how long the virus can survive in milk.
The Texas Health Department has said the cattle infections do not concern the commercial milk supply, as dairies are required to destroy milk from sick cows.
Pasteurisation also kills the virus.
"It is important for people to ensure safe food practices, including consuming only pasteurized milk and milk products," Dr Zhang said.
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It comes after a 21-year-old in Vietnam died of the highly pathogenic avian influenza strain H5N1 after going wild bird trapping.
However, investigations found no evidence of sick or dead birds in the area he visited.
At least 887 people have been infected with bird flu in the past 20 years - and 462 of these died.
Five of these cases were reported in the UK.
In government guidance issued last month, Brits were advised to stay at least two meters away from wild birds because of bird flu fears.
People should avoid all contact with wild birds, which includes pigeons, swans, and seagulls, UK Health Security Agency (UKHSA) officials said.
So far, there is no evidence that H5N1 is spreading between humans.
However, experts fear the sheer scale of the current spread could give the virus more opportunities to evolve, enabling H5N1 to better spread between mammals - and potentially humans.
As for potential vaccines, if required, Dr Zhang said there were some in the pipeline.
"Having candidate vaccine viruses ready allows us to be prepared to quickly produce vaccines for humans if this becomes necessary," she said.
"For this particular H5N1 virus detected in dairy cows, there are a couple of candidate vaccine viruses available."
She said that in the case of a pandemic, close to 20 influenza vaccines are licensed for pandemic use, and they could be tailored to the specific virus strain in circulation.