After the death of the first recipient of a pig heart, doctors announced that they had found a pig virus in the man.
Surgeon Bartley Griffith told the New York Times, however, that it was unclear whether the virus was directly responsible for the 57-year-old man's death in March.
It could also be that it was silent and had no effect on the patient.
The newspaper "USA Today" quoted another doctor from the conducting University of Maryland as saying the virus could be one of several factors that ultimately led to death.
At the beginning of January, 57-year-old David Bennett was the first patient in the world to have a pig heart implanted as a replacement organ.
After the operation, the condition of the seriously ill man was initially relatively stable.
However, after more than a month it had deteriorated rapidly.
The man died in early March.
Experts still see the transplantation as a breakthrough and great success in the field of xenotransplantation – i.e. the transfer of animal organs to humans, which has been researched since the 1980s.
The heart transplant was preceded by a complex procedure: The pig was bred by a specialized company, genetically modified and the organ was repeatedly examined for viruses that could have infected the patient.
According to the New York Times, however, the virus tests carried out are only designed to find active pathogens.
Apparently, this was not the case with the cytomegalovirus found.