Check your currant bushes for thick buds - it could be a pest. Currant gall mites (Cecidophyopsis ribis) have nested in the bud of a blackcurrant.
Six generations can develop within a year. Spider mites, the currant leaf louse and the yellow gooseberry sawfly are also among the common pests that can attackCurrant bushes. The North Rhine-Westphalia Chamber of Agriculture advises breaking off and destroying all heavily thickened round buds - they are particularly easy to spot by April.