The Need for Organs
As of June 2004, over 91,000 Americans are awaiting organs. About 2/3 of those are awaiting kidneys alone. Other organs in need include lungs, pancreas, heart, and intestine. About half of those needing an organ transplant will never receive one, and most of those who do must first wait several months or years. 17 people each day die waiting for organ transplants, and a new name is added to the National Transplant Waiting List every 13 minutes.
At this point, organ failure is largely unpreventable, and many have no choice but to resort to organ transplantation. However, due to the shortage of human organs, xenotransplantation is has emerged as an alternative therapy. Xenotransplantation is the transfer of living cells, tissues, and/or organs from one species to another. Unless the number of organ donors increases, many see xenotransplantation as the possibility to save the lives of those needing organ transplants.
Potential Donor Species
Non-human primates are our closest genetic relatives. Chimpanzees are genetically compatible for transplantation; however, they are not candidates because they are endangered. However, baboons are not endangered and are similarly compatible. The limitation is that baboons only grow to about 70 pounds and have a low frequency of type O blood. Type O donors can donate to anyone, while type A, type B, or type AB donors can only donate to an individual with the corresponding blood type. (A recipient with a blood type known as O negative can receive a transplant or transfusion from a donor with any blood type. However, nearly all of those with O blood are what is referred to as O positive). About 40% of Americans are O positive, and are therefore unlikely to benefit from baboons as donors. Furthermore, because baboons are smaller than humans, their organs would be limited to children.[4]
Recently, interest has turned to pigs as candidates. Pigs are physiologically similar to humans, and are more abundant than non-human primates. In June 2004, South Korea announced that it would spend over 72 million dollars over the next ten years to produce pig organs for transplant. Their hope is to clone human-size pigs by 2005, and by 2007 develop a pig with organs that are compatible with those of the human body. By 2010, South Korea intends to have miniature pigs available for transplant surgery.
Medical Risks
Medical risks associated with xenotransplantation are of foremost concern. Cross-species transmission of known foreign diseases (xenozoonoses) and unknown pathogens could not only infect patients, but hypothetically be transmitted to the public. In theory, a new epidemic could arise. In fact, it is believed that AIDS was first introduced to humans through human-monkey communication. Also of concern are herpesviruses and retroviruses, although after screening, these are thought to be removable from the donor pool.
Yet the concern of viral transmission is well substantiated. In a recent study conducted by the Mayo Clinic in Rochester, Minnesota, researchers found that pigs developed human and hybrid (half pig, half human) cells after being injected with human blood stem cells. The hybrid cells contained porcine endogenous retrovirus, a pig virus much like HIV that was able to pass on to normal human cells.
It is also important to note that viruses known to be harmless in animals have proven harmful in humans. Macaque herpes from monkeys is an example of this. And unknown diseases are also potential risks. Some xenotransplant antagonists consider xenotransplantation a form of involuntary human experimentation, of both the patient and the public.
Finally, the possibility of immunologic rejection and/or infection in the patient is likely. Donor recipients will have to commit to a life long regime of immunosuppressive drugs. Nevertheless, progress is continuously being made in breeding pigs with genomes as close as to the human genome as possible, and immunorejection will therefore assumingly be a decreasing problem. Furthermore, improvements are being made in the development of immunosuppressive drugs.
Economic
Some argue that xenotransplantation is not cost-effective. Pigs or primates must be reared under “germ free conditions” which is extremely expensive. And it may also cost over $50,000 to test each animal for known bacteria or viruses. According to the 1996 Institute of Medicine Report, xenotransplantation could increase annual transplant costs from $3 to over $20 billion. However, it must be remembered that this increase reflects the same number of human organ transplants, plus additional organs taken from animals. And for many, the cost is great, but the cost of life is greater.
Animal Welfare
Additionally, animal rights activists point out that the fact that pigs are killed for food doesn’t justify breeding and killing increased numbers to harvest their organs. Furthermore, there is concern that transgenic (an animal with genes from two species) species have been seen to have multiple physical abnormalities. Such include weakened immunity, stomach problems, and impaired vision, among others. And confining the animals to a sterile, artificial environment for life is additionally upsetting.
Finally, some are against the idea that pigs are bred with human genes (so that the human immune system accepts their organs). These individuals point out that this process creates a creature that is half pig half human, and consider this practice a violation of nature.
Ethical and Legal Issues
Informed consent is critical to the success of xenotransplantation. There may be unknown risks, such as viral infection, that patients must fully understand before consenting to receiving a transplant. The desperate nature of the period when one is in need of an organ also makes questionable the emotional stability and therefore capacity of the patient to make the decision. Th
erefore, the issue of surrogate consent arises.
Human worth issues also arise. Is the worth of a donor recipient lessened because he or she is not technically 100% human? Will all people be able to afford xenotransplant organs, and if not, how marked will the socio-economic inequality be? And finally, because of unknown epidemic risks associated with xenotransplants, the public could potentially be at health danger on account of air-borne or other communicable pathogens. The question then becomes: is the potential public welfare more valuable than the lives of patients in need of organs?
Further Reading