> Since it must be extremely contagious to go from 8 hives to 350 in a month

I would suggest that it has been in all 350 for some time.   EFB has been 
called the disappearing disease because clinical symptoms can disappear and 
then re-appear.  They usually appear at the start of a flow when many bees 
are recruited to foraging and larvae are fed less generously.

In the UK it is a notifiable disease and can only be treated by our Bee 
Inspectors.

For many years it was treated with OTC - rather unsuccessfully (27% 
re-infection rate) - with compulsory destruction for bad cases (over 50% 
visible infection), or at the beekeeper's discretion in light cases.  In 
recent years treatment has moved to shook swarming, where all the bees are 
shaken into a clean hive with new frames and foundation.  A queen excluder 
is placed under the brood box to for a few days to prevent absconding.  At 
first the shook swarms were fed syrup with OTC, but the thinking now is that 
OTC is not needed.  It is recommended  that whole apiaries are shook 
swarmed - not just the colonies showing clinical symptoms as the thinking is 
that it will be in all the colonies.  Re-infection rates are now around 4%.

More information here:
https://secure.fera.defra.gov.uk/beebase/index.cfm?pageid=89


Best wishes

Peter
52.194546N, -1.673618W 

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