I'm praying this is not Mareks...

[Nambroth]

Could I tell if it is Mareks by his poop?

No, I'm afraid not. That would sure make things easier for all of us, I think.

It could be Marek's, but I am concerned that he went downhill extremely rapidly; I would be worried about toxins, myself. You are certain he couldn't have gotten into any rotting vegetation, flesh (I know, gross) or stagnant puddles? Mold is another fast moving toxin depending on the species.

Marek's can move rapidly but it's important to explore all options first.

What vitamin deficiency problems did you have before?

 

[SilkiesForEver]

Quote: Darn. His poops are normal, for all that I could tell. A lot of liquid, but that is probably because his diet is mostly fluids now.

I can't say for sure, but it is possible that he could have... How should I treat him if he did?

I had wry neck in one of the other chicks.

 

[SilkiesForEver]

He's been the same for days now. I'm going to have to put him down, as he has shown no improvement.

 

[horsesandboots]

He's been the same for days now. I'm going to have to put him down, as he has shown no improvement. 

As someone who has had Mareks in her flock. I feel after 24 hours you should have culled him/her.
Unfortunately he/she has been suffering and that is no way to keep a chicken because you can't separate your feelings from the process.

 

[horsesandboots]

Could I tell if it is Mareks by his poop? 

I wish it was that easy. You can only tell by the symptoms he/she displays.

 

[SilkiesForEver]

Quote: I made the decision to wait a few days to see if it was a vitamin deficiency. Now I have ruled that out, so I will put him down.

 

[horsesandboots]

I made the decision to wait a few days to see if it was a vitamin deficiency. Now I have ruled that out, so I will put him down. 

Ugh! That sucks! Sympathy goes out to you. :/

 

[Nambroth]

As someone who has had Mareks in her flock. I feel after 24 hours you should have culled him/her.
Unfortunately he/she has been suffering and that is no way to keep a chicken because you can't separate your feelings from the process.

As someone else that has Marek's in my flock, with respect, I'm glad I don't follow that rule. My cockerel had paralysis for a while and now is doing well back in the yard with his girls. I don't make them suffer if their quality of life is diminished, but as long as they stay bright and eat well, they get a chance longer than 24 hours at my house.

 

[Nambroth]

I made the decision to wait a few days to see if it was a vitamin deficiency. Now I have ruled that out, so I will put him down.

Your first post was two days ago; is he suffering? Is he eating well? Vitamin deficiencies can sometimes take a few weeks to "fix"... of course, it matters how much time you want to invest into the bird. If you can't do a few weeks of care, I understand truly, and you must do what is best for the bird and your family.

 

[Nambroth]

Darn. His poops are normal, for all that I could tell. A lot of liquid, but that is probably because his diet is mostly fluids now.

I can't say for sure, but it is possible that he could have... How should I treat him if he did?

I had wry neck in one of the other chicks.

I'm sorry I was not around to respond more quickly. If you are concerned it was botulism toxins, read this to see if it fits and it also discusses treatment: http://beautyofbirds.com/botulism.html
At this point it has been long enough that flushing his system would not really help. Just supportive care.

If you suspect mold or bad feed, read this: http://birdhealth.com.au/flockbirds/poultry/diseases/mould_infections.html

Unfortunately, none of us here can tell you for sure what is going on with your bird. A skilled avian veterinarian that is familiar with poultry might be able to, but otherwise you must make your best, well informed choices based on what you are seeing there, in person.