But determining when, and in whom, the mutations first appeared requires many more virus samples from farm workers, local residents and mink, collected before and after the outbreak. “That data doesn’t exist,” said Arinjay Banerjee, a virologist at the University of Saskatchewan.

Throughout 2020, testing was difficult for Americans to access and few patient samples were being sequenced. Surveillance in animals was even worse; until this spring, federal officials explicitly recommended against routinely testing animals for the virus.

“Widespread testing wasn’t available, then there became a shortage of certain supplies,” Dr. Behravesh said. “So we didn’t want there to be, you know, a mad rush to test animals.”

Without more samples, it’s impossible to rule out the possibility that the variant emerged in humans, who then spread it to mink, scientists said.

A bigger puzzle is how the taxidermist and his wife got it. The most likely possibility, several experts said, is that the variant was circulating more widely in the human population than was known, and the couple caught it from another infected person.

Another, more speculative, possibility is that they picked up the variant from another animal species. “Taxidermists deal with other dead animals,” said Linda Saif, a virologist and immunologist at Ohio State University.

But because the cases were detected “weeks to months” after the two fell ill, testing any animals they may have been in contact with “was either not feasible or not indicated,” said Lynn Sutfin, a spokesperson for the Michigan D.H.H.S.