Marek's is prevalent in the environment. Fact.
Many if not most birds have been exposed to Marek's in their lifetime. Fact.
Marek's is a herpes virus that causes tumors to develop. It depends on the strain of Marek's as to what kind and where the tumors develop. Neural tumors create the classic paralysis (one leg forward, the other back) and unsteady gait. Skin and follicle tumors create odd lumps and bumps and cause grey looking eyes. Internal tumors can be a silent killer that affect internal organs causing a bird to slowly waste away.
Certain lines and breeds are simply more resistant to the tumors that develop. Many birds keep the tumors at bay until some stressor lowers their immune system. It is similar to the "cold sore" appearing when a human's immune system is lowered. The virus always lives in the body, but only appears at opportunistic times.
Vaccination does not prevent "catching" Marek's only the prevention or lessening of the tumor formation.
Marek's is morphing and becoming more resistant to the strains of vaccine given, so many in the industry believe it is better to breed for resistance rather than vaccinate to avoid the "super strains" which can wipe out whole flocks and whole industries.
I would not destroy my flock again unless I had a Marek's vulnerable flock. If you have a few birds that survive with vigor, breed from them!
Keeping your flock as stress free as possible with strong immune systems is the best defense. Use good husbandry (clean litter, water, feed), no crowding, high quality feed, regular parasite management (internal and external), boosters to keep the gut flora healthy (ACV and probiotics such as yogurt or kefer).
Supposedly St. John's Wort has some evidence of helping Marek's tumors, but I've not seen enough evidence to warrant giving it to the birds on a regular basis.
What I have gleaned in my own research.
LofMc
