- Lei Ruipeng School of the Humanities Huazhong University of Science & Technology Lxp73615@163.com
The remarkable half-century transition of organ transplantation from experimental intervention to standard clinical practice has resulted in a growing disparity between the number of persons who could potentially benefit from allotransplants and the availability of transplantable human organs. This disparity inspired initial attempts to explore alternative therapies for organ failure, among them xenotransplantation, which involves the use of living, nonhuman animal tissues and organs in humans.
No field of medicine has stimulated more excitement and controversy than xenotransplantation. The excitement stems from the possibility that transplantation could finally be extended to all patients who need it. The controversies arise from the immunological hurdles to xenotransplantation, and from the possibility that infectious agents might be passed from the animal source to the human recipient and, potentially, more broadly in the human population. However, in Chinese media and scientists' talks focus seems to be on how successful efforts have been made to prevent the immune rejection by knocking out a certain gene in pig. No word is on the cross-species infection unintentionally or deliberately. The question is: is the cross-species infection morally relevant in xenotransplantation? Xenotransplantation products may infect human recipients with zoonotic and other infectious agents that are not endemic in human populations. Immunosuppressive regimens intended to improve survival of xenotransplantation products may also inhibit immune response to infection. Despite all kinds of measures, xenotransplantation products may still transmit infectious agents to the human recipient. Infectious agents may not be recognized and eliminated either because they are not known to exist (e.g. Nipah virus prior to 1999), diagnostic tools are inadequate to detect them (e.g. prions), or they can not be removed by means currently available (e.g. endogenous retroviruses).
Pig DNA contains endogenous retroviruses (the same class that causes AIDS), and these infect human tissue culture cells. Postmortem analyses of two patients who died 70 and 27 days after receiving baboon livers showed two simian viruses that replicated after transplantation. Our state of knowledge is far too incomplete for us to breed totally virus-free animals, because we probably don't even know all the viruses that need to be eliminated. We do know that viruses jump species even without our help, and there are enough frightening precedents-as far back as the 1918 swine influenza pandemic that killed 20 million-to scare anyone contemplating xenotransplantation. More recently, millions of chickens had to be slaughtered in Hong Kong because of the unexpected jump to humans of an avian influenza virus, and thousands of English cattle have been destroyed because of the jump to humans of bovine spongiform encephalopathy ("mad cow disease"). The spectacular pandemic of AIDS resulted from a virus given new routes of entry: widespread increases in certain lifestyle practices provided a conduit for efficient transmission. HIV-1 probably also resulted from a simian to human virus jump. Deadly Ebola virus is another virus transmitted to humans from primates, and there are at least another 10 primate viruses that infect humans, including a deadly form of herpes. Pigs aren't much safer: about a dozen pig viruses can be transmitted to humans, often with serious results.
It is vital that the risks and benefits of xenotransplantation are properly assessed and communicated. To protect patients and medical/nursing participants, the use of external risk and expert assessment would provide assurance that there is no untoward pressure or expectations. There is some disagreement what the level of safety and the degree of risk necessary should be before xenotransplantation takes place.
Some people argue that the precautionary principle should be the starting point suggesting that the consequences are likely to be harmful or unknown, we ought not to proceed. If taken literally and to the extreme, then little or no scientific advance would take place. Rather, all those involved in deciding about xenotransplantation must be satisfied that the risk to the individual recipient, their families, the medical and nursing teams, and the general public are minimal and controllable. In the end, there will be no absolute guarantees but the scientific evidence must be clearly objectively clear that there are no real risks to those involved.
Part of such risk assessment recognizes that some degree of risk will be acceptable, especially to desperate patients and their families. Likewise medical research staff may regard a degree of risk acceptable in order to establish the validity of a treatment. However, the overwhelming need to gain and maintain public confidence must mean that no xenotransplantation ought to be approved until and unless there is a high level of assurance about safety as possible. Xenotransplantation of organs from animals is a seductive but inherently dangerous idea. The risk is not just to individual patient, who will probably die shortly afterwards; the stakes are much higher, because the entire human population is put at risk. Although there is no specific proof yet of safety or danger currently, independent research on potential viral risk, which isn't sponsored by those biomedical, companies any more is needed. In risk assessment, absence of evidence cannot be read as evidence of absence. That is the case for a moratorium on clinical trials.