By Allison Prang

A man who had the first transplant to replace his human heart with a genetically-modified pig’s heart without immediate rejection died Tuesday afternoon at the University of Maryland Medical Center in Baltimore, two months after the groundbreaking surgery.

David Bennett’s condition began deteriorating several days ago, the University of Maryland Medical Center said Wednesday. The 57-year-old transplant patient had terminal heart disease that led to the surgery on Jan. 7.

It wasn’t immediately clear if his body rejected the organ.

A spokeswoman for the University of Maryland School of Medicine said there was no obvious cause of death and that Mr. Bennett’s physicians plan to conduct a thorough review on the matter.

“We couldn’t have asked for a better organ,” said Dr. Bartley P. Griffith of the University of Maryland Medical Center, who transplanted the pig heart into Mr. Bennett.

While he said the patient couldn’t overcome debilitation from past heart failure, he also touted the transplant.

“It’s a collective plus for the field,” Dr. Griffith said.

Researchers have been working for decades on xenotransplantation, or the transplantation of an organ between two species, amid an organ shortage. Roughly 106,000 people are on the waiting list for organ transplants, according to the U.S. Department of Health and Human Services’ Organ Procurement and Transplantation Network.

While Mr. Bennett only lived with the pig heart for a couple of months, Dr. Parsia Vagefi, UT Southwestern Medical Center’s chief of the division of surgical transplantation, said people shouldn’t view the transplant as a failure and that he hopes it serves as a “new beginning” for xenotransplantation.

“I think what this shows is just the enormous amount of progress that’s been made and hopefully it’s just the beginning that we continue to grow on,” he said.

Mr. Bennett wasn’t eligible for a more typical heart transplant because he didn’t comply with doctors’ orders or attend follow-up visits. Several transplant centers—including the Maryland one—declined to list him for the chance to get a human heart, according to David Bennett Jr. , Mr. Bennett’s son. He also didn’t regularly take his medication, the younger Mr. Bennett previously said.

The U.S. Food and Drug Administration had granted Mr. Bennett’s operation emergency authorization on New Year’s Eve. “It was either die or do this transplant,” he said the day before his surgery, according to the University of Maryland Medicine. The handyman and father of two called the transplant his “last choice.”

Dr. Griffith had called the transplant “a breakthrough surgery” and had said it “brings us one step closer to solving the organ shortage crisis.”

Advances in xenotransplantation have been made in recent decades. Human heart valves, for example, are regularly replaced with ones from pigs. They have in particular been a promising organ source because some of their organs are similar in size to humans’ and they also have large litters. But others have criticized the practice, calling the transplants dangerous and unethical.