I've had three chickens die of the same symptoms several months apart. First they get lethargic, then they lose their balance and can't stand up, then they just lie down in a corner and a couple of days later die. They've all been different ages when it happens, from about 2 months old to 5 months old. Now my 6 month old pullet has the same symptoms. No nasal discharge or other respiratory symptoms. They all came from the same breeder (a small independent farmer). Any thoughts are appreciated!