A 57-year-old Maryland man who survived for two months with a heart transplanted from a genetically altered pig carried signs of a virus that the animals are known to carry, according to the surgeon who performed the first-of-its-kind procedure.

The disclosure bolsters one of the most pressing objections to animal-to-human transplants, which is that widespread use of modified animal organs may facilitate the introduction of new pathogens into the human population.

The presence of viral DNA in the patient may have contributed to his sudden deterioration and death on March 8, Dr. Bartley Griffith, a transplant surgeon at the University of Maryland School of Medicine, said during a presentation to the American Society of Transplantation.

Griffith’s comments were first reported by MIT Technology Review.

The pig was genetically modified so that its organs would not prompt rejection by the human immune system. The heart was provided to the patient, David Bennett Sr., by Revivicor, a regenerative medicine company based in Blacksburg, Virginia.

Company officials declined to comment Thursday. University officials said the animal had been screened for the virus, called porcine cytomegalovirus. But the tests pick up only active infections, not latent ones in which the virus may hide quietly in the pig’s body.

Bennett’s transplant was initially deemed successful. He did not show signs of rejecting the organ, and the pig’s heart continued to function for well over a month, passing a critical milestone for transplant patients.

A test indicated the presence of porcine CMV in Bennett 20 days after the transplant, but at such a low level that Griffith said he thought it might have been a lab error. At 45 days after the surgery, Bennett became acutely ill, and subsequent tests showed a precipitous rise in levels of the virus, Griffith said.

“So we started thinking that the virus that showed up very early at Day 20 as just a twinkle started to grow in time, and it may have been the actor — it could have been the actor — that set this all off,” Griffith told other transplant scientists.

“At Day 45, he looked really funky,” Griffith said. “Something happened. He looked sick. He lost his attention. He wouldn’t talk to us. He lay in bed breathing hard, and was kind of warm.”

The heart transplant was one of several groundbreaking transplants in recent months that offer hope to the tens of thousands of patients who need new kidneys, hearts and lungs amid a dire shortage of donated human organs.

But the prospect of unforeseen consequences — and particularly the potential introduction of an animal disease into the human population — may dampen enthusiasm for the use of genetically modified organs.

Many scientists believe that the coronavirus pandemic originated with a virus transmitted from an animal, as yet unidentified, to people in China.

This article originally appeared in The New York Times.