> but we still AFB (and any lack of it is more likely due to burning 
hives than by treating them with antibiotics)

I beg to differ.

I suppose we have to decide what constitutes a lack of AFB to properly 
analyze this statement.

I've never burnt a hive and I have bought badly diseased equipment, then 
cleaned it up to the point where it did not break down even without 
medication.

For some, I suppose that lack of AFB would be defined as a complete 
absence of AFB spores in a hive (hard to prove).   For most of us, 
though, I think it is safe to say, lack of AFB means no dead brood from 
AFB, no observed ropey, decaying cells, and no apparent scale.

Does 'lack of AFB' mean all hives in an apiary must always be clean for 
all time, or almost all?  How big is an apiary?  1 hive or 1,000?  
Obviously, this AFB-free state is more likely to occur in smaller 
samples, and  easier to observe than in large ones.

Of course, the norm is a state somewhere between zero observable AFB 
over many years and a full-blown apiary-wide outbreak right now.

Reasons for lack of observed AFB can be many, from lack of exposure to 
highly hygienic bees to prophylactic drug treatment or a combination.  
Strategic use of antibiotics can be another. Replacement of older combs 
contributes.  Time and coating with wax and propolis also diminish the 
viability of spores, and with the best bees the window of opportunity 
for the spores to germinate and infect is short.  The longer hives are 
free from breakdown, the less likely they are to break down from 
internal spore load.

Personally, in spite of having supposedly AFB resistant bees, I saw some 
AFB the year before last and treated those hives with Tylosin. (I have 
little respect for the efficacy of OTC, having experimented extensively 
with it in the past and OTC requires more diligence and repetition in 
application than most of us can muster).

Since then, I have seen zero AFB until this spring when I saw some 
suspicious brood in two hives out of 60 or so.  The brood was not 
sufficiently decayed for a positive ID but I just treated those two 
hives once with Tylan and the condition cleared up.

I am examining frames in 110 hives monthly as I split and verify queen 
presence, and I see zero AFB.

 From long experience, including deliberately placing scale into hives, 
and reports going back to Phillips, who recommended buying diseased 
equipment since it is so cheap and then curing it with drugs (sulfa at 
that time), I am convinced that strategic application of a drug by a 
knowledgeable beekeeper can result in hives that do not show any sign of 
AFB even long after the cure.

For me, no visible AFB and no signs of AFB is 'lack of AFB'.

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