me&thegals :
greyfields--I'm appreciating your obvious experience and knowledge. I have one more question for you. Are birds like humans in that all of us can be exposed to an organism but just the least healthy develop the disease?
Cocci is caused by a protozoa, so it's not a 'disease' as much as it is a parasite. So the mechanism is different, which limites the ability of parents to pass resistance along to their progeny.
But, you are absolutely right. In every crop you raise, some birds are going to get sick from it, while others won't even slow down. The extent of which this is passed along to offspring is what I'm not certain of.
According to ATTRA publications, you can produce a cocci resistant flock as long as you do your own breeding (rather than buying in chickens as day olds). So, I tend to follow their advice. The only sustainable solution is to breed your own hens from birds which showed natural resistance. Broilers, though, is another matter since you can't yourself really breed commercially acceptable meat chickens.
me&thegals :
For example, do you know if free-ranging birds exposed to cocci would have lower levels of infection compared to confinement operation birds exposed to the same?
I do not know how confinemnt vs free ranging would affect it. You would think confined birds would be less exposed, however broilers farms certainly have to use all kinds of medicated feeds just to ensure their crop loss is no more than 25%.