Because there are not enough donors of organs, in the future people may be helped by transplants of organs from animals. As yet no one has received a heart, liver, or kidney from a pig, but the topic of "xenotransplantation" is now being hotly debated by doctors, patients, and lawyers.
At the moment, heart valves from pigs are routine. And medical researchers are now studying whether cells from animals can be used to help the human pancreas produce insulin. It also may be possible soon to use skin from pigs to help people with severe burns.
Behind this interest in using animal organs for medical purposes is a shortage of human donors, which means people must wait a long time for a transplant. The group Eurotransplant reports that as of May 1, in Germany alone 11,199 people were waiting for one of five human organsheart, liver, kidney, lung, or pancreas. Last year, there were just 3,130 transplants of such organs.
At a recent symposium of the German Xenotransplantation Working Group (DAX) and the Robert Koch Institute in Berlin, a range of medical specialiststransplant surgeons, immunologists, and virologistsdebated with lawyers and ethics experts the question of animal organs.
Pigs are currently the favored animal because their metabolism is similar to that of humans. But the experts agree that it is still too risky to consider transplanting an entire organ from a pig, and that much more research must be conducted to reduce the possible dangers to people.
The first experiments using pig skin for people with serious burns have not been very successful, said Ralf Toenjes of the Paul Ehrlich Institute. And tests with insulin-producing cells to help people with diabetes have also not been very satisfactory.
But some success has been achieved in using pigs' heart valves and in treating patients whose livers stopped functioning. The cells from pigs' livers are not implanted, but instead the blood of a patient is put into circulation outside the body and cleaned up with the aid of liver cells from pigs. This helps a patient to bridge the period while waiting for a human liver.
Scientists say that at least three problems must be solved if people are going to be helped with a complete animal organ transplant.
First is the question of the human immune system. It is already problematic getting the immune system to accept an organ from another human. Patients must take medications so that the new organ will not be rejected, or less violently so, by their own immune systems. The risks involving an animal organ will be all the greater.
Second, there is the question of to what extent a pig organ could function in a human body. Humans and pigs followed separate evolutionary paths about 90 million years ago. Both still have roughly the same mass of kidney, but in humans this organ excretes a different amount and composition of uric acid than its counterpart in pigs.
Finally, there is the danger of infection from pig viruses. While veterinary medicine has eliminated many of them, porcine endogene retroviruses remain in pigs' genetic code, which could invade human cells. Joachim Denner of the Robert Koch Institute believes there are at least three such retroviruses capable of doing this.