The world's first pig to human heart transplant is "hugely thought provoking" but the patient faces unpredictable complications for the rest of his life, a Scots specialist has said.
David Bennett, 57, received the organ from a genetically modified pig during a seven-hour procedure in Baltimore in the United States.
The transplant was described as a "gamble" by the team and the last hope of saving Mr Bennett's life, though it is not yet clear what his long-term chances of survival are.
Doctors at the University of Maryland Medical Center were granted a special dispensation by the US medical regulator to carry out the procedure, on the basis that Mr Bennett - who has terminal heart disease - would otherwise have died.
Surgeon Bartley Griffith said the surgery would bring the world "one step closer to solving the organ shortage crisis".
The average wait for an adult who needs a heart transplant is nearly three years and many patients die before a match is found.
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A pig’s heart is similar in size, weight, and structure to a human’s heart but not identical, obviously and experts say the bigger issue is organ rejection.
Scientists altered ten genes in the pig whose heart was used for Mr Bennett's transplant so it would not be rejected by his body.
Dr Jonathan Dalzell, a consultant cardiologist at NHS Golden Jubilee said it is not yet clear if the development will translate into any "viable and feasible" clinical treatment in the future.
"This is hugely thought provoking," said Dr Dalzell, who leads the Scottish National Advanced Heart Failure Service (SNAHFS).
"Any development that has the potential ability to circumvent donor organ shortages and large transplant waiting lists will spark huge interest amongst the transplantation community and beyond.
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"However, this is a one-off and highly experimental procedure.
"The patient is very sick and has undergone a massive operation. The team in Baltimore will be monitoring for and dealing with any of the potential myriad of accepted and well known complications following a transplant procedure.
"They also have the very difficult task of trying to be as ready as they can possibly be for any completely unforeseen complications that could result from stepping into the unknown in this way.
"This will be the case not just for now, but for the rest of the patient’s life."
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Doctors have been trying to use animal organs for what is known as xenotransplantation for decades, with mixed success.
However, pig heart valves have been successfully implanted in humans for more than 30 years.
Under sterile conditions, the valves are removed from the pig’s body with excess tissue and myocardium removed. They are then “sized” so they are appropriately fit when implanted into a human.
The pig valve is typically mounted to a stent (frame) that can be reinforced with Dacron cloth and sutures.
Mr Bennett's treatment has re-ignited a debate over the use of animals for human transplants, which many animal rights groups oppose.
People for the Ethical Treatment of Animals (PETA) described the procedures as: "unethical, dangerous, and a tremendous waste of resources".
The charity added: ”The risk of transmitting unknown viruses along with the animal organ are real and, in the time of a pandemic, should be enough to end these studies forever.
“Animals aren’t toolsheds to be raided but complex, intelligent beings.”
In October 2021, surgeons in New York announced that they had successfully transplanted a pig's kidney into a person. At the time, the operation was the most advanced experiment in the field so far.
However, the recipient on that occasion was brain dead with no hope of recovery.
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