Xenotransplantation ("xeno" is greek for stranger) is the transplant of cells, tissues or organs between species. It commonly refers to transplants of animal organs into humans.
Experiments have been performed on transplants of bone marrow, hearts, neurons and other tissues from baboons, chimpanzees and pigs, with limited success in terms of patient survival or organ functionality. However, pig and cow heart valves are commonly used in human patients.
It is a common misconception that xenotransplants involve animals that are most like humans, i.e. chimpanzees. In fact, chimps are endangered, expensive to raise and grow too slowly to be of use for the thousands needing transplants. Chimps are also so alike humans that the risks of transplanting an infectious virus are too high. In 1999, the FDA banned clinical trials with non-human primates. Pigs are a promising doner species because their organs are similar in size to humans, they have large litters, grow quickly, and are far enough from humans on the evolutionary tree to minimize infection risks. They are also more socially acceptable by many, since many in our society also regularily consume them for food.
Other considerations and concerns that arise from the use of animal organs include rejection issues and the risk of transmission of animal diseases to humans. The human immune system reacts violently to pig organs and pigs contain ubiquitous retroviruses which may adversely affect humans. Many biotech companies are working to resolve these and other ethics issues that contribute to the xenotransplantation debate.
Xenotourism and less stringent controls on xenotransplants in other countries might affect North Americans, should a viral epidemic arise. In 2000 the OECD and WHO proposed a watchdog to monitor trials and study the risks of xenotransplantation.