This shubunkin definately has dropsy is not normally infective to other fish so you dont have to Quarentine your shubunkin .
Dropsy can be just down to fluid retention in the body, there is a parasitce that can attack your fishes kidneys eventualy leading to kidneys becoming enlarged and eventually leading to kidney failure the there is a bacterial infection which will cause organ failure lastly there is a viral infection which dropsy can be induced by.
Once dropsy has been identified then the first treatment for dropsy should be to salt the pond which should be applied at 5 or 6 kg per 1,000 litres of pond water [11to 13ib to 220 gallons Imperial] work out the US gallonage as I dont work with it and dont want to make a mistake .
This should be for a period of 3 to 5 days , the water temperature should be slowly raised to over 25c [77f] in some cases even as high as 39c [86f].
To the salt treatment it is advisable to add a good anti bacterial that is safe to use alongside salt i;e Acriflavine is my prefared anti bacterial here making sure you follow the directions to the medication of which ever anti bacterial you choose is completed couse wise .
Sadly dropsy cannot be cured all the time and in the fish cure it almost always make a reapperance a year or so later and the fish die.
So ask yourself is it worth the stuggle of the first cure only to loose it the second time around, a sad choice to make I know but this is down to you .
Dave