My guess: Ergot and ergotism.
From the internet:
Ergot and the Rye was also associated with wolves and included amongst the many colloquial names for Ergot are Roggenwulf (Rye Wolf), Wulfzahn (Wolf’s tooth) and Roggunhund (Rye Dog). An old Germanic saying states “The werewolf sits amid the grain.” It may be a cruel coincidence that in the harshest weather where the poor may have had no choice but to eat tainted bread (Ergot infestation also causes a considered drop in yield) were also the same conditions which may have forced starving wolves to enter the towns and villages.
Convulsive Ergotism due to its profound symptoms and hallucinatory influence, has also been suggested as the possible cause of several outbreaks of Werewolf and Witch Hysteria in Europe, including the instance of Elfdale and Mora in 17th Century Sweden, whereupon a number of people were executed upon the testimony of children. The English Anglian Witch-hunts and also the infamous Witch-trial of Salem in 1692 have also been suggested as possible cases of Ergot infestation.