I use two things to keep my chickens, louse, lice, flea, and mite free.---well 3.
1. Use DE liberally (food grade of course). I use it in their bedding, on the chickens (all new additions get two good dustings before incorporation with the flock), in their feed or treats, and I put it in their favorite dusting areas (thereby continually "dusting" themselves with DE).
2. Use Orange Oil on all the wood in the coop. I use "Orange guard"...it's pure orange oil and is now available at ALL Ace Hardware stores. I find mine in the pest control aisle next to all the poisonous stuff--top shelf where you can hardly see it (grumble grumble).
3. I've had several chicken rescues come with leg mites (have one in quaranteen with advanced stages--her scales are not only lifted but falling off, and I think they are also swollen--poor girl). For those mites, I mix mineral oil with tea tree oil and rub it onto the legs--making sure I get it well under the scales that are raised. Anything oily works in general, but tea trea oil has antibiotic and anti fungal properties, so it also takes care of any secondary issues that often come up with scaley leg mites.
Good Luck
Sandra