Well, I'm going to call it on this one: these chicks are immune. They've been in the damp pen now for 7 days with no symptoms, and have been consuming only home-mixed feed for a few weeks now. If they were going to get cocci they would have.
Last night we had so much rain the roost shed was knee deep in water, as was the lower end of the pen. Poor chicks were sodden and drenched. I make them a little umbrella-roost out of a trestle table with a plank underneath, and they spent the night drying off above the waterline.
Today amazingly they're none the worse for wear, all eating to their hearts' content. Typically cecal cocci takes 5 days between challenge and symptoms, so I'll be watching them closely given the drenched pen.
Well, here they are, the pullets out of the recent chick purchase. They're now around 8-10 weeks of age and I feel fairly confident that they're cocci-immune.
Having said that, a major challenge might overwhelm them even now, so I've taken the precaution of laying wood chips down in the pen the meat hybrids were living in.
Before laying the new surface I also took out the top 5cm layer of soil from the central part of the pen where most of the droppings had accumulated. (This removed soil is now happily growing beans, tomatoes and other seedlings in a few garden beds.)
Surprisingly I've ended up with 8 pullets, so of the 20 bought, only 12 are cockerels. Given that some of the female ISA browns had almost certainly been removed, this is a pretty good average.
I'm still going to be watching them for cocci signs, especially now that rain is forecast and warm weather is already here. But they've been off medicated starter now for a couple of weeks. Thus even though this pen is my dampest and is the one I'd be most concerned about in terms of oocyst build-up, I'd be surprised to see any harm now.
The cockerels are staying in the tractor, as unless I can find homes for some of them, their destiny is the table. As usual I get a bit attached to the handsome little things, and it's worth putting in the effort in case there are any breeders around who want utility above looks. Commercial hybrid roosters can increase egg laying in a backyard crossbred flock, though it's rare that anyone wants to bring in their good points without going to a purebred first. But still, it's worth a shot. I remember when I was looking for a commercial layer cockerel and either couldn't find one at all, or couldn't find one that hadn't been exposed to respiratory disease. And these birds have the further good point (for anyone with similar core values to mine) that they've been reared on pasture with, since their fourth week of age, natural vitamins in preference to artificial ones.
Best of all, I can see from the rich green shine in the black birds' feathers and the good rate of growth in all that the diet of ground, cracked and sprouted grains with soy meal, oilseeds, minerals, grass and kefir is agreeing with them.