EXPERTS insist virus could 'jump' species.
Trials of pig tissue transplanted into humans to treat diabetes, Parkinson's disease and blindness are 'imminent' but solid organ transplants -- hearts, kidneys, livers -- are still several years away', say experts.
The world is on the cusp of a medical revolution which will see tissue from genetically engineered pigs used to treat a complex range of diseases.
Experts from the Thomas Starzl Transplantation Institute at the University of Pittsburgh have said that human trials could begin within two to three years.
But an expert challenged their findings saying the safety issues were still "unresolved".
The fear among medical experts is that a retrovirus could jump from pigs to humans and trigger a new pandemic -- HIV is thought to have started from a similar form of cross-species infection.
In a review of xenotransplantation -- using animal tissue and organs to replace human ones -- experts say the technology has made "great strides".
During the mid-1990s, Imutran, a small biotechnology company in Cambridge, became the first in the world to transplant pigs' hearts into monkeys, demonstrating that cross-species transplantation was possible.
However, immune system rejection problems proved more difficult than expected and safety fears dampened enthusiasm.
One decade on, attention has focused on the transplant of cells and tissues rather than organs -- pancreatic islets for diabetics, brain cells for Parkinson's disease and corneas for the blind.
Problems remain, including loss of transplanted cells, blood clots and other rejection problems.
The hope is that the new, genetically modified pigs will shortly become available which could overcome these issues and make human trials of transplanted tissue "imminent".
The US scientists discount safety fears saying retroviruses "do not pose a substantial risk".
That was challenged by Robin Weiss, professor of virology at University College, London, and an expert on retroviruses. He said yesterday: "I don't think the risk issues have been resolved.
"That doesn't mean clinical trials shouldn't go ahead but there would need to be very close monitoring of patients to ensure they were not picking up something harmful from pigs."