Right: A team of surgeons and technicians perform an animal-to-human transplant, using an organ from a genetically-engineered pig.
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Arguments against using animal organs and tissues for human transplants
1. Using animal organs for human transplants risks organ rejection
Opponents of animal-to-human transplantation stress the high risk of organ rejection, that is, the immune system of the human body receiving the animal organ will mount a massive immune response against the foreign tissue.
Large numbers of current studies have noted the great danger to patient survival posed by the human body's immune response. A report published in Science Daily on March 14, 2019, noted the significant problems that organ rejection currently poses for patients receiving human organs. The report notes, 'One third of organ transplants are lost to transplant rejection. Although acute transplant rejection responds relatively well to steroids, chronic rejection (which is mainly mediated by antibodies) has no effective treatment.'
The problems of organ rejection associated with transplanted animal organs are potentially greater. In 2020 Bernard Rollin, emeritus professor of philosophy, animal sciences, and biomedical sciences at Colorado State University, stated, 'The most significant issue with using animals for a source of transplanted organs (xenotransplantation) for humans is immunological rejection of the organ, with the human immune system recognizing the foreign organ as "not-self" and...rejecting it. In what is known as "hyper-acute rejection", the body begins to reject the organ virtually as soon as it is implanted.' The same problem was described in the 1996 Nuffield Council on Bioethics report 'Animal-to-Human Transplants: the ethics of xenotransplantation'. The report states, 'The major hurdle in the way of successful xenotransplantation is preventing the rejection of transplanted animal organs. This is a problem even with human organ transplantation: the recipient's immune system mounts an attack on the transplanted organ, which it sees as foreign. The immune response to organs or tissue from a different species is much stronger. Two main approaches are being used to overcome this problem. First...the use of baboons is being investigated, on the basis that baboons are closely related to human beings and so the immune response to baboon organs or tissue will not be too strong. The second approach is to use pigs that have been modified genetically so that their organs do not cause such a strong immune response when transplanted into human beings.'
Critics note that the long-term effectiveness of genetic modification to reduce the likelihood of rejection has not been demonstrated. It has been claimed that the problems associated with acquiring a genetically modified organ grown within an animal (usually a pig) have not all been solved. In 2018 , EMBO, the journal of the European Molecular Biology Organization published a report titled 'Ethical rejections of xenotransplantation? The potential and challenges of using human-pig chimeras to create organs for transplantation.' The report states, 'To prevent tissue rejection, the transplanted organ has to contain at least 90 percent human cells, which will make it necessary to generate a human organ with a human vascular system. Second, we do not know how many pig chimeras [pigs with genetically modified organs] will be needed to ensure that a proper patient-specific organ is obtained for transplantation. Finally, heart transplant candidates require intense medical care and usually receive a transplant within 6 months. Will it be possible to generate a human heart in human-pig chimeras within 6 months or will more time be necessary that may place these patients at a higher mortality risk while waiting for the donor heart?'
2. Using animal organs for human transplants risks animal diseases entering human populations
Those who oppose animal organs being transplanted into human beings stress the risk of currently animal-specific diseases being acquired by transplant recipients and the people with whom they come into contact.
On January 18, 2022, The Australian published a report from Rhys Blakely, the science correspondent for The Times which examines some of the negative consequences associated with animal organ transplants. Blakely explains, 'Pig DNA contains viruses that have become embedded inside the animal's genetic code. These porcine [pig] endogenous retroviruses (abbreviated, memorably, to "Pervs") could potentially break out of the pig's DNA to become independent infectious pathogens. Retroviruses can lurk in the body for years, decades even, before they cause symptoms. They're not to be trifled with: another pathogen in the same class is HIV.'
Many medical authorities have stressed the potential for disease transmission when human beings receive transplant organs from animals. The United States Food & Drug Administration (FDA) has stated, 'The use of xenotransplantation raises concerns regarding the potential infection of recipients with both recognized and unrecognized infectious agents and the possible subsequent transmission to their close contacts and into the general human population. Of public health concern is the potential for cross-species infection by retroviruses, which may be latent and lead to disease years after infection. Moreover, new infectious agents may not be readily identifiable with current techniques.'
In January 2001, the American Society for Microbiology published a paper titled 'Infectious Disease Issues in Xenotransplantation'. The paper states, 'Infectious diseases passed from animals to humans under natural circumstances are called zoonoses...[T]he breaches of normal host defenses inherent in xenotransplantation may enable infectious agents that are unable (or poorly able) to infect humans under normal circumstances to cause infections... The risk for such infections would presumably increase with both increased immunosuppression [treatment to reduce the immune response so as to avoid organ rejection] and possible introduction of potentially pathogenic microorganisms through the xenotransplantation product, bypassing the normal defensive mechanisms.' The same point was made in a paper published by The American Journal of Transplantation on October 1, 2017. The paper states, 'From a theoretical standpoint, PERV (porcine endogenous retrovirus) represents the most serious challenge because it is present at multiple copies in the pig genome, and thus unlike other viral pathogens cannot be eliminated by breeding.' Other, non-medical commentators have expressed similar concerns. In a statement first released by PETA (People for the Ethical Treatment of Animals) on October 21, 2021, the organisation states, 'Even before the COVID-19 pandemic, public health experts-including a Centers for Disease Control and Prevention researcher and researchers with the World Health Organization-raised concerns about the potential of xenotransplantation to spread zoonotic and other infectious pathogens. Their warning? These transplants are dangerous to humans, as pigs carry viruses and other infectious pathogens that could potentially be introduced into human populations.'
