If you buy prepared feed for your chickens it usually has food-grade DE in it. They add that to keep the feed dry and it helps chickens rid their intestines of any nasty critters that might be lurking. I have had mixed results using it to control lice and mites. The mites went away after several thorough coop cleanings, a little permethrin in the bottom corners and continuous, daily applications of DE to the chickens themselves. I would say it took a couple of weeks to get that mess under control. The lice are much more stubborn and I'm going to try Poultry Protector today. Seems only one of my hens has lice and she's unfortunately the broody one who is mothering two adopted chicks now.
Side note: If you do use DE, make sure it is the food-grade type, not the type used in pools. Feed stores usually carry the food-grade, but make sure the bag says that clear as day.