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Why would anyone ever consider transplanting an animal organ into a human being?

Click below to see the waiting list for donor organs in:


FAQs on Xenotransplantation

  1. What is xenotransplantation?

Xenotransplantation (pronounced ZEENO-transplantation) is the process of transferring (transplanting) organs, tissues, or cells from one animal to another. The term comes from combining "xeno", the Greek world for stranger, with the word "transplant".

  1. Transplantation between which animals?

Xenotransplantation may apply to transferring (transplanting, implanting, or infusing) organs, tissues, or cells from one species of animal to another, such as from a pig to a baboon, or from an animal (such as a pig) to a human.

  1. Are there different type of xenotransplants?

Yes. There are four basic types.

  • Solid Organ Xenotransplant: removing an organ, such as a kidney, liver, lung, or heart, from a donor animal and transplanting it into a recipient animal (or human)
  • Cellular or Tissue Xenotransplant: grafting tissues or cells (such as islet [insulin-producing] cells) from a donor animal and grafting or implanting it directly into the organ of a recipient (such as the pancreas of a person with type 1 diabetes)
  • External Therapies: filtering/purifying human blood cells outside of the body through an animal organ (such as a kidney) or cells in an external device
  • Human/Animal Hybrid: human cells grown in culture with non-human animal cells that are transplanted back into human patients

In addition, there are non-living animal tissues, such as pig heart valves, that have been used for repairing hearts in humans for many years.

  1. Why would anyone ever want to perform animal-to-animal xenotransplantation?

Reasons include:

  • Treating Cancer: human tumor cells for breast cancer and other diseases are often injected into mice as a "preclinical model" for determining the effectiveness of an anti-cancer treatment.   At present, although the use of animals for these kinds of experiments may be minimized, it is difficult, if not impossible, to eliminate them altogether.
  • Experiments for Transplanting Organs into Humans: many, if not most, animal-to-animal solid organ xenotransplants that have been performed to date have been to prepare the way for transplanting kidneys, livers, and hearts from animals, typically pigs, into humans
  1. Why would anyone ever want to resort to using an animal donor's organ? Don't people donate enough organs?

The shortage of available, healthy human organs for transplantation is great and shows no signs of decreasing in the foreseeable future.   Click on the links below to see the waiting list for (the):

  1. What kinds of animals are potential donor organs for human beings?

Pigs (including miniature swine)

  1. Why not use (non-human) primates, such as chimpanzees and/or baboons, for donor organs? They are closer to human beings than pigs.

It is precisely because they are, in an evolutionary sense, closer to us than pigs. The reasons include safety, ethics, and economics:

  • Safety: Because primates are similar to us, they are more likely to harbor diseases, including deadly viruses that might easily jump from their species to ours.   Because pigs are substantially different from us on the evolutionary scale, the thought is that the infectious agents afflicting pigs would be less likely to affect humans.
  • Ethics: Because primates are similar to us, there is generally greater reluctance at using them as source organs. The term "cannibalism" has been mentioned.
  • Economics: Baboons and chimpanzees require years before becoming fertile and produce smal litters.
  1. Why use pigs (including miniature swine) for donor organs?
  • They can reproduce large litters in months
  • Their organ sizes are generally similar those in humans
  • They are easier to rear in conditions that are free of disease-causing organisms
  • Their risk of infecting humans with pathogens is much smaller than those of non-human primates
  • They can be genetically manipulated to reduce the risk of organ rejection
  1. What health problems could xenotransplantation solve?

People with organ failure and in desperate need of a:

  • Kidney (such as patients on dialysis and/or in end-stage renal disease)
  • Liver
  • Heart
  • Lung
  • In addition, cell or tissue xenotransplants could be used to treat:
  • Diabetes
  • Alzheimer's disease
  • Parkinson disease
  • Huntington disease
  1. Are solid animal organs being transplanted into human beings today?

No. (At least none have been authorized or reported in sanctioned medical or scientific circles.) Research today is all preclinical, that is to say, xenotransplants of organs from one animal to another---primarily, from miniature swine or pigs to baboons.

  1. When were the last xenotransplanted organs in people?

In 1984, Dr. Leonard Bailey transplanted a baboon heart into a newborn infant dubbed "Baby Fae". They baby died after 20 days. In 1992, Dr. Thomas Starzl at the University of Pittsburgh transplanted a baboon kidney into a patient with AIDS and hepatitis B. The patient died after 70 days. In 1993, the procedure was repeated on a patient with hepatitis B, but the patient never regained consciousness.

  1. What are scientists having difficulty making xenotransplanting organs work in people?

There are two main challenges:

  • Keeping the human body's immune system from rejecting the animal organ and
  • Ensuring that the new organ functions after xenotransplantation

To date, overcoming the human immune system appears to be the greatest challenge.

  1. What immune problems does xenotransplantation face?

The human immune system is the product of hundreds of millions of years of development. Provided by Nature, it protects us against infection but reacting to anything it doesn't recognize as part of the human body (self). We live in an ocean of microorganisms (bacterial, viruses, etc.), both helpful and harmful. Without an immune system, however, every microorganism is potentially lethal and without constant effective treatment, we would die within days. But the human immune system was designed to attack anything it recognizes as "foreign". It was never designed in mind to accommodate potentially life-saving transplanted organs.

