The ethics of xenotransplantation are relatively unworrying. People already kill pigs both for food and for sport; killing them to save a human life seems, if anything, easier to justify. However, the science of xenotransplantation much less straightforward.
Import an organ from one animal to another and you may bring with it any number of infectious diseases. That much is well known. However, coping with this danger is not merely a matter of screening for obvious ills such as parasites. Many diseases that could harm humans may be both undetectable and harmless in their natural hosts. Diseases that have been dormant for years may suddenly become active if they find themselves in a new environment, such as a human recipientss body. After that , they may start to infect other people.
The risk of this happening should not be underestimated. The DNA of every organism carries within it hundreds of ghosts of infections past. Such retroviruses which include HIV, the virus that causes AIDS always incorporate themselves into the DNA of their hosts. Many retroviruses also incorporate themselves into their hosts eggs or sperm, and are passed passively from parent to child. Although it is true that most retroviruses gradually lose their infectious powers, some retain their ability to leap out of the host DNA often much later. Certain pig retroviruses are probably among these.
Of course it is possible that none of the retroviruses will be harmful to humans: possible too that scientists will eventually isolate all prospective trouble-makers. But at a time when thousands of British cattle are being slaughtered because of the suspicion that they have a disease that may be transmissible to humans, it seems a reckless gamble to take.
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