I've had chickens for 12 years and my chickens get gleet vent the older they get. I keep everything clean, no moldy food, etc. I think it is just something that catches up to them. They are free range in a fenced in area so sometimes in the spring the ground in the shade has mold/green on it. I keep their bottoms cleaned up as much as I can, give them probiotics, and spray bottoms with Banixx. It helps but doesn't get rid of it completely.
