Animal Magic

      Throughout the world there is a shortage of human organs for transplants. In Britain, for example, 6 O00 people are waiting for organs -- 5 000 for kidneys, and the remainder for hearts, lungs and livers. However, only about I 750 kidney, 500 heart or heart and lung, and 650 liver transplants are carried out each year. And the waiting list increases at five per cent a year. In the United States only half of the 30 000 in need of organs receive them.

      Hence the great interest in animal-to-human transplants, known ax xenografts or xenotransplants (from the Greek word xenos, meaning strange or foreign), which most scientists believe ix the only long-term solution to the organ shortage. There have been attempts at xenotransplants since the beginning of this century but none has been successful; the longest survivor was a 20-day-old baby called Fae, who in 1984 was given the heart of a baboon.

      The main problem with any organ transplant is that the immune system of the patient receiving the organ sees the transplant organ as an invader. The immune system therefore launches a massive attack on the invader, activating enzyme'>enzymes known as complement, which attack the foreign body, eventually killing the patient as well. This rejection has been overcome when transplanting organs between humans by the close matching of tissue and the long-term use of drugs known as immuno-suppressants, first introduced just over ten years ago. With xenotransplants, however, rejection is even more severe; a normal pig's heart, for example, transfused with human blood can be destroyed in 15 minutes.

      So for the past few years a lot of research has gone into preparing animal organs by raising animals that have been genetically altered; they are known as transgenic breeds. Animal embryos are being injected with a human gene to produce human complement inhibitors, which control the release of complement, so that after a transplant the human complement "sees" the transgenic animal organ as if it were human.

      The preferred animals for xenotransplants are not other primates, once considered the best candidates for xenografts, but pigs. Ironically, other primates resemble humans too closely; the same viruses that infect monkeys and apes may infect humans, and a primate organ transplant could pass on fatal diseases; Aids probably originated as a monkey virus that somehow infected humans. Pigs' organs, though, are close in function and size to those of humans, yet distant enough not to carry viruses fatal to humans. And pigs are also cheap and easy to breed.

      A British biotechnology company called Imutran, which is based in Cambridge, is planning the world's first pig-to-human heart transplant some time this year as part of its clinical trials. Last year the Cambridge scientists successfully transplanted transgenic pigs' hearts into monkeys; some of the monkeys were still alive after 60 days, beating the previous best results of scientists in the United States who only achieved a maximum survival rate of 30 hours.

      If Imutran's research continues as expected, transgenic pigs' organs, including kidneys and lungs, could be generally available by the year 2000. '~This will give hope to hundreds of thousands of patients around the world who would otherwise die waiting for a heart, lung or kidney," said Christopher Sampler, Imutran's Chief Executive. The transplants of pigs' livers are not being considered at this stage.

      However, xenografts will still have to overcome ethical as well as scientific barriers. Already, pig insulin is used for diabetes and pig heart valves are used in heart repair surgery, so the use of animal organs is not likely to cause uproar. But it is expected that animal rights campaigners will not accept the rearing and killing of large numbers of animals for human medical use. And, as Les Ward, Director of the Scottish group Advocates for Animals, has pointed out, what will happen to the rest of the pig after the organs have been removed. Will the meat be used for human consumption? "If so -- and bearing in mind that the pig contained human genes --is this not a form of cannibalism?"