Good question but 1 annual inspection is unlikely to control an outbreak whenever it occurs. Scales are easier to see during low brood conditions but scales mean you have a big problem and it probably has already spread to other colonys. Very young brood are the only bees that will show active infection so spring build-up is when you will find the most brood. Here in the US we are permitted to treat active infection with a veterinary prescription but I believe that that is a very poor solution. Put me in the camp of zero tolerance and the only cure is a match. Masking the evidence with prophylactic treatment is a no-win in the long term. 
 
Hygienic behavior may well clean up very low and early contamination. My only evidence is that I have not seen or treated any AFB for over a decade. I keep my eye out but with each passing year no longer scour every brood comb looking for it. So empirical evidence would suggest that once an operation is rid of any spores new infection is not a given. Perhaps a fool's paradise.  

Paul Hosticka
Dayton WA

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