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A modified pig heart used in a groundbreaking—but ultimately unsuccessful—surgery in January was found to have been infected with an animal virus, researchers revealed yesterday.

Dave Bennett, 57, was near death when he underwent the xenotransplantation, or cross-species operation. The donor heart came from a 1-year-old pig that had been genetically modified to remove a sugar molecule common to most mammals known as alpha-gal (listen to overview), which typically causes organ rejection. Bennett swiftly recovered from the surgery, but his health deteriorated after about six weeks and he passed away after two months. 

The findings suggest Bennett’s death may not have been linked to the procedure itself. The operation marked the culmination of decades of research, potentially opening a new source for organ transplants. More than 100,000 Americans are on transplant lists, and 6,000 patients die each year while waiting.