cocci and genetics

I am having an ongoing problem with cocci hammering some of my chicks. The affected chicks are American game from family line kept in Inidiana. I presently live in central Missouri and brought some of that line with me. In Indiana, virtually all chicks were free ranged and were never medicated for cocci. Cocci there was not a problem for those games even though it did affect our meat and egg production chicks. Now that I am trying to breed games in same manner as done in Indiana, cocci are effectively preventing my games from reproducing with nearly 100% loss of chicks. Half-siblings of same chicks, regardless of which parent is game, are largely unaffected. Crosses not intentional for most part but involved either red jungle fowl or dominique. This forcing me to consider introducing new blood / genetics for the purpose of disease resistance. Sometimes a closed flock is not so good, especially if too few birds make it up. I hope to find games in Missouri with resistance to the local cocci so I can fix problem. Man am I ticked!