We purchased chicks from a local breeder (NPIP). We purchased 40 since our birds are older and not laying. We raised them like all our other chicks (we've raised a lot). They were active, healthy chicks. We moved them into a coop that is new, we added some DE to the ground, and put shavings on top. They were mostly feathered out but not totally, so we didn't let them in the run. The were OK for a while and then around 5-6 weeks old we went out and there were 13 chicks dead in one night. They were fine when we fed them that evening, dead in the morning. No wounds, lumps, bumps, etc. We had just fed from a brand new bag of feed. We thought it might have been bad feed. We changed the feed (new brand, bag etc) and removed all feed from the pen. The next day we lost a few more. They'd be fine and then fall over and start dying. Some would be fine and then just dead. We have changed feeders, waterers, feed, cleaned the coop, replaced bedding, etc. I have no idea what is going on. I am scared it is Marek's disease... We've never had issues with chicks or chickens. Should we cull all of these? IF it is Marek's, is my whole area contaminated? Can I steralize it or will everything I ever get have to deal with this. I know I can vaccinate but I don't want to deal with losses and carriers. Please help!! I answered the questions on the stickies below as well. 1) What type of bird , age and weight: Problems started at around 5-6 weeks old. Have been continuing for two weeks still. 2) What is the behavior, exactly. In one night, we lost 13 birds. Just dead in the morning. Seemed fine when we fed them that evening. 3) How long has the bird been exhibiting symptoms? Some are just dead. Others have leg issues or just fall over and are never able to get up again. 4) Are other birds exhibiting the same symptoms? Just the birds from this "batch". 5) Is there any bleeding, injury, broken bones or other sign of trauma. None 6) What happened, if anything that you know of, that may have caused the situation .I am suspecting Marek's.. 7) What has the bird been eating and drinking, if at all. Medicated chick starter crumbles. All seem to be eating and drinking normally. 8) How does the poop look? Normal? Bloody? Runny? etc.We haven't noticed anything "off" with poops. 9) What has been the treatment you have administered so far? We have switched feed, gotten new feeders/waterers, put new bedding down, put down diatomacious earth, checked for mites, dusted chicks with light coating of DE. 10 ) What is your intent as far as treatment? For example, do you want to treat completely yourself, or do you need help in stabilizing the bird til you can get to a vet? I want to know what we can do. My main concern is if this is Marek's, what do we do for future birds? 11) If you have a picture of the wound or condition, please post it. It may help. 12) Describe the housing/bedding in use- Enclosed coop, dirt floor covered with big pine shavings (not the little ones the can eat) feeders and waterers are off the floor and kept clean.