Early signs of Mycoplasma

Hi, i've had my chickens since May 2010 as a newcomer. I have 10,  8 girls and 2 boys. I had just the 6 girls last year in June time and they all suffered quite badly with Myco, one of the girls eye's blew up like a golf ball and the vet thought she'd be a gonna but she managed to pull through with a couple of injections. The others got better as well in the end but the medication I was given to put in the water didn't work i just bit the bullet and got them all injections. Since then I hatched 2 girls and 2 boys under one of the girls that went broody.

Now in the last week or so the 2 boys have sounded a bit odd after they do their call, its just after they cockel doodle do as they breath back in i can hear what is almost a bit of a half rattle half very quiet cooooing sound, and i'm sure i heard one of the girls rattle a bit the other day.

What can i do to prevent the myco coming on again?

Many thanks in advance.

Brad 

Thanks Joyfull, i have just re-located them to another part of my back garden with fencing round them now so that could have been trigger i guess.

As Joy said these birds will be carriers of Mycoplasma Gallisepicum (MG) or in other words Chronic Respiratory disease and you should not ever breed from them.  A/bs are the only control NOTE not cure.  If a vet confirms the diagnosis a cull and clean up and start again 4 weeks later with healthy stock is the only way to rid yourself of it and in my view this is a case for the head ruling the heart.  

Otherwise you must isolate these birds and be prepared to repeat the a/bs at regular intervals.  Sorry to give you bad news but there's no cure.

HF