>Is it tropical?  As it supposedly comes from cerana, cerana has a range from Indonesia to Japan.  Hot to cold range.

> In my colonies it appears to disappear under hot conditions, and thrive during the cold winter months.

My bad, Apis Cerana is native to Asia but definitely not confined to the tropics. But honestly, there is no proof whatever that Apis cerana is the original host. 

In various Canadian studies, Nosema was not correlated with winter loss

> No differences in Nosema infection levels were found between colonies that died and those that survived. All the factors studied had a significant effect on colony mortality, except Nosema infections. 

> The most striking result regarding variation of the factors studied was the explosive growth in Nosema infection levels (317-fold) after the winter. 

> Nosema disease and tracheal mites were apparently the least damaging factors, while higher fatality cases were associated with varroa mites either alone or in combination with other factors

> The relative weight of nosema disease in the mortality of overwintered colonies is a controversial matter. Higes et al. attribute a major effect to Nosema infections in the mortality of colonies. However, there is no conclusive evidence to affirm that Nosema is an important factor in the recent honey bee die-offs worldwide.

Guzmán-Novoa, E., Eccles, L., Calvete, Y., Mcgowan, J., Kelly, P. G., & Correa-Benítez, A. (2010). Varroa destructor is the main culprit for the death and reduced populations of overwintered honey bee (Apis mellifera) colonies in Ontario, Canada. Apidologie, 41(4), 443-450.

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