BBC Science Correspondent Sue Nelson reports

The report claims patients who receive major animal organs in transplants will cease to be wholly human because the animals' DNA would infiltrate every part of their bodies.

Two animal welfare organisations - Compassion in World Farming and the British Union for the Abolition of Vivisection - have sponsored the report which is based on a review of research literature dating back to the early 1990s.

Although tissues from animals have been used in human operations for several years, it is the breeding of genetically altered herds for use in whole-organ transplants that has intensified the scientific and ethical arguments.

Immune system

The UK biotechnology company Immutran is preparing a request to the regulatory authorities for permission to carry out a procedure in which a pig's liver is given to a human patient as a temporary measure while a suitable human organ is found.

Immutran is preparing to carry out a liver operation

The company says it has modified the pig's genetic make-up so that the organ will not suffer hyperacute rejection when it comes into contact with the human immune system.

The UK government has just updated the rules governing xenotransplantation - animal to human transplants - to allow such a request to be considered.

Its Advisory Group on the Ethics of Xenotransplantation told the government in 1997 there should be no ethical obstacles to such operations.

Transplant dangers

However, this new report says the use of animal tissues is dangerous. It believes:

• there are serious "infectious risks to patients and the public";
• there is "scant evidence" that animal organs can sustain human life;
• genetic engineering is "not fully developed" to deal with the problems that exist;
• the transplants could produce human-animal hybrids or chimeras - a reference to the mythical beast that was part lion, part goat and part snake.

To illustrate this last point, the report refers to a case in the early 90s in which a man was given a baboon organ: "This was shown in the 1992 experiment by Dr Thomas Starzl, in which a patient received a baboon liver to replace his own failing organ.

"For the 70 days of his survival, this man was a baboon-human chimera, formed by a process which Starzl refers to as 'baboonization'.

"Baboon DNA, representing traces of white blood cells, was found in every tissue of the patient's body which was tested, including the heart, lungs, kidneys and lymph nodes."

Dual species

Zoologist and neurochemist Dr Gill Langley, one of the authors of the report says this issue of chimerism has so far been ignored by many in the debate.

"It's now clear that a human xenotransplant patient will become a literal chimera, a pig or baboon human hybrid.

"Not only will an animal organ produce animal factors which will circulate around the blood stream, but cells from the animal organ will travel all over the human body and to every organ, including the heart, lungs, kidneys, bone marrow and lymph nodes."

She said it may even be necessary for xenotransplant patients to become chimeras with a "dual-species immune system" to avoid long term rejection of the animal organ, she says.

Organ shortage

The report also covers the more publicised concerns about the possibility of animal viruses jumping the species barrier. 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.

Dr David White: We will abide by the rules

Immutran scientist Dr David White dismissed the report as a scare story. He said nothing would be done outside the regulations.

"We have set up very specific studies to allow us to assure ourselves and our patients and the regulatory authorities that the procedures we are undertaking are indeed safe.

"We are responsible to the Xenotransplantation Interim Regulatory Authority which looks at all of these ethical issues as well as the issues of safety and efficacy, and we're very happy to abide by them."

Dr David White: Studies will evaluate safety

Dr White said there was a desperate need for donor organs and the ethical reservations were not as great as some people were suggesting: "We have been using pig heart valves to help people for decades. What's the difference between that and a pig heart."