I think that once you clean out their coop well and burn any of their old bedding , you should be fine. The only symptoms were that they would be lethargic, retract their necks and the eyes would get puffy. Then within a few days or so of that they would die. I treated with Tetracycline and...
When I had sudden death the county came out and we found they had Salmonella and I had to treat the whole flock immediately with an antibiotic in their water from the farm store