MG can be tested for, you might even try calling your state veterinarian or contact someone at the department of agriculture, often ag programs at state universities will do this sort of testing cheaply or for free. It is a disease that has serious impact on commercial poultry, so quite a bit of research has been put into it. And yes, you can treat the symptoms of MG, but the birds wil forever be carriers and experience reactivation episodes during which they become more contagious. But the same comments are true of quite a lot of infectious diseases of chickens. Whenever you experience disease in your flock, you have to make a larger decision about how you intend to deal with it in the future. The 'general precautions' approach is to assume that any bird showing symptoms of infectious disease will forever be contagious (it's not always true, and it's an extreme assumption, but just go with it for a minute). When you look at it that way, you really only have two choices: cull these birds now to spare the rest of your flock, or expose the rest of your flock to the disease and expect to lose other birds instead. And you're right, the choice is entirely yours, and also a hard one. At the very least, it's good to practice the best hygeine possible while you're still making your decision, so that you don't accidentally carry the disease from one bird to another yourself.
Incidentally, MG is ubiquitous in many parts of the world. Wild birds can carry it, so if you free range your flock they could just as easily contract MG that way. It's a stubborn disease. It doesn't usually have a high mortality rate, but it can cause a drop in egg production, and results in condemnation of many meat birds at slaughter time due to airsacculitis. Birds that are weakened by MG can get co-infections with other common opportunistic pathogens. So it's not so much that it's a terrible disease in itself, but it is extremely frustrating!
