I hatched my original flock from hatching eggs and also added 6 hatchery chicks that were mareks vaccinated. This was approximately 2 years ago. About six months in, I purchased 2 separate adult trios from a breeder at a show. Those 6 birds have never shown a sign of being ill and up until that point all of my other chickens were fine. They were kept separate but much later I found they had mites which did spread to my others by some pen moving I did. About 6-8 weeks later a few of my originals became ill. Some got stinky diarrhea became lethargic and died. Others had more neurological symptoms with partial paralysis or twisted necks before dying. I have had two hatches since and I usually lose about half at the 3-4 month mark and then another handful at the 6 month mark. I keep thinking that those who survive will have better immunity and eventually I won't loose as many. Long story cut short, what am I even dealing with here? Can I sell their hatching eggs? Can the larger boys be eaten safely? As a new chicken person I am trying to learn as I go, but I feel I am not learning fast enough. Is there anything I can treat the symptomatic birds with?