The world is on the cusp of a medical revolution which will see transplants of tissue from genetically engineered pigs to treat a range of human diseases, US scientists say.
Human trials could begin within two to three years of tissue taken from specially bred pigs to help the millions of sufferers from diabetes, brain diseases and blindness.
But a British expert challenged their findings, saying the safety issues were still "unresolved". The fear is that a retrovirus could jump from pigs to humans and trigger a new pandemic -- HIV is thought to have started from a cross-species infection.
Experts from the Thomas Stazl Transplantation Institute at the University of Pittsburgh say the technology has made "great strides".
During the mid-1990s, a small biotechnology company in Cambridge became the first in the world to transplant pigs' hearts into monkeys. However, immune system rejection problems proved more difficult than expected.
A 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.
Bur rejection problems remain. New, genetically modified pigs will shortly become available which could overcome these issues.
The US scientists say retroviruses "do not pose a substantial risk".
That was challenged by Robin Weiss, professor of virology at University College, London. 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."