Thank you for your rapid reply, Randy.  Helpful!  I can't read the Swedish but I did find Fries' 2009 paper "Nosema ceranae in European honey bees (Apis mellifera)" published in Journal of Invertebrate Pathology that references freezing. https://www.researchgate.net/publication/38086103_Nosema_ceranae_in_European_honey_bees_Apis_mellifera

See Feb.3. "The effect of freezing on N. ceranae spore viability is dramatic." Her data do not show a 100% extermination of spores.  What the small number of surviving spores means for re-infection (or re-establishment of infection) is probably still a question.  I wonder, too, if increasing the amount of time in a freezer at ~-18 deg. C increases spore mortality such that it reaches 100% mortality at moment 'x' in time.

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