The World Health Organization (WHO) called Wednesday for a strengthening of global detection networks for the H5N1 bird flu virus which infected a large number of animal species.

Earlier this month, the H5N1 virus was detected in very high concentrations in raw, or unpasteurised, milk from infected cows -- a surprising development for experts because they were not thought to be susceptible to this type of influenza.

While the surveillance of birds and poultry is already very developed, "what we really need globally is strong surveillance in different animal species," Maria Van Kerkhove, who leads pandemic prevention and preparedness at the WHO, said during a press conference in Geneva.

The increased surveillance must be extended to milk and dairy products to ensure "that people are protected", Van Kerkhove stressed.

She noted that pasteurisation, which involves heating milk to kill microbes, is recommended and safe.

"I don't think detection in dairy cattle fundamentally changes our risk assessment. But this is concerning," Van Kerkhove added.

There is no evidence of human-to-human transmission at present but health officials fear that if the virus spreads widely it could mutate into a form that could pass between humans.

"Every opportunity this virus has to continue to circulate, to continue to mix with animal species... it has the potential to cause an epidemic and outbreak and potentially become a virus that has that pandemic potential," she said.

From 2003 to April 1 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.

But since 2021, there have only been 28 cases of human infection, Van Kerkhove said.

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.

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.

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 exposed to wild birds.