One controversial aspect of the proposed US guidelines on xenotransplantation is the lack of an explicit ban on the use of organs from non-human primates, such as baboons. Although these are less susceptible to rejection because of their close similarity to human organs, they are nonetheless widely considered unsuitable for transplantation because of their much higher perceived disease risk, and the fact that it would be impractical to breed the large numbers of ‘clean’ animals that would be needed.

Such considerations led an ethics panel set up by the UK government to rule out the use of primates as donors on the grounds that pig pathogens are better characterized and the animals are easier to breed in large numbers under clean conditions.

But many scientists are unhappy about the lack of a US ban on the use of primates, given their unsuitability. US agencies appear to have felt that the issue was not the species used, but rather the level of disease risk. Applications for clinical trials would be judged on the basis of how well-defined the pathogens of a particular species were, how easily they could be removed, and on the risks that they harboured unknown viruses, says Louisa Chapman, an official at the US Centers for Disease Control and Prevention. She argues that in practice non-human primates will have much greater difficulty in meeting these criteria than pigs.

Such assurances are met with scepticism by critics who point out that the US Food and Drug Administration (FDA) approved a controversial trial of baboon bone marrow in AIDS patient Jeff Getty in 1995 (see Nature 378, 756; 1995).

Suzanne Ildstad, director of the Institute for Cellular Therapeutics at Allegheny University of the Health Sciences in Philadelphia, who oversaw this trial, is keen to continue with further trials.

At least one other surgeon also has plans to transplant solid organs from non-human primates. Leonard Bailey, from the Loma Linda University Medical Center in California, one of the country's top heart transplant surgeons, says he intends to apply to the FDA to transplant hearts from the centre's baboon colony into children. “We don't want to risk the public health, but we don't think we need to hold back on the basis of speculation about risks to public health.”

Baby Fae: despite failure, new operations with baboon hearts are still being planned. Credit: SIPA PRESS

In 1984, Bailey carried out the most celebrated xenotransplant operation, placing a baboon heart into a two-week old baby — Baby Fae. The child died three weeks later after her immune system destroyed the organ. Bailey now claims to have obtained “prolonged survival” in animal studies (see World Journal of Surgery 21, 943-950; 1997) and intends to try again.

But another surgeon who also pioneered early baboon xenotransplants, Thomas Starzl, from the University of Pittsburgh, says he has decided not to proceed for the time being. Starzl carried out a series of unsuccessful baboon-to-human kidney transplants in the early 1960s and again in the 1990s. But he says lack of scientific understanding means that “we are too far from being able to do anything [clinically]; we are tremendously interested but we think the research endeavours are going in the wrong direction”.

Concern about the use of non-human primates has been heightened by a loophole in the guidelines that would seem to risk allowing the use of virus-laden wild primates. The revised guidelines do not explicitly ban the use of these, saying only that departures from ideal husbandry would need to be justified by the trial sponsor.