This came up in a discussion at the honeybeeworld.com forum and I thought it worth posting here.
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> Assuming there is AFB in these boxes, when should I expect to see AFB symptoms appear in my hives? Will it show up pretty quick, or will it be delayed since larvae will be fed with incoming nectar and not honey from these boxes?

You may never see AFB, especially if you have good stock that has some resistance to AFB.  In fact, I doubt you will see any.  But you will have to stay alert if you don't medicate at all.  If you see a cell or two here and there, then you must be really vigilant.

I have friends with 8,000 hives who have not medicated for years and find 20 or less AFB hives a year and simply burn them when they find them.  Obviously all their hives have some degree of exposure, but only a few break down.  I've inspected their hives numerous times and never see any AFB.

I also have a friend who has more than that who has been clean for years and quit medicating due to export considerations.  This year, I saw a cell or two in one of the six hives in each of four yards I visited.  I gave him a heads-up and his only option, given the fact that it is everywhere at low and increasing levels, is to medicate the whole outfit for a while at least, and scrupulously pull any breakdown he finds.

AFB does not usually hit all at once.  It generally starts with a few cells, then more cells, then reaches a point where the bees cannot -- or do not -- deal with it and then the whole frame and adjacent frames break down.  That process takes weeks and months from the initial cells.

It's simply a matter of odds whether a hive of bees that happens to have no resistance due to a genetic fluke or poor breeding happens to be challenged with some equipment with enough spores to push them over the edge, or they rob out some infected hive -- or not. 

 Most hives handle the occasional larva death to AFB without showing symptoms and their spore levels drop over time.  Susceptible colonies, though will go the other way and avalanche into full-blown AFB.

Good bees are hard to infect if you are at all careful -- and lucky.

> If these boxes are infected, if I were to give my bees a treatment of terramycin in early spring before being supered, would that give them resistance and help reduce background spore levels? 

What spring prophylacic treatment does is prevent breakdown at the time of year when the bees are old and over-extended and vulnerable due to weather and the expanding brood nest with many cells of brood per bee and comb which did not have brood recently being used for brood for the first time.  After cells have raised healthy brood, they are not likley to cause breakdown.  Once the brood nest has expanded to its full size and begins to contract, the risk is much less.

> And then next year see if AFB shows up if I don't treat? Or am I better off not treating now, and see if AFB shows up?

I'd either give them a feew spring dustings, or go without, depending on your preference.  In the latter case, you'll have to inspect every frame regularly for a while to see if you will have a problem or not.  It's a good idea in either case, but mandatory in the latter instance.

> While it won't sterilize the boxes, will a treatment of terramycin even knock down spore levels? (Especially if I inspect every comb afterwards to make sure there aren't any AFB scale?)

It is not just scale that you need to worry about.  Scale only happens when the bees do not clean out a dead larva and it dries out.  At that point, you are a long way down the road.  What you need to watch for is the earlier stages where a few larvae die and turn brown, then gluey.  That means that the bees are not able to deal with the challenge and clinical AFB is emerging. 

The longer there is no breakdown -- cells with brown, ropey larvae --  the less likley a breakdown becomes.  Spores deteriorate with time, get lost in the wax, and are carried out of the hive in debris.  Some spores hatch, but are unsuccessful and die.  The only way to get new spores is from robbing, adding old equipment -- or a breakdown, so avoiding or dealing promptly with a breakdown is paramount.

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