I did a lot of research on dust baths (really on anything I do, very helpful sometimes, but I can get bogged down in details). I read a very interesting study by the University of California at Riverside, on the effect of sulfur in dust baths on mites and lice.
It killed them both for many weeks, and because of the kicking chickens do while bathing, even killed the pests on chickens who didn't bathe. Wow!
Here's a link to the study:
https://www.ncbi.nlm.nih.gov/m/pubmed/22458590/
I do have the advantage of a completely covered dust bath, it's never exposed to rain, so it's always really dusty, which the chickens like. It's also hanging, which keeps other stuff from getting kicked in it, and it's a very popular place for my 12 hens and one rooster.
They love to roost on its edge, bathe in it, just lay in it, and use the extra space it provides in the tractor to run across and play, it's like a mezzanine for them. It's a few inches lower than the adjacent roosting bar, they jump back forth between them all the time.
I'm not scientific about it, I just add a good big splash of solfur when I add another 50 pound bag of Home Depot Play Sand, if I have some wood ash from the wood stove, I'll use that too. I also add a splash of diatomaceous earth, another parasite killer. According to the study diatomaceous earth is good, but not as effective as sulfur on killing mites and lice, although diatomaceous earth may do more for worms, I don't know, but I like using both together.
My secret ingredients!
Who would have thought that many hens could fit in one 2 foot x 3 foot dust box! OK, Yankee Stadium ladies room, I get it, same deal!
My flock is only 10 months old now, but I've been using the sulphur and diametrious earth for several months now, and I've never had a sick bird. Now maybe it's the daily free ranging, or the more than usual amount of light the Flying Fortress Chicken Tractor provides, but I'm having crazy good egg production.
In the dead of winter, yesterday, I had 11 eggs out of 12 hens. It ranges from 8 to 11 every day. Maybe I've been lucky, OR maybe I've spent a lot of time reading and learning from you guys!!
As anyone who's been there knows, we make our own luck with hard work and knowledge!
Good luck with your problem, it's painful, but you're way smarter now because of it. Education is expensive, that's for sure. Ignoring the lessons costs more though, so you're on the right track!