The science magazine this week reports what it calls "the first compelling evidence that dormant pig viruses do not spread to humans", based on two small studies in the US.

The first, by the US Centers for Disease Control and Protection, shows 10 Swedish diabetic patients who received pig pancreatic cells have not gone on to be infected with diseased pig DNA.

Pigs have built up dormant viruses related to HIV, known as porcine endogenous retroviruses (PERVS), over thousands of years and there are fears they may be passed on to humans who receive transplanted cells and organs.

If viruses did cross the species barrier, some scientists fear the creation of new diseases which could have a devastating effect on human life - what the New Scientist calls the "doomsday scenario".

On the other hand, pigs are genetically the most suitable animal for transplants and the ability to use their organs and cells could solve the worldwide shortage of organ donors.

Pig cells

Scientists from the CDC are expected to unveil the results of their study of Swedish patients at a meeting of the UK Xenotransplantation Interim Regulatory Authority this week.

New Scientist says they show that there was no pig virus DNA, no antibodies and no enzymes linked to PERVs in the blood of the humans tested.

"We looked in blood, lymphocytes (white blood cells) and serum, and found nothing. These are reassuring data," CDC expert Walid Heinene told the New Scientist.

The magazine says the results of another study by US company Diacrin are also expected to be revealed this week.

Diacrin is looking at whether pig viruses pass from patients with Parkinson's and Huntingdon's disease who have received foetal pig cells.

It reportedly shows that none of the 24 patients tested has become infected.

Last month, another study of 25 patients by Massachusetts company Circe Biomedical also showed no sign of pig viruses crossing over to humans given pig liver cells.

Lab conditions

However, a study last year by Robin Weiss of the Institute of Cancer Research in London demonstrated that two pig viruses could transfer to human cells in laboratory conditions.

This study caused the US Food and Drug Administration to temporarily withdraw approval for clinical trials involving animal to human transplants.

Further tests are needed to show if pig viruses can be passed to humans

There are thought to be a large number of studies going on around the world on the potential for cross over between human and pig cells, but the New Scientist acknowledges that transplanting entire organs could present a much greater risk than cells transplants.

UK biotech organisation Imutran, which is seeking permission to use a pig liver as a temporary external support for a British patient, is conducting its own study into the potential for crossover from pig organs to humans.

The findings of the study of 160 people, overseen by the US CDC, are expected to be revealed in the autumn.

Imutran, which says it has trawled the world for patients, is looking at whether the viruses cross over to humans and, if so, whether any harm is caused.

It says the patients being tested range from those who have received pig organs in the last few months and those who have had transplants several years ago.