>If antibiotics have only a marginal effect, why use them?

This is the key question.  In my own operation, the antibiotic
oxytetracycline has impressive effect against EFB, and I use it sparingly
when appropriate.

Tylosin tartrate has a well-demonstrated effect against AFB, yet I still
practice burning, treating the apparently-healthy adjacent hives to prevent
spread of the infection to them.

Re fumagillin, Nosema apis is still present in many areas, especially in
colder climates.  Fumagillin has a long track record of being effective at
increasing winter survival and honey production when that species is
present at noticeable levels.

The outcome for treatment against N ceranae are mixed.  For example, Pernal
found that it appeared to be highly effective against winter mortality in
one year, but had little effect the next.

I'm in complete agreement with you on our responsibility to minimize the
use of antibiotics.  In my own operation, I have yet to use fumagillin
prophylactically.  But I do not feel that we have enough evidence to date
to make that a blanket recommendation to all commercial beekeepers,
especially with the apparent drifting of N ceranae-infected bees to
adjacent hives.
--
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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