So what's your theory on how it got introduced? None of my birds are vaccinated as I suspect most backyard flocks are not. Are the vets recommending that all new chicks be vaccinated? How do you get in front of the problem? We all have migrating bird issues plus the resident magpies so some of the risk factors are out of our control.
CSU giving you an incorrect diagnosis is inexcusable. I've had my issues with the large animal side and don't have a tremendous comfort level with them.
There's a lot of interesting information out there about Marek's and the vaccinations and how they do and don't work. Sounds like even vaccinating them as chicks is not always fool proof, and doesn't stop them from being carriers since they can still get the virus, it just usually won't hit them as bad so they don't show symptoms but are then carriers (and shedders) of it. I'm surprised it's not more prevalent than it is.