Xenotransplantation - Nuffield Council on Bioethics

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Animal-to-Human Transplants : the ethics of xenotransplantation for a xenograft would have to be made aware of, and to consider these risks, about which very little is known at present, along with all the other factors that would need to be taken into account (paragraphs 7.14 - 7.21). 6.4 The possibility must also be considered that should an animal organism infect a xenograft recipient the resultant disease might then be passed on to the public at large. In this way, xenografting may pose a risk to public health as well as to individual health. Many of those who responded to the public consultation emphasised the importance of addressing the risks of disease transmission associated with xenotransplantation. 3 This chapter considers, first, the possibility that new diseases will be transmitted from animals to xenograft recipients. It then considers the risks that such diseases might spread from xenograft recipients into the general population. Finally, the principles by which these risks might be assessed and managed are discussed. The risk that infectious organisms will be transmitted from animals to human beings 6.5 There is evidence that human beings are susceptible to some animal diseases. Such diseases are called zoonoses. For example: human beings in contact with monkeys can become infected with monkeypox virus, which is related to the smallpox virus; contact with macaque monkeys can lead to infection with a macaque form of herpes B virus which causes encephalitis in human beings which is rapidly fatal; the human immunodeficiency virus (HIV) virus that causes AIDS is very similar to the simian immunodeficiency viruses (SIV) found in primates. One view attributes the emergence of the HIV virus and the disease AIDS in human beings to the transmission of SIV viruses from primates to human beings. There is evidence that the SIV virus can, indeed, be transmitted from primates to human beings although, as yet, there is no evidence of disease symptoms in SIV infected human beings. 4, 5 3 These included Animal Aid, the British Union for the Abolition of Vivisection, the Christian Medical Fellowship, the Church of England, the Church of Scotland, the Church in Wales, Professor David Onions, the Genetics Forum, the Jain Academy, Professor John Polkinghorne and PPL Therapeutics. 4 Chapman L E et al. (1995) <strong>Xenotransplantation</strong> and xenogeneic infections. New England Journal of Medicine, 333:1498-501. 5 Thanks, but no thanks. The Economist, 21 October 1995, pp 17, 137-9. 68
Transmission of infectious diseases 6.6 <strong>Xenotransplantation</strong> is one way by which disease-causing organisms could be transferred from animals to human beings. Because xenotransplantation involves the direct introduction of animal organs or tissue into the human body, many of the natural barriers to infection are by-passed. Xenograft recipients are also likely to require immunosuppression to prevent transplant rejection. Since immunosuppression lowers the body’s resistance to disease, the possibility of infection of a recipient with animal diseases may be increased further. 6.7 It is sometimes argued that the risk of transmission of animal diseases is overestimated, since immunosuppressed recipients of human organ transplants who are in contact with pets or farm animals, do not contract animal diseases. This argument, however, ignores the major difference between close contact with animals and the transfer of animal organs or tissue directly into the human body. The potential importance of this difference is illustrated by the following example. Cats are susceptible to infection by feline leukaemia virus. Dogs or human beings, in contact with cats, do not become infected with the virus. But if young, immunosuppressed dogs are transplanted with infected cat tissue, the dogs become infected with the virus and develop tumours. 6 Infectious organisms 6.8 Xenografting provides a potential route for the transmission of disease. So it is necessary to consider the organisms that infect possible source animals and to assess whether they are likely to cause disease in human beings. Important categories of infectious organisms include viruses, bacteria and fungi. Prion proteins are another type of infectious agent that are thought to cause diseases such as bovine spongiform encephalopathy (BSE, or ‘mad cow disease’) and human Creutzfeldt-Jakob Disease (CJD). There is evidence that prion diseases can pass from one species to another. 7 Thus, prion disease transmission is another risk that needs to be considered when contemplating xenografting. 6.9 Most concern about the risks of infection from xenotransplantation focuses on viruses. This is because: 1 viral infections are difficult to treat with drugs; 2 viral infections may have a long latent period during which the person has no symptoms of the disease. This is the case with HIV infection: it may take 10 years for an infected person to develop AIDS. If a new disease were to 6 Onions D (1995) Comments made at an Institute of Medicine conference on Xenograft transplantation: science, ethics and public policy,Washington DC. 7 Prusiner S (1995) The prion diseases. Scientific American, 272(1):30-7. 69
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Preface Preface Since the N
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Working Party on Xenografts Working
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Xenotransplantation - Nuffield Council on Bioethics

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