Thanks for the question.

Sorry to hear about your currant bush.  We have been hearing from other gardeners with a similar experience.  There is some discussion occurring as to whether any existent insecticide would be effective.  Bacillus thuringiensis (Bt) has been suggested as a possibility but that should be applied now.

The adult form is a type of sawfly.  Even if most of the foliage is removed, the plant should still survive.  I would not do any pruning at this time since the plant is actively growing.  Eggs over-winter in the soil so be sure to remove all foliage and plant debris from under the current bush.  This should be done around the end of June.  Discard into the trash.

The following sites will give you information on these and other points, including some remedial actions.

https://apps.extension.umn.edu/garden/diagnose/insect/garden/caterpillars/medium/importedcurrantworm.html

https://pnwhandbooks.org/insect/small-fruit/currant-gooseberry/currant-gooseberry-imported-currant-worm

http://stellaotto.com/imported-currant-worm-on-the-rampage/

https://extension.psu.edu/insect-pests-in-currants-and-gooseberries-in-home-fruit-plantings

http://pnwhandbooks.stage.extension.oregonstate.edu/insect/small-fruit/currant-gooseberry/currant-gooseberry-imported-currant-worm

Good Luck