We have lost 2 chickens so far and a 3rd is going down quickly. They start off by being lethargic just not waning to move at all, then they get a bad limp looking walk when you do make they move. After about a day or two like that they just die. There is nothing out of character as far as the physical looks of the chickens. What could it be and how can I save the rest of my chickens?