Recent research suggests that in the absence of varroa wounding, DWV is a
relatively benign virus that a bee's immune system can largely suppress.
Reference #31 from the study that Ghislain cited appears to support this,
since few of the queens has DWV copy numbers of even 1 million per
queen--which appears to be a rather trivial amount (counts in workers don't
appear to be serious until they get into the tens or hundreds of
*billions).*

What the paper doesn't discuss is the oral infection route of bees via the
jelly from infected nurses.  This would clearly expose developing queen
larvae in the cell builder hive.  From practical experience, we get better
queen cell production success from cell builder hives that don't have high
mite levels (although I have no evidence that that has to do with DWV).

And since a queen's diet consists solely of nurse-produced jelly for the
rest of her life, she will be continually exposed to DWV virions in the
jelly (along with dsRNA produced by the nurses, which may help to suppress
DWV).

Once reaching adulthood, queens are apparently rarely wounded by a mite, so
they seem to reach equilibrium with DWV.

A worthwhile study to read is:  Patterns of viral infection in honey bee
queens   DOI 10.1099/vir.0.047019-0
Two quotations from the paper:
  The ovaries had the lowest infection counts and the lowest viral titres.
Hence, it is possible that the ovaries may be protected to preserve the
vital reproductive functions.
  In summary, our results suggest covert viral infection in queens from
most healthy colonies. In colonies with severe varroa mite infestation, the
queen may eventually be infected. Thus, queens are not entirely immune to
viral infections, which appear to be transmitted from their worker
offspring rather than via the often-discussed sexual transmission during
multiple mating. The protective mechanisms behind this pattern remain to be
elucidated.

Dick brought up two salient questions:
1.  Is this part of the reason that a hive that has high DWV symptoms is
pretty much doomed even if the mite population is knocked to zero?
A colony is not doomed at all, provided that one doesn't wait too late in
the season.  We routinely bring back badly mite and DWV-infested colonies
in August (using a strong formic acid blast).  But by September it is
typically too late.

2. Does this imply we need to think about knocking mite numbers lower thru
the whole year to keep virus counts down rather than use an economic
threshold mite count as the guide to what is an ok mite count?

Absolutely!!!  This is the way that the most successful commercial
beekeepers (as far as colony survival) run their operations.  It is also
how we run our own 1500-hive operation.  Our treatment threshold is 6
mites/half cup of bees from June on--lower than that from December through
May.

The most important thing is to get mite counts to close to zero in August,
so that the colony can produce a generation of "winter bees" that have not
suffered from varroa feeding, and that have low titers of DWV.

-- 
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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