>> I'm not sure whether it is simply because I am running more hygienic 
>> bees, because there is less feral population, or what.  But it sure is 
>> curious.

> May be the widespread adoption of Tylosin. If most commercial beekeepers 
> are using it and have stamped down foulbrood in their outfits, this would 
> tend to reduce the incidence of foulbrood in general, irrespective of 
> whether smaller beekeepers use it or not.

I agree with Peter.  The loss of sulfathiazole some time back meant that 
there was only one preventative available legally until Tylosin came on the 
scene.  That in itself is a recipe for developing resistance, so having a 
back-up is bound to have an impact.

Tylosin is also very persistent which makes it a good drug for suppressing 
AFB, but also potentially problematic in the wrong hands in regard to honey 
purity in this time of extremely sensitive lab tests and a critical public.

Sulfa was a miracle drug, partly due to its persistence, but its persistence 
was also a curse, especially since some people are very sensitive to it.

OTC (oxytetracycline) is effective against most AFB, but is quite 
short-lived and as a result, under-dosing is common, even when the initial 
dose is as prescribed.  Maintaining the control dose over a brood cycle is 
critical to achieving and maintaining control.  With OTC, this can be 
difficult and OTC was never very good at reliably suppressing AFB.  What OTC 
lacks in persistence has had to be made up in beekeeper persistence and 
repeated applications.

Tylosin, unlike OTC is very long-lived as are its metabolites.  Each dose 
maintains its activity over sufficient time to make it very effective, and 
several dosings are apparently sufficient to drive AFB levels down to where 
even average bees do not break down, assuming that scale has been removed 
and all other things are equal.

As far as I know, we never saw any resistance develop to sulfa, but shortly 
after it was banned, some AFB strains showed resistance to OTC.  Will we see 
AFB resistant to Tylosin?  That is a good question.

In my opinion, the resistance that developed to OTC came about by horizontal 
gene transfer from other, already resistant bacteria, not spontaneously. 
However, IMO, the process was accelerated and somewhat enabled by the fact 
that maintaining a controlling level of OTC in hives at critical times is 
difficult.  As a result of the quick fading of OTC potency over a short time 
under hive conditions, bacteria had ample opportunity to be selected for 
those which could tolerate OTC.

So, yes, I agree that Tylosin has been a factor and also think that the 
breeding efforts aimed at hygienic behaviour have had an effect as well. 

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