> I think if a hive is 1 or 2 prophylactic treatments away from 
clinical symptoms of AFB, then you are not free of AFB.

Agreed, but that is not the case where antibiotics are used 
knowledgeably and new infection is not being introduced by robbing or 
manipulations.

 > if your antibiotic of choice is especially persistent, then you would 
have to adjust this figure...if it is only the presence of antibiotics 
that is keeping your bees from showing symptoms, than they are not free 
of AFB.

Obviously, but if your antibiotic is that persistent you cannot produce 
legally marketable honey in Canada or Europe.  Not sure about the U.S.  
And, for that matter, if your prophylactic treatments are continued and 
completely successful, after a few seasons, barring new infection, 
either from outside or from internal breakdown, your bees will not need 
them and AFB breakdown will be rare and limited in scope.

Persistence is only helpful in that the antibiotic has to be 
continuously present for at least several brood cycles during the 'cure' 
or there is breakdown.  Breakdown re-seeds the hives and the process has 
to be begun anew.  After the cure, the antibiotic must be consumed by 
the bees before the hive can be used in production.

Persistence is a problem for exporting honey producers and Canadian 
beekeepers are very careful about use of antibiotics (other than 
fumigillan, it seems).  Many prefer burning the few cases they find to 
the risk of accidentally having tainted honey.  In large operations, it 
is hard to know exactly what everyone is doing and where treated hives 
might have gone, so simply burning them as they are found is the simplest.

On the other hand, I discovered a few AFB cells in several hives in each 
of four yards in one large commercial beekeeper and reported that to 
him.  He has a low level but widely distributed problem and I doubt his 
solution was to burn.  My guess is that he treated all hives that fall 
and then changed his management to carefully inspect all brood chambers 
routinely until all signs disappear and finds are infrequent.

This sort of problem usually arises when someone is careless or 
untrained in spotting AFB and is assigned to make up brood chambers from 
deadout hives, especially in a dark room.  AFB detection in such 
situations requires very bright light and good vision, plus scrupulous 
attention to each comb.  People tend to let down their guard if AFB 
levels are low and one hive can seed ten or more when the combs are 
sorted.  In a few years of this, the disease is everywhere.

Today's bees seem able to hold AFB back much better than in the past, 
but over time if there are successful infections the AFB spore levels in 
all hives build to where the most susceptible hives break down quickly.

A word of warning to those experiencing drought: Be especially vigilant 
in watching for AFB this year.  Dearths are known to bring on AFB outbreaks.

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