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The debate over the safety of transplanting pig islets into humans became even more controversial in late August. Researchers at the British biotechnology company Imutran Ltd. determined that 160 people from eight countries who carry living pig tissue showed no signs of Porcine Endogenous Retrovirus (PERV) infection. According to Associated Press (AP) news wire reports, 36 of the 160 patients had a high risk of PERV infection because they had very weak immune systems. The study, the largest yet of people treated with pig tissue, was confirmed by testing at the U.S. Centers for Disease Control and Prevention (CDC).
Xenotransplantation is the term used for transplanting cells, tissue and organs from one species to another. Some doctors and scientists feel that xenotransplantation will one day save thousands of human lives because there is a worldwide shortage of donated human organs. With around 60,000 people in the United States on waiting lists for organ donations and 1 million type 1s needing islets, pigs can provide an unlimited supply of organs, tissues and islets to alleviate the shortage of human material.
Xenotransplantation, however, has aroused fears of new epidemics. Several scientists point to the AIDS virus as the primary example because it is believed that AIDS was first contracted in humans from viruses carried by monkeys. In addition, two years ago, British virologist Robin A. Weiss demonstrated that PERV can infect human cells in a test tube. Although PERV has never harmed pigs who carry it, there is speculation that it might hurt humans who contract it. As a result, Britain and a few other countries have declared a moratorium on xenotransplants.
Transplant Patients Show No Signs of PERV
According to the August 20 issue of the journal Science, 160 people carrying living pig tissue showed no signs of PERV. Patients in the study had been treated up to 12 years previously with pig islets for diabetes, pig skin grafts for severe burns and other pig organs for various maladies.
Camillo Ricordi, MD, chief of cellular transplantation at the Diabetes Research Institute in Miami, calls the Imutran study a very important study, but stresses continued caution.
"It remains to be demonstrated that PERV infection of humans is possible following transplantation of [pig] tissue," says Ricordi, who feels that despite the findings of the Imutran study, there is still the possibility of infection in a situation of prolonged exposure with a more substantial number of islets.
Jonathan S. Allan, a virologist at the Southwest Foundation for Medical Research in San Antonio, Texas, echoes Ricordi's sentiment. Allan was quoted in the August 20 issue of The New York Times as saying, "There is still a potential that a pig virus could be expressed and infect the patient at some later time."
Dr. Louisa Chapman of the CDC cautions that the findings of the Imutran study are "not proof that xenotransplantation is safe."
75 Years Since the Last Big Advancement in Diabetes Treatment
Alastair T. Gordon of the Islet Foundation feels that xenotransplantation of pig islets would be good for the advancement of diabetes treatment. In January 1998, he wrote that the last significant advance in the treatment of diabetes was the discovery of insulin over 75 years ago by Banting and Best.
"Their practice of 'injecting the filthy juices of dogs and pigs into children' was ridiculed and opposed back in 1921," writes Gordon. "Fortunately, science prevailed as it must today for all our sakes, and for the sakes of our children."
Pigs Cleaner Than Humans
Gordon says the risk of PERV infection has been present for hundreds of years, pointing out that humans have butchered pigs over the centuries, with literally millions of opportunities for viruses to enter human bodies through wounds and lesions.
"PERV infection of a human being has never been a public health issue," Gordon says, adding that as organ donors, pigs are a much safer source than human cadavers. "Pigs can be bred in a specific, pathogen-free environment to optimize the safety of their tissue for transplants. We have no control over the lives led by human donors, or the safety of their organs after brain death."
Some groups have always questioned the ethics of sacrificing pigs to cure any human disease. Gordon retaliates by saying, "Surely it is more ethical to sacrifice a pig to spare a child the horrors of diabetes, than it is to slaughter pigs for bacon, suede and paint brushes."
What About Another AIDS Epidemic?
Deb Butterfield, executive director of the Insulin Free World Foundation (IFW) says that the problems surrounding xenotransplantation are more political than scientific. Butterfield's IFW is "very pro-xenotransplantation," and she feels that the reason more studies have not been done on xenotransplantation is because of the fear of new epidemics like AIDS.
"To have a fear of something and not go further, because there is this unspoken fear, just seems like it's going to hold back a lot of progress," says Butterfield. "It's really a matter of getting the [U.S. Food and Drug Administration (FDA)] and everybody to say it's okay that we test these things."
Waiting for the Green Light
British regulatory authorities as well as officials at the FDA say they want better evidence that pig tissue is safe before allowing tests of whole-organ, cross-species transplants to proceed.
Gordon writes that every medical advance and procedure carries both risk and benefit, arguing that when the benefit outweighs the risk, there is progress.
"To deny islet xenotransplantation to people with diabetes is to ignore the profound benefits while irresponsibly exaggerating the undemonstrated risks," he says.
Allan feels xenotransplantations should be limited to small numbers of closely monitored patients, but adds that the Imutran study should not be a "green light" for xenotransplantations on a mass scale.
Ricordi feels the same way, calling the Imutran study one of "a series of intermittent red lights, were you have to stop, proceed with caution, stop again and so on."
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