Although the WHO stressed that the cases were isolated and had no impact on the way the A(H1N1) swine flu pandemic evolved in humans, the UN health agency said recent findings may indicate broader potential for flu viruses to mix and mutate.

Pigs have traditionally acted as virtual mixing vessels for different flu viruses, allowing them to mutate into new forms that could be passed to humans.

"These recent findings further suggest that influenza A viruses in animals and humans increasingly behave like a pool of genes circulating among multiple hosts, and that the potential exists for novel influenza viruses to be generated in animals other than swine," the WHO said in a briefing note.

"This situation reinforces the need for close monitoring and close collaboration between public health and veterinary authorities," it added.

"When influenza infections are detected in farmed animals, WHO recommends monitoring of farm workers for signs of respiratory illness, and testing for H1N1 infection should such signs appear."

Currently, countries are only required to notify avian influenza cases in birds to international authorities, although some nations have tougher domestic rules that apply to other animals like pigs.

But the WHO said that since the rulebook also required reporting of "any emerging disease" in animals to the World Animal Health Organisation (OIE), "this would include infections with the pandemic H1N1 virus or other novel influenza viruses."

The WHO emphasised that laboratory tests had not detected signs that the A(H1N1) pandemic virus had mutated to a more virulent form.

"These isolated events have had no impact on the dynamics of the pandemic, which is spreading readily via human-to-human transmission," it added.

The recently detected cases involving animals have included swine flu appearing in pigs.

"As human infections become increasingly widespread, transmission of the virus from humans to swine is likely to occur with greater frequency," the WHO noted.

Other A(H1N1) infections have been reported in turkeys in Chile and Canada and in pets such as a cat in the United States, and the WHO also highlighted the progress of highly virulent H5N1 bird flu in recent years.

Another case raised by the global health watchdog involved a "novel H3N2 influenza virus" found recently in mink on several mink farms in Denmark.

That virus was formed by a combination of human and swine genes that had not been identified previously in circulating influenza viruses, according to the WHO.

Testing of farm workers there detected no spread to humans.

"However, the incident demonstrates the constantly evolving ecology of influenza viruses, the potential for surprising changes, and the need for constant vigilance, also in animals," the Geneva-based health agency added.

An official told AFP this week that the WHO is focusing on dealing with the ongoing pandemic and is leaving it up to other researchers or insitutions to track down the source of the A(H1N1) virus first uncovered in Mexico and the United States in April.

On Wednesday, the Paris-based OIE had insisted that there was no evidence that animals had played "any particular role in the epidemiology or the spread" of the pandemic virus.

"It does not come as a surprise that notifications of infection in new animals species are received; on the contrary it demonstrates animal disease surveillance is efficient and functioning to the benefit of all," OIE Director General Bernard Vallat said in a statement.

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