LA PAJUELA, Peru — It started as just another farm chore for Elvira Flores, a teenage shepherd in the northern Andean highlands. On Sept. 8, 2013, Flores drove her flock across a dirt road that crosses her family’s rocky green fields and down to a stream. After the sheep drank their fill, something went wrong. “All of a sudden they started to jump, kick their bellies and hit their heads against the ground,” the shy 16-year-old recalled on a blustery afternoon a year later, clutching her sweater to ward off the chill. “White foam came out of their mouths and noses.”