I am not suprised
>This idea is based on the assumption, reinforced by discussions with some researchers and others
> that viruses do not last long outside their hosts 
Yes in most cases this is true, BUT not for ccd which can last 2-3 years

>The other explanation is a virus, but I would be surprised if one could come on so fast.  
 ccd can spread quite quickly if there is a large infective load,
through a WHOLE yard in 2 months, just from drifting
Dead out equipment is worse, no drifting needed

>My mite drops on one of these hives has so far shown zero! mites over almost a week.
As I said before: mites are only a minor part of this problem

>I'm wondering what to do next.

On a warm day do a ramp test.  

If you see rear leg symptoms, then feed vitamin C, ??  1 gm/3x week ??
until the ccd is under control, then 1 gm/week, for a month, then 1gm/every 2 weeks
Each feed is ~ 1 gm at a time(per hive)

OR do an experiment, feed half (of yard) VC, and half feed only

My method of feeding VC is thus: disolve 10 gm in minimum water, use recycled ketchup bottle,
add VC to bottle, add honey, warm and shake well,
feed by top-bar drench (run a bead of honey(like a welding bead) along length of top bar, all bars)
It may be my imagination, but I think the bees prefer this, even over straight honey

dave

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