A cow is milked at a farm in Germany.The World Health Organisation has called for close monitoring of farm workers and animals for influenza A viruses, following recent cases in a wider range of creatures than pigs.(AFP/File/John Macdougall)

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, it said recent findings may indicate greater 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."

The UN health agency emphasised that extensive tests by laboratories 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, with "limited evidence" indicating that they were directly infected by humans.

"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 been reported in turkeys in Chile and Canada and in pets in the United States, and the WHO also highlighted the progress of highly virulent H5N1 bird flu in recent years.