OTC has the desirable characteristic of degrading fairly rapidly in moist environments (as in a bee hive). Therefore, it fit the bill of allowing refuges of susceptible bacteria (and beneficial competing bacteria) to survive.
Tylosin, on the other hand has a very long life in the hive—on the order of several months or years (Kochansky 2004). That is why it is currently such an effective antibiotic against AFB—it just keeps killing and killing the bacteria. This persistence was noted in the process of its registration for bee hive use, so the label specifically prohibits its use as a prophylactic measure, or its application in sugar syrup.
Of course, many commercial beekeepers now routinely (and illegally, at least in my state) feed tylosin in sugar syrup as a prophylactic measure against AFB! It is a “box movers’” dream—no need to inspect for foulbrood, nor loss of AFB-tainted equipment—just treat ‘em all with tylosin. I strongly question this practice! We do not know the long-term effects of a persistent antibiotic upon symbiotic honey bee gut flora or those in the bee bread. Of even more concern is the imprudence of such practice—tylosin is an incredibly effective tool for the control of AFB. The routine use of it will predictably soon render it ineffective as tylosin-resistant bacteria evolve. Those misusing the product will ruin it for the rest of us! This is not a matter of “laughing with the sinners or crying with the saints”—it is rather a shortsighted folly.