The H5N1 bird flu virus strain has been detected in very high concentrations in raw milk from infected animals, the WHO said Friday, though how long the virus can survive in milk is unknown.

Avian influenza A(H5N1) first emerged in 1996 but since 2020, the number of outbreaks in birds has grown exponentially, alongside an increase in the number of infected mammals.

The strain has led to the deaths of tens of millions of poultry, with wild birds and land and marine mammals also infected.

Cows and goats joined the list last month — a surprising development for experts because they were not thought to be susceptible to this type of influenza.

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.

"The case in Texas is the first case of a human infected by avian influenza by a cow," said Wenqing Zhang, head of the global influenza program at the World Health Organization.

"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," she told a media briefing in Geneva.

It was only the second case of a human testing positive for bird flu in the United States, and came after the virus sickened herds that were apparently exposed to wild birds.

"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," Zhang said.

"The virus has also been detected in milk from infected animals."

Zhang said there was a "very high virus concentration in raw milk", but experts were still investigating exactly how long the virus is able to survive in milk.

The Texas health department has said the cattle infections do not present a concern for the commercial milk supply, as dairies are required to destroy milk from sick cows. Pasteurization also kills the virus.

"It is important for people to ensure safe food practices, including consuming only pasteurised milk and milk products," said Zhang.

Recent human cases mild

From 2003 to 1 April this year, the WHO said it had recorded 463 deaths from 889 human cases across 23 countries, putting the case fatality rate at 52 percent.

Zhang noted that the human cases recorded in Europe and the United States in the past few years — since the virus surged — have been mild cases.

So far, there is no evidence that A(H5N1) is spreading between humans.

And Zhang stressed that the A(H5N1) viruses identified in cows and in the human case in Texas showed no increased adaptation to mammals.

As for potential vaccines, if required, 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."

In the case of a pandemic, there are close to 20 influenza vaccines licensed for pandemic use and they could be tailored with the specific virus strain in circulation, she said.