I read a thread on here and a poor guy lost his WHOLE flock TWICE to MG... very bad stuff...... he HAD to cull EVERYTHING.... peacocks, turkeys, chickens, ducks........
The first time he bough some birds and introduced them without quarantine the second time.... he didn't know how he got it. He ordered NPIP eggs (they don't test for everything, just a couple of things) so it was a mystery to him. MG can be transmitted in eggs.
I just read about this in an issue of backyard poultry. Scary stuff.