Marek's, to the best of my knowledge, Karen, is not a respiratory ailment. It usually attacks the central nervous system and typically will cause paralysis in the legs and feet. With some strains of Marek's, there may even be blindness, though I don't know if it causes the pupils to become distorted. The "death rattle" as you call it could be IB [infectious bronchitis] and if this is the case, you need to isolate him from the rest of the flock as soon as possible. As the name implies, it is highly contagious!
How old is your boy? Are there any other symptoms besides listlessness, lack of appetite and the sounds of respiratory congestion? Let us know. When we all put our heads together, we can, and often do, come up with an answer!
