What do the birds look like when dead? Any posturing? I had this trouble last year only with my Mille Fleurs and then it was gone as fast as it started. I did necropsys and found nothing.
Maybe you could do a necropsy on a bird. I am pretty sure there is a link somewhere that you can see what to look for. Well look for spots on any of the organs, and look in the cecal area to see if it is infected. Gail Damerows book has good B&W pics and good diagnosis tools. The Chicken Health Handbook. I think Speckled hen posted something about a good color diagnosis for necropsies, do a search on her and see if you can find anything. Check out the links at the top of the page. Check out Dianas pages,,,,,I cant think of her online name. hmmm she is dubbed the human encyclopedia
There appears to be a rash of this going around lately.
Sorry for your losses.