WASHINGTON -- U.S. pork groups have issued specific precautions about swine flu after an unusual new strain of influenza killed at least 81 people in Mexico and popped up in the United States and, possibly, Britain and New Zealand.

Although it is called "swine flu" there is no evidence that any of the cases stemmed from contact with pigs, said Liz Wagstrom, a veterinarian who works on public health issues for the U.S. National Pork Board.

"As far as we know, it's never been seen in pigs worldwide," Wagstrom said in an interview.

But because people and pigs can pass flu viruses to each other, the pork board and the National Pork Producers Council said pigs and barn workers should be vaccinated for seasonal flus, and sick employees should stay away from barns.

Farmers should also restrict visitors from barns, especially people who have recently been in Mexico, Wagstrom said, and maintain "biosecurity" measures like keeping birds out of barns and ensuring water is treated.

Swine flu, first found in 1930, causes fever and coughing in pigs, but is not usually severe enough to kill them. The virus is common in U.S. barns, so farmers routinely vaccinate herds.

In the United States, there have been 12 cases since 2005 in which humans caught swine flu after being in contact with pigs, Wagstrom said. "It's very uncommon to see it spread from pigs to people," she said.

But as concerns have grown about flu pandemics, and U.S. officials increased the number of labs able to test and type flu viruses, officials have also begun to pay closer attention to swine flu, Wagstrom said.

PIGS A 'MIXING VESSEL' FOR VIRUSES

Pigs can catch human and avian flus, so are thought to be a so-called mixing vessel for the viruses - a host in which viruses can swap genes, creating new strains.

Last year, the U.S. Agriculture Department and the U.S. Centers for Disease Control and Prevention began work on a formal surveillance project for swine flu - a project that began collecting samples last week, Wagstrom said.

"The timing was pretty exquisite," Wagstrom said.

The project will work with labs to keep an eye out for any unusual strains of the virus, and help trace any cases of swine flu in humans who have contact with hogs, she said.

Since the mid-1990s, there have been more incidents of swine flu viruses mixing with other types, but researchers are not sure why, said Christopher Olsen, a veterinary expert.

"Modern animal production facilities are actually much more biosecure than in, quote unquote, the old days," said Olsen, associate dean at the University of Wisconsin's school of veterinary medicine.

"If anything, you would expect less potential for mixing of viruses across species than you would have in the days when pigs and ducks were raised outside," he said in a telephone interview.

The virus behind the outbreak contains elements of European and Asian viruses, which is especially puzzling, he said.

At some point in its evolution, the new virus probably came from a pig, but it would be "a stretch to say there was a direct transmission from pigs to people," Olsen said.

"You'd have to consider other intermediary hosts as well on its way to people," he said.