I have a problem that's not really similar, but in the research I've done trying to place what it is, I have managed to locate something with symptoms like yours that are listed.
I have found that Coryza fits both my symptoms and yours. Seems to be more of a pest than Mareks, I guess and can be very resistant to anti-biotics. The one hen I have with eye symptoms has foamy, frothy eyes that are very glassy, but she looks alert otherwise. I have found the majority of the birds I have that are coming down ill come from a specific batch of eggs I hatched; I've considered culling all of those birds from that particular hatch since they've all come down with something or other.
If it is Coryza, it's also one of those pass-it-on things, that eggs that are hatched out by an infected or carrying hen are born as carriers with their immune systems ramped up to fight the illness. This is my understanding at least, more experienced Chicken-ers may be able to help us both out by informing us if they know this to be otherwise.
I'm going to resort to my usual cure-all -- ACV (with mother) in the water (1 tbsp per galon) for 14 days before merging down to every few days here and there, and 1 dropper full of oil of oregano to every 2 cups of plain, all natural yogurt served to the chickens delight for 7 days. I usually premix 2 or 3 containers and just divide them over 7 days, usally dumping a Tbsp full into their water as well.
Oil of oregano has been my go to, as it's a natural, highly potent anti-biotic. I've used it on my cats, dogs, family and friends with much success, and it helped me battle off Cocci. with a decent result.
Keep us posted on your treatment choice and how it goes? I'd like to be able to keep your info on hand as well