Maybe every full moon, the weresheep would perform a ritual and infect another sheep. Next full moon each sheep would infect another sheep, and their numbers increase exponentially, until they are numerous enough to start attacking people. You would not see the ritual, just the lamps would flicker for a few seconds you heard an ominous laugh coming from no particular place. You then have to search for the weresheep and kill it before it infects the rest of them. A weresheep would have a slightly different behavior from the rest, and you could not catch it with traps. It would stay near the shady areas whenever possible, and watch your village from a distance. It would be hard, but not impossible to stop a weresheep.
Of course, this would not happen too often, maybe 1/10 of the time at the start of a new game, but if not it would happen within the first 30 days of your village.