Joining in on this thread, as I had a rooster succumb to Mareks, 100%confirmed via gross necropsy and DNA testing.
https://www.backyardchickens.com/t/752543/rooster-with-mass-in-neck-possible-tumor-thymus-response
https://www.backyardchickens.com/t/752543/rooster-with-mass-in-neck-possible-tumor-thymus-response