I went to a chicken show a month or so ago... one of the birds that went with me came back (not sold). Although he was not put back in a pen with the rest of my chickens, he was in a pen nearby. He went to another home a few days later and that person contacted me to tell me he was sneezing. She put him on antibiotics and he apparently recovered fine. Then about two weeks later, some of my birds started sneezing. One wheezed for literally one day. I put them all on Duramycin for ten days. Another of these birds became lethargic and stopped eating. I separated her and she died a few days later. All of the rest of these birds are now asymptomatic. No one sneezing, no one wheezing. Everyone's eyes are clear and normal looking. These are all silkies, about 10 weeks old.
Now in the pen next door over (marans, 8 weeks old), a few of those birds started sneezing. I started them on Duramycin, too. One of these birds has squinty eyes with what looks like a very small amount of discharge. Another bird has little black spots on its lower eyelids. No other spots anywhere else and I'm not even sure the black spots on its eyelids aren't its usual pigment (though I think it isn't). All of these birds are active, eating and otherwise behaving normally.
I've looked at tons of pictures of foul pox and these spots really aren't looking like pox to me. I should have taken the one bird that died for necropsy, but I didn't.
So... I know Marek's affects eyes, too. These birds have lived in my basement and garage since I hatched them. The only outside exposure they had was this chicken show I went to. I have wild turkeys come through my yard on a regular basis and I usually open the garage for fresh air for the grow out pen. I have an outdoor coop with adult birds and some chicks that do not appear ill at all.