Here is some info about Mareks and botulism:
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/
http://www.thepoultrysite.com/diseaseinfo/19/botulism/
When you hear about neurological issues, lameness, and not being able to hold up the head, many will at first think of Mareks disease or botulism, or a poisoning. Check your food or scratch for mold or an odd color or odor. Look for any plants they are eating which could be poisonous.
But I...