As Seminole has said, it'll be there on your property, but for the most part it is anyways. I don't believe there could be anyplace on this earth where it isn't found. If this is the case, then how do some birds not succumb and others do? I believe it is strictly a genetics issue. I've seen people here who have culled and gone on to have no other problems even though they didn't replace with vaccinated birds. Wouldn't have been my first choice, but there ya go. Others have tried the same thing and some even bringing in all new vaccinated birds and have still lost their second and sometimes even the third flock. So what's the answer? Cull and wait another year before starting again? Keep the birds you have and hope for the best? To me, the idea of culling a flock isn't a viable option. I would much rather keep the ones who survive the outbreak and know they have resistance to the virus. Breed them and get more birds that are even more resistant. Marek's is still here as could be seen by my poor girl that died recently, yet before she was picked on, she was thriving. This girl was from my first control group of unvaccinated birds from the survivors of the outbreak here.
I will tell you this, it isn't fun either route you take. I know what it's like to stand there and choose who lives and who goes. It leaves you with a sick feeling inside that takes a long time to heal. Yeah, yeah...they're only chickens. Chickens that were put into my care. On the other hand, if you decide to keep them, the process isn't without it's own heartbreak. Some will be fine and never show symptoms, some will slowly fade away as you watch helplessly. Some may even have episodes of paralysis off and on through out their lives, as do a couple of mine. This is where the Super B Complex comes into play. I am looking into the Turmeric as well, to see if it might be a player too. It certainly can't hurt as it's loaded with vitamins, but it also may just be a weapon against the internal spread of the disease. Turmeric acts as a blood cleanser, simplified, and it work on both the red and white systems. Marek's lives in the spine during it's lifetime and is released into the body during times of stress from illness or physical means. What would happen if the system were given a boost to clean itself?
Just food for thought.