Last year British scientists found that a virus called Porcine endogenous retrovirus (Perv), whose genes are found throughout a pig's genetic make-up, is shed by pig kidney cells which can in turn infect human cells.

Pigs organs are about the same size as those in humans

Now the UK's Lancet journal reports a German research group has found that the Perv virus is produced by cells from pig aortas, livers, lungs and skin as well as from the kidneys. These are all tissues likely to be used for transplants.

The group said producing Perv-free pigs would be "difficult or impossible" and the findings suggested "a serious risk of retrovirus transfer after xenotransplantation".

The Lancet warns: "The discovery sparked fears that if pig organs were used for transplantation they could introduce new, possibly deadly disease-cutting viruses into the human population."

Pigs are reckoned to be the most practical species to use for transplants into humans because they are the right size, are easy to raise and until recently were thought to be fairly free of pathogens that could threaten transplant recipients.

Findings disputed

However, the findings conflict with a US/Swedish research group who revealed earlier this month they had found no evidence of Perv infection in blood samples from 10 Swedish diabetes patients who had received transplants of insulin-producing cells from pigs.

Jonathan Stoye of London's National Institute for Medical Research: Risk of infection very low

A third research group has also said they could find no evidence of Perv infection in two kidney-failure patients who had their blood passed through pig kidneys.

Jonathan Stoye, an expert from London's National Institute for Medical Research, said that only with limited clinical trials, which are awaiting regulatory approval, would it be possible to test "long-term xenograft survival and function".