I still see it once in a while in a few cells from a weak hive; burn the
frames, scorch the boxes.  Nebraska's longtime beekeepers at a recent
meeting reported similar in an informal discussion. For years, we used to
treat prophylactically with terramycin on the top bars which mask the
problem.  You should be able to find an older study of honey that
demonstrates the spores are probably endemic.  I think the focus on varroa
moved our thinking and also just regular changing out old wax frames helps a
lot which we didn't used to do as often.
Dave

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