I battled a round of cocci this spring in my outdoor grow out pens. Amprolium (Corrid) did nothing for it so had to use Sulfadimethoxine. I lost 2 chicks that were over 5 weeks old. Because I must have a bad kind here, I've started giving my chicks one round of Amprolium at 4 weeks before they go outside and 20 days later a round of the S drug. I may alternate them another time or two especially if I have any new cases.