This doesn't answer the question, but at a lecture at the National Honey  
Show last year the speaker (Steve Ryan I think) said (if my fallible memory  
serves) that the action of propolis in combatting bacteria was different to that  
of the usual antibiotics in that it didn't kill the bugs but enveloped them,  
preventing reproduction.
 
If this is the case then the same mode of action might be effective against  
Nosema spores.
 
Chris

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