The understanding of Nosema seems to be in the very early stages.   
Similar ambiguity and confusion happened with tracheal and Varroa  
mites.  I remember a well respected and experienced bee researcher at  
a large beekeeper conference suggesting that we did not have to worry  
much about tracheal mites given past experiences in England.

Figuring out the effects of Nosema on colonies may be even more  
complex than the for the other two maladies given the large range in  
intensity possible in any individual (spores or DNA copies per bee).   
This has a big influence on a composite sample, which is the only time  
efficient way of currently gauging infection.

And then there is the question of what infection levels are possibly  
harmful.  Even in Spain, there seem to be different views on whether  
high levels are common and whether they cause collapses.  More  
globally, levels may be different at different times of year in  
different areas, and similar levels may have different effects in  
different environments.  Clearly high levels are associated with  
colony mortality in some areas with long winters and treatments may be  
necessary.  In areas with milder winters there have been at least a  
couple of field experiments in which Nosema ceranae copy number per  
bee correlates with weakening or slower growth of colonies:

https://www.ars.usda.gov/SP2UserFiles/Place/64133000/PDFFiles/501-600/518-Villa--Negative%20evidence%20for%20effects.pdf

http://www.ars.usda.gov/SP2UserFiles/Place/64133000/PDFFiles/501-600/529-Rinderer--Functionality%20of%20Varroa-Resistanrt.pdf

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