A US medical team on Thursday announced it had carried out the second-known kidney transplant from a pig to a human, the first inside the body of a brain dead recipient.

The procedure, which was described in a scientific paper, comes on the heels of the successful implantation of a porcine heart into a person earlier this month.

It is hoped that advances in the field of so-called xenotransplantation, or cross-species organ donation, could one day solve the chronic shortage of organ donations.

"Today's results are a remarkable achievement for humanity and advance xenotransplant into the clinical realm," said Selwyn Vickers, dean of the University of Alabama at Birmingham (UAB) Heersink School of Medicine, which carried out the procedure.

The first pig kidney was transplanted in a human by a team at New York University (NYU) Langone on September 25 2021, and involved a brain dead patient on a ventilator whose family had given permission for the proof-of-concept experiment.

That procedure involved attaching a kidney to blood vessels on the top of one of the patient's legs, so that scientists could observe it and take biopsy samples.

The same team carried out another similar experiment on November 22.

- Fully connected -

The newly announced surgery took place on September 30, 2021. It involved placing two kidneys from a genetically-modified pig inside a person, 57-year-old Jim Parsons, who had wanted to be an organ donor but his organs were deemed unsuitable.

"The transplanted kidneys filtered blood, produced urine and, importantly, were not immediately rejected," UAB said in a statement.

The kidneys remained viable until the study was ended 77 hours later, and the findings were published in the peer-reviewed

"American Journal of Transplantation."

Moreover, because the kidney was fully connected inside the body, the UAB team says their procedure is a step closer to becoming a clinical reality. They plan to soon move into human trials and then seek regulatory approval.

The donor pig had 10 key genetic modifications to make its organs suitable for human transplant.

All donor pigs involved in the four known pig-to-human transplants came from a herd from Revivicor, a subsidiary of the biotech company United Therapeutics Corporation. Previous research has found such pig transplants suitable for non-human primates.

According to official US data, there are nearly 107,000 Americans awaiting an organ -- 90,000 of whom need a kidney. Seventeen Americans die each day while waiting for an organ.

Early xenotransplantation research focused on harvesting organs from primates -- for example a baboon heart was transplanted into a newborn known as "Baby Fae" in 1984, but she survived only 20 days.

Today, pig heart valves are widely used in humans, and pig skin is grafted on human burn victims.

Pigs make the ideal donors because of their organ size, their rapid growth and large litters, and the fact they are already raised as a food source.