A blinding epidemic?????

[cluckinafarm]

I am tossing this out to everyone with advanced knowledge about complete blindness occurring within your flock. I have searched for over an hour now and have found many articles about blindness, but all of the articles I found have other symptoms beyond the blindness associated with them. In our flock, the blindness occurs swiftly (within a day or two) and the eyes are clear, un-swollen and the chicken otherwise healthy. If placed in a small enclosure, given food and water they can find easily, they live fine with their blindness. If left out with the rest, they simply sit around until dark when I would then place them back in the hen house for the night, but they will not seek out food or water and would starve to death or die of dehydration. Every bird we have that has turned blind is healthy in every other way and I believe those that died, died from lack of food and water.
Do any of you know what this could be? If their was any other symptoms present, we could figure it out, but they simply stop acting like a regular chicken and sit down when the blindness occurs.
ANY IDEAS?