3. Using animal organs for human transplants treats human patients as test subjects
Critics of animal organs being transplanted into human beings argue that this procedure is too dangerous, and the human patients are being used as test subjects in what are merely experiments.
Critics point to earlier periods of animal to human transplantation and highlight the experimental and unsuccessful nature of the procedures. They suggest they were not undertaken with the real intent to cure the patient or prolong their lives and instead were experiments. It is further suggested that patients were not adequately informed about the procedures and other options which may have existed.
In the 1960s, Louisiana surgeon, Keith Reemtsma, transplanted 13 human kidneys into human patients. Twelve of these patients died of either infection or rejection within two months and the thirteenth died nine months later of acute electrolyte disturbance. Despite a lack of success, such animal to human transplants continued sporadically until the 1980s.
On October 26, 1984, Dr. Leonard Bailey and his associates at the Loma Linda University Medical Center in California implanted a heart from a seven-month-old baboon in a human infant born with a life-threatening heart condition. Baby Fae, as the newborn child has become known to preserve anonymity, lived for twenty days before dying on November 15 due to rejection of the foreign tissue and kidney failure. Ethical reviews following the child's death raised questions about the experimental rather than therapeutic nature of the transplant; expressed doubts that the procedure offered a greater chance of survival than any less radical alternative; and questioned the quality and extent of information provided to baby's mother regarding other treatment options, the risks of the procedure, and prognosis for the child. Dr. Paul Terasaki, director of the California Regional Organ Procurement Agency, has stated, 'My feeling is that they should have tried harder to get a human heart. I think they did not make any effort to get a human infant heart because they set on doing a baboon.' Concerns about the effectiveness and ethics of animal to human transplants following the Baby Fae transplant saw such procedures all but cease.A related concern regarding animal to human transplants is that the organ recipient, as well as being inadequately informed, may not be able to consent freely to the transplant. It has been noted that patients are generally only offered an animal organ as a transplant when they are in immediately life-threatening ill health. In these circumstances, their desperation for a treatment may make it difficult for them to make a fully considered decision. The United States Department of Health and Human Services outlines some of the information that must be available to the patient if consent is to be reasonably given. These include: background and history of the particular procedure, including previous related trials and outcomes and relevant results from animal studies; a description of the procedure(s) to be followed, including identification of those that are experimental; a description of the risks and potential benefits, if any, of the procedure; available alternatives (both accepted medical practices and other experimental approaches), including their comparative risks and benefits; possible social, economic, psychological, and/or medical consequences to the subject and his or her family.
It has also been noted that because animal-to-human transplants involve the risk of the recipient contracting one or more diseases which can then be passed on to family, friends and other contacts, these people should be informed of the risks they are facing.4. Using animal organs for human transplants transgresses animal rights
Those who oppose transplanting animal organs into human recipients also stress that this practice is morally questionably because it ignores the rights of animals and often involves inflicting pain upon them in addition to taking their lives.
One of the major arguments against using animals to supply organs for human transplants is that this is a total violation of animals' right to life. It treats animals as though they exist only to provide materials for human beings. It reduces them to the level of objects or apparatus with no right to their own existence. This is made even more the case because most animals likely to be used for transplantations will have been breed expressly for this purpose. Clare Palmer, a British philosopher, theologian, and scholar of environmental and religious studies, is currently a professor in the Department of Philosophy at Texas A&M University. Professor Palmer has explained, 'From a rights perspective, this treats pigs merely as means to our ends, almost as a kind of mechanism, whose entire life is created and disposed of for human use.'
Another objection to the use of animals for human transplants is that the process involves suffering for the animals concerned. Before animals such as pigs are used to supply transplant organs for human beings, they are used in cross-species transplant experiments. To lay the groundwork for pig organ-to-human transplants, xenotransplantation researchers and regulatory agencies agree pig organ transplants must first show survival rates of three to six months in nonhuman primates. Thousands of monkeys, chimpanzees and baboons have been experimented on and killed in the course of this cross-species transplant research.
