Mareks is almost everywhere according to information I have read. Many chickens seem to have natural immunity to it, but I still like to get vaccinated chicks. Half of my flock is vaccinated while the others were not, and I have never seen the disease. Occular Mareks is one of the four types of the disease, and your flock has been exposed if she has been with them. Have you ever had one die of a tumor or paralysis, or for no apparent sudden symptoms? There is not anything that can be done about the disease, but I would just continue to watch her and make sure that she gets enough food and water. Since I've never treated Mareks, I'm not sure if both eyes become affected, or if one does, if that is truly Mareks. Here are some good links to read about the disease, and hopefully it hasn't affected your chickens:
Eye issue? - Page 2
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My chicken has been scratching her eye/ear area for a while. She has a little bump on the bottom of her eye lid and the eye area swollen as shown above. Recently it really became worse so I started looking on BYC to find answers. I threw everything at her and she is better. I gave her Tylan for 3 days in case it was a respiratory thang. I put wormer in her and diluted in her eye. I put ivermectin on her in case it was a tick or mites. So she is better but I don't know what did it. I think it was the eye wormer in the eye because I just did it yesterday and it is better. :) Will keep monitoring her and the others. So thankful for this web site. Dingbat lives on!