The juvenile stages settle in the gills of Koi, when they mature they mate and the male leaves the Koi, the fertilized female settles on the body of the Koi and continues to grow, becoming the familiar worm shape.
The female buries into the skin and underlying tissue to hold on. The damage caused can become a target for bacterial or fungal infection which can spread.
Lernaea lay eggs which can lay undetected in the pond and can hatch when conditions and water temperatures are right.
Treatment is by manual removal of the parasite with tweezers under anesthetic, ensuring that the whole parasite is removed. To be sure of complete removal, dip a cotton bud in strong potassium permanganate solution and dab the worm with this solution whereupon it will release its grip immediately. Pond treatments include Dimilin or Paradex.