I've read most of the other threads about your issues and the loss of chickens over the past few months. I'm sorry to hear you are having so many mysterious deaths. In the other threads it looks like you have dusted them for mites (what with?); treated them with Wazine (for how long?); and given Sulmet (for how long?). Any other meds, antibotics or treatments given?
You also mentioned that they freerange (how often?) and there were several references to bringing them the feed bucket. Do they have food available at all times? 24/7? If not, they should. You mentioned the ones that died were very thin.
At this point, I would confine them all (no more freeranging); provide good healthy food and clean water at all times; and eliminate all meds.
It could be any number of things - as has been suggested - possible mold or contamination or poison they are getting into - or - possible over medicated.
Get them back to the basics and watch them carefully. Also, please try and get one of the deceased birds tested.
You also mentioned that they freerange (how often?) and there were several references to bringing them the feed bucket. Do they have food available at all times? 24/7? If not, they should. You mentioned the ones that died were very thin.
At this point, I would confine them all (no more freeranging); provide good healthy food and clean water at all times; and eliminate all meds.
It could be any number of things - as has been suggested - possible mold or contamination or poison they are getting into - or - possible over medicated.
Get them back to the basics and watch them carefully. Also, please try and get one of the deceased birds tested.