Hi My free range chickens are being decimated by a disease that starts with a leg going lame, followed by a general malaise and an inability to walk, culminating in them not eating and gasping for air before they die. The disease is slow to spread as out of 60 free range hens only 4 have had it. I live on a tropical island and these are my island mixed breed hens. They sleep in trees at night and run around during the day over 3 acres. Possibly this is why so few are getting the disease. My vet does not know what this is. I think it must be viral, nothing seems to cure them. But it takes over 6 weeks for the symptoms to get bad and then they die. I have been told we do not have Marek's disease here but possibly that is not right? Any thoughts?