snippet from the 1940 L. H. Watkins bit Pete posted:
"
In the United States it is common knowledge that E. F. B. occurs in serious proportions only in certain localities, and that it may appear in a large percentage of the colonies of these areas year after year, while apiaries as little as fifteen miles away will have but a few scattered cases, or none at all. It has also been noted that once the disease has appeared in a territory it remains there ; and though there may not be a single  case for years, let the conditions be right and it will surely appear. "

1940 is the salient bit...so much less bee mobility driving spread. No widespread off label/prophylactic meds use driving resistance and altered virulence. I wonder too in the absence of test kits if simple stress via chilling/starvation was often labelled EFB. Chilling and starvation both improve with better weather and forage/feeding. The EFB I have experienced does not.

BTW in the back and forth on medication, there was a lot of talk on dosage but no reference to prep used. Here we (used to) be able to buy OTC in different preps which fell into two categories, OTC already mixed with sugar (premix) and OTC powder that you had to mix with sugar/syrup.

Figuring out the per-colony dose for at least one of the preps required an advanced math degree...package directions were geared to preparing vast commercial quantities of medicated syrup.

The prep I see most often available is Oxysol-62.5.

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