 and the primary transmission vector, which is the common face fly. So for pinkeye to get started, the eye of the animal must become irritated in some fashion, whether that be getting scratched, getting something stuck in it, or some other irritation that causes the eye to weep. So tall pasture grass is a prime culprit for scratched eyes. Once the weeping begins, face flies have an attraction to the moisture, and the flies then transfer M. bovis to the irritated eye of the animal, and the pinkeye sets in.