Afraid that my whole flock had a Summer (or more like a year) of continually getting it. This included my ram, ewes and lambs. I treated with Alamycin LA and orbenin cream and was even thinking of culling the whole lot as it just seemed to be going on and on. Anyway, (fingers crossed) but I am happy to report 3 years or so with no outbreaks, and my flock of ewes is more or less the same. In my opinion it might have been spread by flies as it coincided with a very mild Winter and I seem to recall that that that Summer was terrible for fly-strike. If I have it in my flock again I might consider using a fly repellant like Spot-on to see if it makes any difference. I feel your pain, I shudder when I look back at my meds book and see all the entries and the fact that I had some animals getting it more than once. In my opinion you do need to treat promptly as they look so miserable when they have it and the damage can be long term if left untreated.
Helen