I had a BO cockerel that was treated for cocci with several others. Except he took months to catch up in size, probably due to the intestinal damage that SpeckledHen and Sonew talked about. But after treatment, he never looked sick, just small.
I may just try treating your hen again with something different.
Marek's is not always paralysis or tumors. I've had 6 spread over the last 2 years that had wasting away over a period of months, starting with a mis-diagnosed roo that must have had Marek's. They looked like they were eating , just not enough. Half of them had a history of treatment for cocci. What got me thinking of Marek's in my flock-after 2 years- was that I had hatched 10 chicks under my birds but the eggs were gotten by mail. The eggs don't carry it. So it had to be my flock. 7/10 chicks, half had the wasting. The other half had the paralysis. One by one.
Wasting away is the most common symptom. Paralysis is the most noticeable. Or they can even have no symptoms at all. The only way to know would be a lab necropsy. Marek's is probably the most misunderstood disease. It can affect any nerves, most commonly the wings or the legs. Some of mine seemed to have some digestive nerve paths affected. It can also affect the respiratory nerves. Some of mine had a sporadic gasp. There's just so many different symptoms.
If you have any chickens that were exposed to any other chickens , vaccinated or not, you can have Marek's in your flock. She may peck at the treats, ground, food, but is she really eating it? Is her crop full or empty at night? Does she separate herself from the flock, hanging out outside the group?
Still, my first move would be treating her with a different cocci treatment. Separate her and see if she's really eating and drinking.
Sorry.