The human immune system has been shown to have four different rejection processes:

  • Hyperacute: The body quickly destroys the organ, often within hours, because it recognizes a specific sugar molecule called gal [galactose-(alpha 1,3)-galactose] and other key molecules on the organ cells that it considers "foreign".
  • Delayed (vascular): Over months, the blood vessels of the transplanted organ are attacked by antibodies and immune cells. The cause it not fully understood.
  • Acute (cellular): Over months, the T-cells of the immune system attack the transplanted organ.
  • Chronic: The progressive destruction of the transplanted organ over months to years, possibly due to antibodies to the organ. The process is not fully understood.
  1. What's being done to overcome the immune problem?
  • To make animal organs more compatible with the humans, animals are being genetically modified to "knock out" genes that produce cell parts which the human immune system would reject. There has already been some success: the gal gene has been knocked out of pig DNA.
  • Pigs being cloned to produce new lines of pigs which are closer to human size (miniature swine) and have genetically more compatible organs.
  • For some cell xenotransplants, cells are being encapsulated so that they are far less likely to be rejected
  1. Does xenotransplantation pose a risk to the community as a whole (that is, people other than patients)?


The greatest danger is from what are known as "porcine endogenous retroviruses" (PERVs).   PERVs are retroviruses that are embedded in pig DNA and could be potentially transmitted to patients, and spread beyond just patients, following xenotransplantation, especially from solid organs.

As described by the Australian Government's National Health and Medical Research Council, PERVS are "...present in almost all strains of pigs and cannot be removed by raising pigs in sterile conditions. Although PERV is inactive, and therefore harmless in pigs, there are concerns that transplantation into humans may activate the virus, creating a new human disease that could spread to those close to the transplant recipient and eventually to the wider community. PERVs can infect human cells in the laboratory, suggesting that they could infect humans through xenotransplantation..."

Worse, retroviruses do not always initially cause obvious signs of a disease. If a retrovirus were present in a xenotransplant organ were to infect the human recipient of that organ, it could spread to close contacts, caregivers, and even the general population before it had even become obvious that an infection had occurred.

On the other hand, a study of some 150 patients who have had pig transplanted tissue or had their blood pass through pig cells have shown no evidence of infection with PERVs. Another study has shown no transfer of PERVs from pig to human cells in cell cultures. However, there have been no studies demonstrating that the risk from PERVs is minimal or can be entirely eliminated. Tests are available to test for the presence of PERVs, and new tests are being developed. However, these will only be able to test for those elements of PERVs for which they are designed; they cannot prove the absence of all PERVs.

The risk from PERVs is sufficiently serious that the US Food and Drug Administration Guidelines call for establishing a national database for xenotransplantation, maintaining specimens from animals and xenotransplant recipients for as long as 50 years, lifetime monitoring of xenotransplant patients and mandatory autopsy upon death, repeated monitoring of health care providers, and all (intimate) contacts of the patient for life.

  1. Have viruses ever "jumped" from animals to people?

There are many examples of viruses moving from one species---some with widespread, deadly consequences.   Probably the best examples are:

  • HIV (AIDS-causing virus) which appears to have originated in non-human primates from SIV (simian immunovirus)
  • The 1918 influenza that killed millions worldwide appears to have originated from bird (avian) viruses. (For years, it had been thought to have originated in pigs. In either case, the deadly virus jumped from animals to people.)

As well, many flu strains that arise annually appear to originate in animals, often in the far East.

  1. What is "xenotourism"?

"Xenotourism", is defined by the US Secretary Advisory Committee on Xenotransplantation (SACX) as describe personal travel outside of a country of residence for the purpose of participating in xenotransplantation programs or attending clinics to obtain therapies not presently available or acceptable in the home country." In short, it describes people who go to another country to obtain an organ (for transplantation) to circumvent the waiting list that exists in their home country. If xenotransplantation becomes an acceptable practice, xenotourism could lead to severe health problems, both for the recipient and the community at large because of the enormous number of safeguards that would be required for this technology.

  1. What are the ethical arguments for and against the use of (solid organ) xenotransplantation?

The arguments, on both sides, are numerous, complicated, and often based on personal beliefs. Unlike most medical procedures in which arguments are often based on weighing the risks and benefits for the patient, we also need to consider the risks versus the benefits for the community at large . Animal rights issues often come into play.

Pro-xenotransplantation arguments include (but are not limited to):

  • Hundreds of thousands of lives could be saved
  • Patients (such as with cancer) who might not otherwise be eligible could receive organs
  • Minimal time on waiting lists which might lead to patients (in improved conditions) having a better chance at survival
  • Easier to obtain a second organ for transplantation
  • Could eliminate many, lengthy, poor quality of life situations for patients, such as kidney dialysis
  • Decrease likelihood of receiving "partially damaged" organs
  • Could eliminate "black market" in human donor organs

Anti-xenotransplantation arguments include (but are not limited to):

  • Potentially create and spread serious disease(s) from animals to humans, perhaps developing into plagues that may inflict the community at large
  • Long-term monitoring as proposed by FDA and other regulatory agencies likely unenforceable, leading to potential abuse, and leading to new global diseases
  • Animal rights issues (e.g., cruelty, inappropriate use of animals)

There is an excellent, debate on xenotransplantation available here.

  1. What are the alternatives to xenotransplantation?

There are an insufficient number of human organs currently available for donation, and there are no projections that increasing the donor rate will cover the growing shortage. Alternative technologies to xenotransplantation currently being evaluated globally include:

  • Stem cell research (limited in the US by Federal legislation)
  • Gene therapy
  • Artificial organs

Want to know more on xenotransplantation?

Background Information on Xenotransplantation

Scientific/Scholarly Sources of Information on Xenotransplantation

Government/Regulatory/Guideline Documents on Xenotransplantation

Transplant-Related Professional/Medical Organizations

Organ Transplant Donor/Recipient Networks and Registries

Industry & Companies Involved in Xenotransplantation Research

Organizations Whose Members Might Benefit From Xenotransplantation

Organizations Supporting Xenotransplantation

Organizations Opposing Xenotransplantation

Animal Rights Organizations Opposing Xenotransplantation

Bioethics: Sites/Articles/Perspectives

Xenotransplantation Articles on the Web

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