Chickens with possible coccidiosis?

Our customer wrote:-

I have a couple of chickens (hens) with what I suspect to be coccidiosis. They have been separated from the main flock but I have seen no improvement. The birds are kept outside but within a secure run. Can you recommend a preparation I can use to treat the birds and should I treat the whole flock or just those with symptoms of the disease?.

Our reply:-

If these are young birds, or new to your set-up, then it might well be coccidiosis. If they are all mature adults and there have been no movements of birds or other changes recently, coccidiosis is not so likely.

The standard treatment for coccidiosis is Baycox, but it is prescription only. I am not sure that you have enough birds for it to be worth buying that. Backyard keepers of a few birds will sometimes use Coxoid (which is intended for pigeons). If you use Coxoid in chickens there is a 28 day withdrawal period for eggs (eggs should not be eaten for 28 days after treatment finishes) and 7 days for meat. For coccidiosis you should improve the hygiene, reduce stocking density if possible, and treat the whole flock.

Ideally you would consult your vet and submit samples to a lab (and post mortem any bird which dies) to properly establish the cause, as you could be treating for the wrong disease.