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There is a worldwide shortage of organs for clinical transplantation and, sadly, many patients due to receive new organs die on the waiting list. As well, a number of cases occur where there is little chance of a person actually receiving a transplant, so the person is not added to the list, causing an underrepresentation of the shortage. Recent advances in understanding the mechanisms of transplant organ rejection have brought science to a stage where it is reasonable to consider that organs from other species, probably pigs, may soon be engineered to minimize the risk of serious rejection and used as an alternative to human tissues, possibly ending organ shortages. Other procedures, some of which are being investigated in early clinical trials, aim to use cells or tissues from other species to treat both life-threatening and debilitating illnesses such as cancer, AIDS, diabetes, liver failure and Parkinson's disease.

At this point, there are only a couple known successful xenotransplants. A few patients who were in need of liver transplants were able to use pig livers that were on a trolley by their bedside successfully until a proper donor liver was available. Some recipients of pig neural cells with paralysis due to stroke and Parkinson's have experienced dramatic improvements.

The most difficult challenge for xenotransplantation at this point is that of rejection. The problem exists even for human to human transplants (known as allotransplantation), but is much more serious for transplants between different species. All cells have markers which define what they are, to which antibodies decide whether to invade as foreign or not. The more different the genetic code between the donor organ and recipient, the greater the difference between a "safe" marker and a "foreign" marker. Many companies today are working on genetically modifying animals such as pigs (causing them to be transgenic), with some success, so that the markers they produce are actually human.

Another worrisome element of xenotransplantation is that of infectious disease spreading from the donor animal. One example is porcine endogenous retroviruses (PERVs) which are viruses within pigs that pigs are immune to, but some researchers have found that they can infect humans. Some recipients of pig neural cell transplants have had to agree to never donate blood, take frequent blood tests and use safe sex methods for the rest of their lives due to the risk of spreading such viruses. However, the patients who have received these pig cell transplants have yet to show any PERV-type infection. What may hold for other animals is unknown at this point.

End-stage organ failure is one of the most important public health problems facing the industrialized world today. However, xenotransplantation raises many novel medical, legal and ethical issues. The effects of xenotransplantation on the human gene pool and possible long-term genetic problems offer a note of caution. Equally, any permanent alteration to the genetic code of animals is a cause for concern.

See Also: PBS Special on Pig to Human Transplants