Peter B can probably dig out the old papers from before Bill Wilson got everyone on the antibiotic band-wagon.
I've found that both AFB and EFB can lurk in comb for years, re-emerge when bees tear down and rebuild brood comb.  There are layers of spores, one for every brood round, that are 'sealed' into the wax.
I couldn't find a quick reference to longevity of EFB, although my Parasites in Social Insects, Paul Schmid-Hempel, 1998, says both are long lived.  For AFB - it can be up to 35 years according to Haseman, 1961.
Overall, it's oft reported that foul brood bacterial spores are difficult to kill, simple fumigation or bleaching isn't sufficient.  Presumably, it's because the spores can be buried deep in the wax, only proliferate when re-exposed to the host.
We've used formic, can't say I've noticed any effect on either foul brood.  EFB, I've seen colonies 'grow out-of-it, but the trade-off is in how far behind the EFB survivors are compared to non-EFB colonies, which is an important consideration in our short, northern, summers.
Antibiotics, irradiation work.  The latter tends to be hard to find (geographically) and expensive in US.  In the 1980s, the INEL environmental specialists, without my knowledge, tried analyzing honey in one of their  Idaho reactors - it exploded, leading to one very expensive clean-up.  We were using bees to collect samples around their nuclear facilities and burial sites.  They saw the honey - thought - what the heck, let's try it.  Bad idea.


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