External parasites that feed on chickens do NOT feed on humans. Poultry lice and mites are species-specific.
If your guy has creepy crawlies on him, I doubt it's from the chickens or their blood-sucking buddies. (Bed bugs, maybe?)
To actually answer your question.... 5% Sevin dust is my go-to. Some people say food-grade Diatomaceous Earth works, but I've never really had any luck with it. I found it to be irritating to my chickens' respiratory systems.
If you have 100s of birds, you're going to have to go with a bulk option. You can get three-packs of Sevin at Lowes or Home Depot... they're 1-1.5lbs each, from what I remember.
As for requirement and frequency... Technically, no, you don't *have* to dust your birds on the regular. "Wild" chickens get a long just fine without it... Access to clean dirt is usually sufficient to control parasites through dust bathing. If you notice a bird acting sluggish or lethargic, it can't hurt to hit them with dust, though.
I worm regularly, and I dust when necessary.
If you're selling them to process, I'd be more worried about potential stunting and anemia if you have a growing bird playing host to vampire bugs. But a bird like that would show symptoms that you could treat with the dust and some high-protein treats. :)
Hope that helps!!
Edited by MrsBrooke - 12/3/15 at 1:26pm