Hello. I registered specifically cause I saw this post. I just had the sneezing chickens horrors, and hope I can offer something here. Mine was environmental, mould spores in the straw. It was bad. What breed are your birds, because some of my birds dealt with it better than others? I lost 20 of 26 cornish cross, and it happened fast. My red sussex were completely unaffected. Yours sounds like could be biological based (I had no nasal discharge), in which case get them on water soluble tylan. Get apple cider vinegar in their water, and try and get raw grated garlic down them (it helps out their circulation if their respiratory system is under stress). Get the sneezers out of where they are, that'll help to isolate the cause, be it environmental or biological. Get them out of their environment, away from stress and temperature fluctuations, electrolytes in water, ACV, and some tylan to control infection (or secondary infection if it is environmental). The main thing I found was supportive care; if their respiratory system is under stress, you need to reduce further stresses on the lungs and circulatory system. If any die, pull them out and have a look inside (while still fresh if you can), if it is environmental and caused by particles in the air, you will see greyish discoloured areas on the lungs and yellowish creamy coloured areas formed in the air sacs behind the lungs. I spoke with an animal pathologist after my horrors, and was told that biological infection will frequently show as lesions and haemorrhaging in the lungs and air sacs. Hope it works out for you, because my issue was stressful and very difficult to manage effectively. Apparently, this sort of stuff hits the meat birds way harder than the egg layers and dual purpose birds. Best of luck.