Where are you located? Different countries have different wormers. In the UK, Flubenvet is the only good dewormer licensed that I know of. In Canada, levamisole may be more commonly used. Wormout tablets or Gel is another good one in the US and Australia.
http://extension.unh.edu/resources/files/Resource000791_Rep813.pdf
https://www.backyardchickens.com/a/the-great-big-giant-mareks-disease-faq
Many of those symptoms could be worms, coccidiosis, or a lot of things. Mareks can cause a lack of immunity to many common diseases. Here are some links for you to read about Mareks:Quick update, after reading another post about a light Sussex I believe that my brahma may be suffering the same thing, weight loss, tucked in head (never extends) , feathers a mess and sort of tired looking eyes. Could mareks be a possibility?
http://extension.unh.edu/resources/files/Resource000791_Rep813.pdf
https://www.backyardchickens.com/a/the-great-big-giant-mareks-disease-faq