Scientists and regulators investigating the mysterious spread of a deadly virus plaguing the U.S. pork industry are stepping up their scrutiny of what the nation's hog herd eats.

With a dearth of solid leads, investigators are exploring whether something in pig feed could be a conduit for porcine epidemic diarrhea virus, which has spread to 27 states and killed millions of young pigs since it was first identified in the U.S. last April. One focus of the inquiry: porcine plasma, a widely used feed ingredient made from the...