By Julie Steenhuysen

CHICAGO, (Reuters) – A terminally ill American man was fitted with a genetically modified pig’s heart. Three days later, the patient showed signs of improvement, according to his doctors on Monday.

University of Maryland Medicine’s team performed this surgery. It is one of the first that demonstrated the possibility of human-pig transplant.

Researchers hope that the use of pig organs can help to reduce organ shortages if it is successful.

“This was a breakthrough surgery and brings us one step closer to solving the organ shortage crisis. There are simply not enough donor human hearts available to meet the long list of potential recipients,” Dr. Bartley Griffith, who surgically transplanted the pig heart into the patient, said in a statement.

“We are proceeding cautiously, but we are also optimistic that this first-in-the-world surgery will provide an important new option for patients in the future,” Griffith added.

The heart transplant was the last resort for David Bennett, a 57 year-old Maryland resident.

“It was either do this or die. I would like to live. I know it’s a shot in the dark, but it’s my last choice,” Bennett said a day before his surgery, according to a statement released by the university.

In order to move forward with the experiment, the university was granted an emergency authorization by the U.S. Food and Drug Administration (FDA) on New Year’s Eve under its compassionate use program.

“The FDA used our data and data on the experimental pig to authorize the transplant in an end-stage heart disease patient who had no other treatment options,” said Dr. Muhammad Mohiuddin, who heads the University’s program on xenotransplantation – transplanting animal organs into humans.

According to organdonor.gov., approximately 111,000 Americans are waiting for an organ donation. More than 6,000 Americans die annually before they receive one.

Revivicor, an American regenerative medicine business based in Blacksburg (Virginia), provided Bennett’s genetically-modified pig heart. The transplant team took the heart of Bennett and put it in a special container to keep its functionality until the day.

Since their organs are similar to those of humans, pigs can be a promising source of transplants. At the point of slaughter, a hog’s heart is approximately the same size as an adult human heart.

Research is ongoing on other organs that can be transplanted into people from pigs such as the kidneys, liver, and lungs.

Previous attempts at transplants from pigs to humans have been unsuccessful due to genetic differences.

Scientists solved the problem by editing out potentially dangerous genes.

Bennett had three gene mutations that could have led to rejection of his heart. Six genes from the human population were also inserted in Bennett’s pig genome.

Researchers have also removed an pig gene that was causing excessive heart tissue growth.

This work was partially funded by a $15.7million research grant that Revivicor received to assess Revivicor’s genetically-modified porc hearts in baboon study.

Bennett also received genetic mutations to the heart of the pig, as well as an experimental antirejection drug from Kiniksa Pharmaceuticals (NASDAQ) in Lexington, Mass.