I had a particularly virulent strain of mycoplasma synoviae. It was a variant that caused more respiratory than joint issues. I tried controlling it by consulting with the state vet/lab, and with a private avian vet. Over the course of months the flock kept getting reinfected. Some died, some had chronic respiratory issues, some got secondary infections. A few did OK, (Big Medicine, the SLB were more resistant showing mild symptoms compared to the others), but I couldn't bring anything new in without them getting sick. Ultimately I just got tired of sick birds and being a reservoir for avian diseases. I also wanted to be able to share stock with other hobbyist with a clean conscience. And I wasn't wild about all the drugs that were going into my food source. I know getting MS in a flock is not uncommon. Had it not been so persistent, even in the face of professional treatment and meticulous husbandry, I may have tried to carry on and just breed for resistance. If it happens again, I may do that and just never let them leave the farm. I know in commercial flocks they probably would not depopulate for MS, but they have a natural business cycle where they start fresh periodly for other reasons. It presents more of a problem for an ongoing flock.