Oh, sorry, I forgot that you said you were in Portugal. Here are some articles about the 4 varieties of Mareks and the different symptoms you may see:
http://csu-cvmbs.colostate.edu/Documents/vdl-mareks-disease-fact-sheet.pdf
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/
http://csu-cvmbs.colostate.edu/Documents/vdl-mareks-disease-fact-sheet.pdf
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/