Back to the question:

>> It is conceivable that a similar phenomenon may arise in 
>> American-foulbrood-infected larvae receiving sulfa drugs since it has 
>> been mentioned earlier that strains of B. larvae could adapt themselves 
>> to a tenfold increase in the amount of the drug.

This interesting.  I recall listening to a talk by C. Peng in which she 
discussed the window between inefficacy and toxicity to bee larvae.  It was 
fairly narrow.  I don't know about Tylosin or sulfa in that regard.

The reason it is interesting is that back in the 70s and 80s I noticed in 
practice that the recommended OTC dose  applied in sugar as dust  was not 
sufficient for control, but doubling it achieved reliable control.  No 
toxicity was apparent in practice.  OTC extender patties, when prepared and 
used as recommended achieved very uniform control.

> > However, this possibility may not be serious owing to the short period 
> > of contact of the vegetative cells in the larvae with the drug and to 
> > the rapidity with which the adapted organisms may lose their 
> > "resistance" on further growth in the  absence of the drug."

This is very interesting.  I am trying to visualise "further growth in the 
absence of the drug" and imagine what that might look like.

On the other hand, add in the application of another drug with different 
mechanisms, and the barrier becomes much higher to establishing or 
maintaining resistance to either, especially if the resistant individuals 
are less fit in the first place.

>> "To many beekeepers, the announcement of the preventive properties, often 
>> perverted to "curative properties," of sulfa drugs for American foulbrood 
>> was the answer to the problem. Before long, however, with the repeated 
>> observation of disease recurrence after sulfa drug treatment, it was 
>> realized that considerable caution was required in the use of these 
>> drugs, that they did not cure an infected colony since they did not kill 
>> the spores of B. larvae, that indiscriminat and careless use and undue 
>> reliance on their effectiveness might result in masking the disease and 
>> in disseminating it widely

We have been over this many times here on BEE-L over the last two decades 
and although this states the obvious, the significance varies with location, 
situation and goals.

a.) Where there is no background level of infection in the equipment or the 
environment, and prevention is the goal, any movement of disease spores is 
of great concern.

b.) Where there is a background level either throughout all the equipment or 
the environment, the concern is much less, since control is the goal and no 
one is likely to burn all the equipment or the environment.

In recent years, the addition of hygienic stocks to the equation has shifted 
the balance point as well and demanded we take anew look at the issue.

Although a medication regime or maintaining a high level of hygiene in stock 
are both answers to dealing with a background level of AFB spores either in 
the equipment, the environment, or both, each has its problems and its 
costs.  I have played with both over the years.

We have discussed the problem of identifying and maintaining hygienic stock 
and the fact that stock degenerates and that even a few non-hygienic queens 
in an apiary  where the sole line of defence is hygiene can lead to 
breakdown and a consequent increase in the background level of AFB spores. 
Vigilance and decisive action is required on detection of any weakness in 
the line of defence.  This is difficult in migratory commercial operations. 
In addition, when challenged by disease, hygienic stocks, even if they do 
not break down, are heavily burdened with removal of larvae and may be 
uneconomic or fail to winter.

We have also covered the problems associated with using drugs, and 
especially with relying on only one drug, and the product purity issues. 
Generally speaking, use of drugs is simpler and the results are more uniform 
and more reliable across the board and failures are less catastrophic.

In my experiments and my travels, I have concluded that a combination of 
measures, with consideration of the operating environment and the 
constraints is the best compromise.

Obtaining stock with some degree of hygienic properties and supporting it 
with a multi-drug regime where indicated, and hive inspections on a routine 
basis with removal for destruction or radiation of any seriously 
contaminated combs is probably the most reliable, lowest cost, highest 
return strategy at present to deal with AFB and to minimise the impact of 
resistance incidents.

Some combination of the above is pretty much standard practise in the 
industry today and quite likely why we are seeing less concern about AFB. 

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