Crustacean parasite, Lernaea-- aka anchor worms can cause serious damage to a koi. It's actually not the worms themselves that kills the koi, it's the wound caused by them by burying themselves into the skin and underlying tissue of the koi that invites other diseases ( can be bacterial or fungal) to infect the koi.

The symptoms of anchor worms include the koi scratching themselves against everything in the tank and white green threads hanging out with inflamed centres on the body of the fish.