Help blindness in chickens

[cemdsg]

Over the past four weeks I have had a horrid disease going through my coup of bantams. They go blind normally one eye at a time. They are not sneezing excessively and have no other symptoms save lethargy when they can't see. They have stopped laying. I have taken one to the vet who gave me a broad spectrum anti biotic but they are still succumbing and we are having to put them out of their misery which is horrid. Help please! What could it be?

 

[Judy]

http://edis.ifas.ufl.edu/ps044

If you look at the charts near the end of the article, Marek's is the only one that lists blindness. Several list sneezing and weakness, though.