Chickens and wild life are "contaminated" you can only have a "clean flock" if you keep them inside, do not bring any feed or shavings bags near, don't go to the grocery store etc etc. MG is as common as us getting a split end.
If you read the article I compiled, perhaps you would find it easier to handle. Maine's wildlife in the 1980's tested positive for MG.
I choose to keep my own flock, understanding that this is an issue, but a friend can come in with something on her/his shoes.
If you take your birds to a show, fair, you may as well not bring that bird back into your flock, have a "show house" for those that are no longer pure, they will be contaminated beyond belief of things that scare me.
MG is not a fun thing to have in your flock, but you folks show me how many of your birds test negative vs shows no signs. Rodents spread these conditions into our lives by feeding off the grain, at our homes or at the grain mills.
Good luck and sorry you are culling for my opinion nothing. Keep in mind when I began this hunt for MG, MS, ILT etc, it freaked me out too.