Thanks for the question.  The current spanworm, as you discovered, can rather quickly defoliate alpine current bushes.  In early spring spanworm pupae, which have overwintered in leaf litter beneath the bushes, emerge as adult moths.  These moths quickly lay eggs that quickly hatch into larvae.  These larvae are voracious eaters and eagerly feast upon the currant bush during late-May to early June.  These larvae are most vulnerable to control measures.  If these larvae are not killed by mid-summer, they go back into the leaf litter beneath the bush and overwinter there as pupae.  Then in the following spring, the fun begins all over again.  The good news in all of this is that rarely will this defoliation lead to the demise of the currant.

Here is a picture of what the caterpillar looks like during its eating stage:

https://bugguide.net/node/view/1192394

These two articles will provide you additional information about currant spanworms:

https://apps.extension.umn.edu/garden/diagnose/plant/deciduous/ribes/leaveschewed.html

https://extension.umaine.edu/cranberries/grower-services/insects/spanworms/

The most effective time to control this insect is in the spring when you begin to see adult moths.  This will be your signal that these moths will soon lay eggs that will hatch into larvae.  You will want to kill these larvae before they start to eat.  Your treatment should extend into July.

This site will give you some treatment options:

http://www.landscapeonline.com/research/article-a.php?number=13866

In my opinion the two best treatments are:

1).  Application of B.T. (Bacillus thuringiensis).  This is a non-chemical approach that utilizes a bacterial extract that will kill the larvae.  You apply a solution of this to the leaves by spraying.  Be sure to read the instructions about how to make this solution and how often to apply it.  See:

https://www.planetnatural.com/bacillus-thuringiensis/

2).  Application of a spray insecticide that contains spinosad.  This is also a bacterial derived product.  Again, carefully follow its instructions for spraying and for application frequency.  See:

http://npic.orst.edu/factsheets/spinosadgen.html

Remember to apply these products as soon as you begin to observe the emergence of moths in the spring 

Good Luck!!