Still no signs of cocci in any of the chicks that remained here, so that's positive. We've had many days of rain and the pen has definitely been warm and damp, but the chicks are handling it. Good news.
The other day I sold a handful of chicks, and the new owner rang me up the next day to say one was sick. Clearly she has cocci (and after I suggested what to buy, the chick apparently came good again). The stress of transport probably contributed, but I don't think it's a major factor.
Now I confess I've been a bit lazy with the latest chicks, and have put them in the large pen from four weeks of age without taking off the surface litter, and to top it off we've had a week of heavy summer rain. However none of the remaining chicks in the pen now are showing signs of cocci, nor have the other sold chicks succumbed, so the pen isn't the problem in itself, nor is the rain, nor is transport stress.
Normally if one bird has coccidiosis the others will show signs either at the same time or a little later. When you get one chick with coccidiosis while the others are well, it could be a sign that the sick bird has other underlying illnesses (whether congenital or infective).
Which brings me to a new thought. Although I've stated that the chicks were fed bran/pollard instead of sprouts, that's not strictly true -- they did get some. It's just that their staple diet was made up using the bran/pollard. This little girl may have been unlucky enough to eat more than her share of the bad wheat, or to be unable to cope with its effects on her digestive system, predisposing her to secondary ailments. Marek's is another possibility, though I've never seen classical Marek's signs, so it's fairly unlikely.
I'm just putting this 'out there' so anyone reading this blog will realise (if they haven't already) how many co-factors there are in managing coccidiosis; and sometimes it may be that a bird simply doesn't have the immune strength to cope with moderate challenges. If the other chicks had been sick I'd be saying the opposite and going hell for leather to clean up the pen and improve management. But it's not as simple as management in this case, I feel.
Naturally I've offered to swap the sick bird for another, but the new owner is happy to keep her. I hope she stays well.