Clinical trials using animal organs to keep humans alive may be just five years away, an expert said yesterday.

Anthony Warrens, from Imperial College London, said the technical obstacles to successful transplantation of animal organs into humans had almost been surmounted, revitalising research in the area.

"If you had asked me five years ago, I'd have been very pessimistic. But there have been significant developments in the course of the last year that make one feel this is a problem we can get around," he told the British Association festival of science in Dublin.

Dr Warrens said the shortfall in the number of donor organs needed by doctors was intense: for every human organ that becomes available for transplant, there are five people on waiting lists who could benefit.

"If it were feasible to use animals as organ donors, an unlimited supply would potentially become instantly available," said Dr Warrens.

Pigs are the main focus of interest for researchers in this area, because they are of similar size to humans and can be cultivated in large numbers. They are also genetically similar enough for their organs to work in people, but sufficiently distant to reduce likelihood of transfer of infections.

Much of the research into xenotransplantation, the process of using animal organs in humans, was pioneered in Britain in the 1990s, but the work faced problems.

The principal barrier has been the body's immune system. Antibodies in the blood will bind to the surface of any foreign organ, such as a kidney, and reject it within hours.

"We have known for many years that the target of these antibodies is a carbohydrate structure called the 'gal epitope'," said Dr Warrens.

"However, pigs have now been generated which do not have the gal epitope on their cells and early evidence suggests that this manipulation may be effective in limiting damage."

In addition, by mimicking the way the body's immune system learns not to attack its own cells, scientists have further improved the acceptance of animal organs. They have shown that pig transplants in experimental models can survive up to 80 days.

A further problem has been the possibility of infection.

"Is there going to be a pandemic of new organisms transferred from the pig population to the human population if we indulge in xenotransplantation?" said Dr Warrens.

The final dilemma is the ethics.

"Do we have the right to use animals in this way? This is a decision for society."

Currently, there is a moratorium on xenotransplantation, but as biological challenges are solved, Dr Warrens said the only possible next step would be to transplant pig organs into humans in closely monitored trials. This could be done within five years.