I will try and answer your questions as best I can. I'm sort of on a tight schedule this morning but I don't want you to have to wait.
First, I am going to address the vitamins....for some reason, some chicks seem to need more vitamins than others. So, even though everyone is eating the same thing, one will show symptoms that the others don't. Vitamins/electrolytes in his water will help but it would be better if you got poly vi sol without iron and gave him .25mls per day plus a vitamin E capsul, 200 ius, twice per day. You just snip off the end and squeeze it into his beak. It can't hurt to try this and it just might make a huge difference. Plus, it will tell you if it is something else that is going on. If he was on medicated chick starter, change his diet to nonmedicated....medicated contains a vitamin B blocker to help ward off coccidia...
On to the coccidia question....yes, they can get coccidia in the brooder. All it takes is for one of them to have carried it from the hatchery, feed store, etc. I've had several groups of chicks come directly from the feed store with it. Again, it doesn't hurt to treat him if you even suspect it. You do not have to see any signs of it in the feces...by that time, usually, it is too late to treat. So much damage has been done to the intestinal wall that treatment often isn't enough to save them.
Now, for the toughest question....
With Marek's, they start limping like they have injured a leg. This can last for several days. Finally, the bird can not get up. His legs are useless. Although, contrary to popular belief, they still move, kick and wiggle. They just can't stand. They will try and use wings to balance. It's just so hard to explain. I have not had any that stumbled and stepped on themselves as you describe. I'm not saying it isn't Marek's, I'm just saying that the symptoms don't match up and there are other things to consider first and try and correct. And, they came on so suddenly and severly, it just doesn't match up with Marek's.
Please try the vitamin therapy and if it doesn't make a difference...can take up to 2 weeks to see results. If he gets worse and he dies, try and have a necropsy done so that you will know what you are dealing with. It really is the only way to know for sure. If it is Marek's, all your other birds have already been exposed. For your sake, and the sakes of your birds, I truly hope that it isn't Marek's. It is a heartbreakingly devastating disease
