I wonder if the recurrences people have experienced are because, where treatment has started immediately there is no time for resistance to Pink Eye to develop in each animal? Effectively then it would be as if each exposure was the first.
Just a thought.
We found that most sheep needed no treatment and the Pink Eye cleared up within a few days on its own. If it persisted then we used Orbenin until the eyes began to improve, then stopped it again. We used the LA IM AntiB (which if I remember correctly was Terramycin) only on animals whose eyes continued to be bad, which was never many.
The Australian sites quote flies and windblown dust as the main causes of Pink Eye, but here blown hay fragments can be guilty, and untreated flies in places where they are present - we don't get head/eye flies here as it's very open and windy, lucky us.