Id treat with ivermectin liquid 0.1mL per 2.2 pounds of bird. So the average hen I treat with 0.3mL and Ive never had a problem. I put directly below vent on skin, not feathers. I apply it 3 times in total, once a week for 3 weeks. I also remove all bedding from coop and spray it down and all wood with permectrin II, once a week for 3 weeks. Some people use the permectrin II on the chickens but Ive always had better luck just using ivermectin directly on them. Also you can so a permethrin 10% dip on the chickens once a week before you add ivermectin if you wanna be overly cautious.