From what I've learned (after losing my one pet chicken to it), Mereks is a herpes virus that is most commonly introduced by the wild bird population and is practically unavoidable. There are immunizations for it, however the results are very mixed. After speaking with quite a few people who had lost a significantly higher number of their hatching flock WITH immunization (people who make half their living off their stock), I thought it would be best to breed natural immunity/carriers. I acknowledge from hereon out, I will struggle with this disease and will likely lose a few to it.
My boy declined rapidly. He went from having an unexplained limp, to getting better to being unable to walk in the span of a week. He could not move to feed and water himself and that is where I draw the line. It was a hard decision, but he was put down and buried on the property.
Mereks has similar symptoms to botulism (and to upper respiratory and every other chicken disease out there). My understanding is a way to keep your birds 'immunized' is to keep a turkey or two with them. Turkey's carry a form of herpes that builds immunity in the chickens but cannot be caught by them. Those who don't build immunity to it are naturally weak and lacking resistance on the Mereks front. It really is how I feel I have to handle my situation, and there are others who feel the same.
Mereks is easy to see under a necropsy -- lesions form on the internal organs and sometimes the chest cavity. You can look up Mereks Lesions in google. I did so after I cut him open, however even for my very inexperienced eye, I saw something was wrong before I looked up the pictures to compare.
Best of luck.