The BUAV (British Union Against Vivisection) claim that primates used in xenotransplantation research will experience a large number of traumatic procedures, including major surgery, from which many will die; internal haemorrhages; isolation in small cages; repeated blood sampling; wound infections; nausea, vomiting and diarrhoea because of immunosuppressant drugs and kidney or heart failure. Dr. Gill Langley, a Fellow of the Royal Society of Medicine and a member of the Animal Procedures Committee since 1998, has stated, 'It's not just the suffering they [the primates] endure in the laboratories and research establishments. Just getting there can be torture. Studies of primates show them to have complex mental abilities which may increase their capacity to suffer. Supplying the laboratories in the UK imposes huge suffering on the animals. It involves capturing wild individuals, usually in Africa. They're then contained in small, single cages, and transported for very long distances causing deaths, distress, and suffering. A number of inquiries have said that the use of primates was unacceptable and should be limited to very small numbers. Our view is that any number is unacceptable.' Other animal rights activists have explained the suffering that will be imposed on those pigs bred in specifically engineered, hygienic conditions to be donor animals. Daniel Lyons is the chief executive officer of the Centre for Animals and Social Justice, a British animal protection charity. He is an honorary research fellow at the University of Sheffield and the author of The Politics of Animal Experimentation. Lyons has explained, 'One of the most important natural behaviors of pigs is rooting and foraging behavior, and they spend about three-quarters of their waking lives rooting and foraging for food. And obviously, in nature, they would have a virtually infinitely complex environment to explore; they would have room to socialize with their fellows. They're at least as intelligent as dogs. We're talking about very, very intelligent, sensitive animals. But none of this will be afforded to them if they're being factory-farmed. Because of the needs for the relatively sterile conditions, they won't have any of this rooting and foraging behavior.
And the suffering starts way before that, because in order to minimize the bacteria that they'll be carrying, the piglets, or some of them at least, will be born by cesarean section, rather than being born naturally and having a bond with their mothers. The separation of the sow from the piglet normally wouldn't be allowed, because it's very important for the piglet's health, both psychologically and physically, for it to have an early relationship with its mother. But it'll be taken away and reared in incubators. It will be a very, very sterile production procedure.'
5. Alternatives to animal organ transplants are being developed
Critics of animal transplants argue that technologies are being developed which will provide better alternatives without either the ethical or practical problems created by using animal organs. These 3D printed organ substitutes create no ethical concerns and do not create rejection or disease-transfer problems. Like animal organs, once developed they will also be available without any supply limitations.
Scientists have used variations of 3D printing techniques to print mini organoids and microfluidic models of tissues, also known as organs on chips. Some of these models are used by pharmaceutical companies to test drugs before moving on to animal studies and eventually clinical trials. One group, for example, printed cardiac cells on a chip and connected it to a bioreactor before using it to test the cardiac toxicity of a well-known cancer drug.
Some companies have had some success with 3D printing full organs. Robby Bowles, a bioengineer at the University of Utah, has stated, 'There are a number of companies who are attempting to do things like 3-D print ears.' Researchers have already reported transplanting 3-D printed ears onto children who had birth defects that left their ears underdeveloped. Bowles has noted, 'The ear transplants are kind of the first proof of concept of 3-D printing for medicine.'
Advances are being made toward constructing components of the kidney and the whole organ. In 2016, scientists from the Lewis Lab at Harvard University developed a novel bioprinting method that allowed the creation of small segments of the nephron, called proximal tubules. Nephrons are the most basic structure of the kidney and responsible for all the blood filtering in these organs. In 2019, American bioprinting company Organovo announced the successful automated production of kidney organoids. Organoids are self-organising stem-cell-based structures that can be produced in large numbers relatively quickly. In September 2020, the United Therapeutics Corporation partnered with Israeli regenerative medicine company CollPlant Biotechnologies to apply its material technology (rhCollagen) to kidney bioprinting development.
Similar advances are being made toward the construction of other organs. In 2016, researchers from the University of California San Diego were able to 3D print organic tissue that mimicked real liver structures both in architecture and function. Back then, such bioengineered tissues were used by the pharmaceutical industry for drug development and testing. Organovo (mentioned above with their kidney advancements) was also able to 3D bioprint liver tissue patches, and in 2018, it went further to implant them into living mice. The results were very positive, with tissue retention and functionality verified a month post-implantation. Brazilian researchers from the University of S�o Paulo reported successful bioprinting 'miniature livers' in late 2019. These organoid structures were from human blood cells and performed liver normal functions such as producing proteins, storing vitamins, and even secreting bile. According to the researchers, the entire process took approximately 90 days from collecting the patient's blood to the final maturing of the mini-livers.
Supporters of this technology note that the most promising projects of 3D bioprinted organs are for the heart. The heart is one of the easiest organs to recreate because it does not employ any complex biochemical reactions. Rather, its primary function is to act as a hydraulic pump. The Wake Forest Institute for Regenerative Medicine (WFIRM) is an American research institute focused on tissue engineering for various applications, including transplantation. In 2018, a WFRIM research group claimed to have 3D bioprinted functional cardiac tissue using mice cells. In 2019 researchers from Tel Aviv University's (TAU) School of Molecular Cell Biology and Biotechnology produced the first fully-vascularised 3D printed mini human heart. This bioprinted organ was made from human cells taken from a patient and carrier gels. TAU's team is now working to mature the cardiac cells and make them fully functional. The Chicago-based company Biolife4D has also accomplished a similar breakthrough in 2019, announcing its own bioprinted heart. This one, however, shows extended functionally when compared to the one developed at Tel Aviv University and is much larger too. Specialists predict that a ready-to-transplant bioprinted heart could be available within the decade.