im led to understand mg is rare in commercial flock because of vaccination so surely ex batts maybe a better choice than an auction gamble?
I'm not sure though I do believe MG is not controlled in commercial flocks by vaccination as the vaccine for it is very expensive. Where there have been programmes in some countries to eliminate MG from commercial flocks it's been done by antimicrobial treatment of hatching eggs to produce reduced MG infection in breeder flocks and selective means ie culling effected flocks and breeding MG free only.
In the US MG has been all but eradicated. Where there are programmes they have been driven by regulation. In other countries without such programmes MG is on the increase. MG elimination programmes have been implemented in UK breeder flocks and it has been reported that all major breeder flocks are MG free (not sure of the current staus) however MS (Mycoplasma synoviae) which can produce similar symptoms though is less economically significant has not been eliminated.
Whether breeder flocks are MG free is to some extent academic as rehomed birds come from production flocks and can still be infected with MG if a poultry producers biosecurity is not good. If live vaccines are used the birds will automatically be carriers of a milder strain.
Dead vaccines have been developed but they are less effective than live ones using a milder strain and the cost of administration is much higher as individual birds need to be treated and I'm not sure that these are now available commercially. Antibiotic treatments by regular administration of Tilmicosin have shown reduced effects of MG and improved economics but the use of regular treatments of antibiotics is now not encouraged or is regulated against because of the risk of producing resistant strains.
The bottom line is as far as I understand rehomed ex batts can well be carriers of MG and MS and certainly will be if vaccination has been used as a control in an all in all out production system.
HF