GENEVA, Switzerland (AFP)– 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 survives in milk is unknown.

Avian influenza A(H5N1) first emerged in 1996 but since 2020, an exponential growth has been recorded in the number of outbreaks in birds, while an increasing number of mammals have been affected.

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

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 programme 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. Pasteurisation also kills the virus.

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