This is a nasty parasite, not common in koi, but does happen. It is a 'foreign' parasite regarded as one which is monitored in common carp by the Environment Agency (EA) who will not allow movement of common carp between waters unless they have been checked for its presence.
Fluke-Solve is effective but because this tapeworm is so problematic it is recommended that treatment is done in a separate container and that the fish be isolated for 3 weeks to ensure all eggs from the dead worm s are shed and removed from the water by filters. Meanwhile the pond will need to be emptied and sterilised, steam cleaned etc to eliminate eggs and prevent reinfection.
Graphic from the information sheet produced by the EA on Bothriocephalus acheilognathi.