There is a growing risk that pigs will catch the new H1N1 flu strain -- commonly known as swine flu -- from humans, German researchers said.

Widespread transmission from people to pigs could mix up virus strains further, leading to unpredictable changes in the disease.

There have already been a handful of suspected cases of humans passing the current pandemic H1N1 virus to swine. The latest German research confirms it is infectious to pigs and can spread rapidly.

Thomas Vahlenkamp and colleagues from the Friedrich Loeffler Institute, Germany's national research centre for animal health, experimentally infected five pigs with the new flu.

Four days later, the virus had spread to three uninfected pigs housed with them and all the pigs showed clinical signs of disease, they reported in the Journal of General Virology.

"With the increasing numbers of human infections, a spill-over of this human virus to pigs is becoming more likely," Vahlenkamp said.

"The prevention of human-to-pig transmissions should have a high priority in order to avoid involvement of pigs in the epidemiology of this pandemic."

Encouragingly, though, while the virus spread quickly among the pigs, it did not spread to five chickens housed with them.

The World Health Organisation declared a pandemic last month following the spread of the new flu virus, which mixes swine, avian and human elements. It has killed more than 400 people globally and likely now infects millions.