I'm sorry you've been struggling with so many issues. That can be disheartening.
I agree with Old Grey Mare....you've got a lot of separate issues going on.
I would have some questions as well. Do you know it was Mareks? Why do you believe it was Mareks?
Did you isolate the new birds before adding to your flock? You've added a lot of different birds at different times...that could be a source of various infections and issues. You've had babies free ranging on natural ground, that too can cause some issues.
The one bird with lesions looks to have fowl pox which will make its way through the flock unless you isolate her. There are some vaccines for that. Usually fowl pox looks nasty but runs its course and then is done.
The very skinny bird who is eating and laying may have a parasite or worm overload or potentially slow acting Mareks which wastes them away....Mareks can take different forms....the neural paralysis (classis leg splay look) or the slow wasting or the eye tumor. Sometimes follicle tumors. Just depends on the strain.
Don't leap to Mareks unless you know for certain. You won't know for certain if it was Mareks unless a necropsy is done. While Mareks is very prevalent in the environment, if you have one bird who gets it, others may not as they may have natural immunity...that's not a bad thing. The industry is moving away from vaccinations and to natural immunity as the virus continues to morph. Those birds who do not get Mareks symptoms after an outbreak are the birds you breed from. So Mareks is not the end of your flock...however it can be a real pain to deal with as some birds succumb. I like to mix my breeds and vaccinated with unvaccinated/natural immunity so that I don't risk all my flock going down with one strain.
As to your situation....I can give you general guidelines to maximize flock health:
1. Always isolate any new bird for at least 2 weeks, 1 month best...that means out of touch, no fence to fence contact, out of wind, no cross contamination with feces or feeding bowls/cans. Care for your existing flock first THEN take care of new birds with at least different coat and shoes.
2. Worm and parasite treat new arrivals as a matter of course to reduce cross infection to your existing flock. If you are not worried about egg sale nor organic regulations, I prefer Ivermectin pour on as that will rid most external and internal parasites. You can get that easily at most feed stores. Treat small drop at base of neck and vent, then treat again in 7 to 10 days. If you see any infestation at original inspection, treat again a third time being sure to clean litter each week and sprinkle poultry dust for external issues. Pull eggs for 1 week after last treatment. I also like to put new arrivals on medicated (amprolium) feed for at least 2 weeks as they acclimate to the coccidia in your soil to prevent coccidiosis in them. If I have suspicions of outbreak, I treat the whole flock with Amprolium based feed. If I see more signs, everyone goes on Sulmet regimen.
3. Periodically worm and parasite treat your regular flock, especially at times of stress or if you see symptoms of being unthrifty, slowing in laying, diarrhea, worms in feces. I like Rooster Booster Triple Action Multi wormer added to feed as there is no pull date for eggs. I do that seasonally. I also regularly dust with Poultry Dust and keep that in the coops under clean litter. If I have an explosion (like I did this summer with unseasonably long hot spell), I apply the Ivermectin.
4. If anyone shows sniffles or signs of illness, isolate in an isolation coop to help contain the infection until you can figure out what is going on. Often you need to treat the whole flock as mentioned above.
5. I didn't address it as you are already doing it, but good layer feed (or grower feed if chicks), plus oyster shell for layers, then apple cider vinegar (raw with mother) in plastic waterers, and periodic probiotics (yogurt or some probiotic supplements) with clean litter is 99% of caring for chickens. Unless you have an outbreak or new arrivals, then see #1 through 4.
I suspicion you have had different issues raised due to mixing existing birds with new arrivals, during warm/moist months which grows all manner of beasties.
Chickens are a great hobby, but they do take work. When I realized they are not a purchase and watch type of animal, it was easier for me to take the periodic frustrations.
I wish you the best of luck.
Edited by Lady of McCamley - 10/31/15 at 7:21pm