Share 'Lice treatment?'

Hi all, So, I noticed in the summer that my girls had lice - not very badly, but there were a few bugs.  I got some DE from Laura and have been dusting them with it and putting it in their dustbathing spot and around the coop, but it's gotten much worse and they are now losing feathers and looking bald in spots (& not as if they are molting - they are breaking the feathers off from excessive…