Mites or lice can be major problem in the hen house. I have never experienced these parasites, maybe because we are fortunate or because I am proactive. Mites or lice can occur at any time and once they get started they are hard to get rid of.
We have never had mites so I will not claim any great knowledge on how to get rid of them. I take a six step approach to prevent mites and lice.
One: Sick birds are prone to having mites, so if I see a bird not doing well I get rid of it. Sick birds do not take care of themselves. They don’t eat enough nor take a dust bath. This poor girl's comb is blue which means that she has poor circulation.
Two: I regularly inspect the chickens for mites by checking under their wings and around their vent hole. At night is a good time, but I usually check for lice and mites during the butchering process.
Three: I make sure they have ample opportunity for taking a dust bath. Our birds free range so they find plenty places to bathe. Like under trees or in the dry dirt under our front deck.
Four: I do not allow any chicken to roost in a nest box. Overnight they will poop in the box and a dirty box can breed mites.
Five: When I freshen up their nesting boxes with wood chips, I mix in a small handful of diatomaceous earth (DE) and a hand full of wood ash.
Six: I do not introduce chickens from other people into my flock. It is too risky. If a need arises, I have a special coop just for the purpose of keeping foreign birds quarantined until they are returned to the owner or I butcher them.
I trust this is helpful. As for treating mites or lice, I will leave it to the experts to tell us what to do.
Have a great day and thanks for your up-vote, Tim and Joann