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I am new to chickens as well, but I am not new to healthcare (human anyway), and I hope I haven't added to your confusion. I do know that Corid is amprolium and the medicine in medicated feed is amprolium. I know medicated feed bags say to be certain that it is the only source or amprolium, but then again, I am not sure what "too much" would actually be (I am trying to figure that out myself). I am also not sure if you can treat with amplrolium, or if it is more of a preventative. It is a coccidiostat, meaning it helps control the cocci in their systems so that they can gradually get immunity by being exposed, but not overwhelmed by infection (similar to what the vaccine is supposed to do), it is my understanding that it doesn't get rid of a full blown infection though. I think the reason that you don't feed amprolium products to vaccinated chicks is because it will potentially kill the couple of oocyts that the vaccine contains (they give them live oocyts to allow a controled exposure so the immune system will begin to recognize it a foreign and attack any additional oocyts that they come across in their environment).
I think all chickens will, eventually, be exposed to whatever of the 9 strains of cocci are present in your particular environment. The hope is that they are exposed slowly enough to mount an immune response against the protozoa that causes it and defeat it and in doing so become "immune". This happens mostly from exposure to droppings, dirt, and other surfaces that contain the fertilized ova of the protozoa (they can live in soil for a looooong time). They require constant re-exposure to remain immune (unlike viral immunity that often is for a lifetime once you recover). The little ones who have weakened immune systems, or who are exposed to too many of the ova at a given time, get overwhelmed by the infection and may not make it. I worry about live vaccine in any species (human included) when given at such a young age. Most infant creatures don't have much immunity to speak of (only that provided by mom before birth that hangs around for a bit).
I am hoping that someone with more experience can comment on this for you, but when I posted a similar thread on the emergencies/diseases thread, they suggested the sulmet (my
TSC was out, it is about 20 as well and is in the calf medicine section).