Interesting back-up for the view of persistence in colonies, thanks mbc.
As far as I recall the non-feeding of swarms helps reduce the spore load by prolonging the spell in their honey stomach and allowing the proventriculus to filter out particles which are then processed and vented as GG says. No doubt the working of wax when there is no comb ready to use also helps clean their mouthparts of bacterial cells and spores too.
Incoming swarms *might* bring it to an apiary but another way would be outgoing swarms reoccupying contaminated sites and providing lots of inoculum to spread out through robbing or absconding. When cases reoccur at historic sites something like that must be happening. As long as it was properly tidied up by the beekeeper previously of course.
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