Mycoplasmas are EVERYWHERE. Mycoplasmal infections are easily treated with wide spectrum antibiotics such as Tylan (which was specifically developed to combat Mycoplasma Synovaie in Turkeys) and LS-50, if it is caught early.
Mycoplasmas are non-invasive. That means they exist inside an organism, but cause no harm to the host. When the host is stressed, the mycoplasma will start doing their thing.
If a bird's lungs become irritated by its environment, BANG, Mycoplasma causes CRD. You should CLEAN and TREAT.
Killing off your flock will help nothing.
If the "keeping" practices that started the problem are not corrected, when you repopulate, you'll have the same problems. Mycoplasma gallisepticum free flocks are in laying houses and broiler houses, and high intensity raising facilities, not in backyards.
This is my understanding after hours of reading about Mycoplasmal infections; the methods are tried and tested right here. I hope someone can add to this or straighten me out if I'm inaccurate.
If you test your birds for Mycoplasmas, make sure you test yourself. You have it. So does every tweety bird that flies over your property, and most any other critter for miles around.
If birds are coming down with respiratory problems, there's an underlying environmental factor that's causing the outbreak. This time of year (winter), it's likely coop conditions, as in poor air quality in the coops.
You can Tylan and Baytril till you're blue in the face, but if the conditions aren't corrected, you'll do it again in a month or two. An ounce of prevention is worth a pound of Tylan.
Another tip about MG infections... If they are intense or ongoing, MG will destroy or severely damage a pullet's reproductive system.
So... If you have outbreak after outbreak, and can't find the root of the problem, all the Tylan and Baytril in the world won't help. You'll end up with an emaciated, poorly producing flock of birds.