After reading the posts in this thread, I am more confused than ever.
I have access to an avian vet once a month, but is not a specialist in chickens by any means.
He said that in all honesty ALL flocks have probably been exposed to Mareks and it depends on whether you have an OUTBREAK of the disease. He said the only way to control it is to vaccinate day old chicks, never bring adult birds into an existing flock (for any other issues not just Mareks), and culling or treating symptoms is up to preference. Is he nuts?
He also recommended as a preventative to any issues is sanitizing/disinfecting cages, coops, etc. as often as possible with a product like Ken'L-Lan-128. I have been using this in my dog kennel as well because Parvo has been bad around here because of raccoon overpopulation.
Oxine, activated, is probably a better choice for cleaning as it has been shown to kill the Marek's virus with the first application. I've read and been told that the Ken'L-Lan takes a few uses to eradicate all traces.
As for the Vet....my personal opinion and that's all it is, they know what they were taught and what they've seen. Nothing wrong with that, but some of the information they have, if they don't constantly keep up with all the new trials and break throughs, is what they learned in Vet school. My view is this, if you want to bring in new birds, then do so, but make sure you have them isolated for 6-8 weeks so you 'know' they aren't sick and able to infect your existing flock. With the Marek's, I can only give you my experiences. Yes, the best option any of us have available to us, vaccinate your birds!!! I prefer the vaccination that has all 3 vaccines in it that some hatcheries offer, but I currently have some that were only done with the MD-Vac. They are now 4 weeks and counting. We'll see in another few what is going to happen.
I also agree with the Vet about exposure...it's out there. Again, my experiences tell me it's not the little exposures that get them. It's the constant exposure or a bird that is weakened to start with. Then you'll have the disease. So again, yes, I agree with the face value of the statement. If birds always had an outbreak, as your Vet said, there wouldn't be any birds left because Marek's is everywhere. Lol, sad thing, but when it first hit here, I started looking at people in the feed store at the same time I was and wondered what their birds had and what the person was carrying around with them.
Seminolewind called this disease perfectly when she said it was like fighting a ghost. It's constantly changing, moving and you can't see it! All you see are the results. Maybe it sounds weird, but if you have Marek's in your flock already, enjoy the ride. You could cull all and start over after a lengthy wait and constant cleaning or choose to take the chance and see where it leads you. This is what I have done and many before me as well. If you've read the thread, then you know I am currently raising chicks from birds that survived. In fact 2 groups since the original outbreak. At 6 weeks, Control Group #2 is alive and doing very well without any type of vaccination for the disease. I still have a ways to go yet but everyday they live is a good day. That's about all you can do when you start down this path.