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

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

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

Thomas Vahlenkamp and colleagues from the Friedrich Loeffler Institute, Germany's national research center for animal health, experimentally polluted 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 common Virology.

"With the rising 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 taking part of pigs in the epidemiology of this pandemic."

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

The World Health Organization confirmed 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 internationally and likely now infects millions.