We have lost 2 of our adult chickens 2+years old in the past year about 6 months apart. Their bottoms turned red and they lost some feathers. Their vents were red and swollen. I tried soaking their bottoms in epsom salts, cleaned them good and kept applying antibacterial spray for birds. Both died within 1 week of symptoms. Any thoughts on what caused this and how I can prevent it? They were isa browns.