Unfortunately what I've experienced with my flock is that once they show the symptoms of being ill, it's a loosing battle. Their immune systems are so skewered by the Marek's that there is little I can do except cull them to keep them from suffering.
I did a head count this morning. I'm down to 20 standards at this time. I have a few crosses/bantam/standard but they are in their own category. 4 have ocular Marek's but are holding their own. Weight is good, eating well and acting normal in spite of having really weird eyes. I had 38 the first part of the year, rehomed 4 roosters before I knew I had Marek's in the flock but they have since died from a fox attack. So my best estimates is that I've lost 14-15 standards from Mareks and 4 bantams from it also.
SO I have 13 standard hens that are showing resistance, although the one that I lost yesterday was resistant until she got whatever she got that killed her. That is the pattern that I am seeing in the birds (standards) that have survived the primary infection. 3 roosters are currently showing resistance also. Problem is I need them to survive longer than 3 years and unfortunately BO's are listed as being particularly susceptible to Marek's. I cannot see the logic in breeding future chicks from them with that thought in mind.
Earlier this summer I had a perfectly healthy looking Welsummer hen come up to me, sit down at my feet, all fluffed up and immediately pull her head in and go to sleep. I picked here up and like the hen I put down, skin and bones. I culled her immediately.
From the description of the symptoms, I'd say they are succumbing to fungal infections. They get good food that is kept inside and dry but they do get out in the wet so who knows where they pick it up.
My plan is still to introduce vaccinated Fayoumis into the flock. At least 6 or more hens. No cockerels and let my resistant Bantam roosters cross with them. I'd go with a few cockerels but in all honesty, my bantam boys will just beat them senseless so may as well not go there.
@chickisoup, what are you treating your birds with for fungal infections?