Some subs contain potassium sorbate and also sodium propionate, which are fungus and bacterial growth inhibitors respectively. These are very inexpensive (human) food preservatives available from Univar in bulk, and elsewhere in smaller amounts.
Since nosema is fungal, could the sorbate be a factor? I know Gilles Fert mentioned using sorbate in his thin syrup in his book about queen rearing to prevent fermentation, so the use of sorbate for bees has a history.
Nonetheless, I have asked many researchers if they have any knowledge about this and received blank stares. Mike Randall mentioned to me at the IPM Workshop in Edmonton that nosema has been reclassified -- as a fungus, if I got it right. This got me thinking again and I decided this is worth chasing. Being lazy, I am calling out the hounds (you folks) to join in the chase.
Of course, there are those who will find this revelation a further reason to condemn all subs, citing the tremendous importance of each and every microorganism in each and every hive on God's Green Earth, even though only some subs use these inhibitors. Those battling nosema, though, might want to consider making an exception.
Food for thought?
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The current levels of potassium sorbate and also sodium propionate in one well-known proprietary diet is listed as 0.1% (1 part per 1,000)
There is a lot of good info in Allen Carson Cohen's "Insect Diets" as well.
See http://www.amazon.com/Insect-Diets-Science-Technology-ebook/dp/B000PWQMWS
I've scanned just two pages and think I can share them here as "fair use", since anyone who looks at them is going to want to rush out and get the rest of the book.
