As a former State Bee Inspector, I have seen more of this stuff than I care to recall. I would never say never. 

But, swarms, drifting bees, and shook swarms are the least likely way to get AFB. The beekeeper is usually the one at fault, either by letting hives die of AFB, which spreads by robbing -- or by adding diseased frames or combs to healthy bees. 

That said, personally I would suggest that giving a dose or two of TM to swarms of unknown origin is cheap insurance. 

PLB

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