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Lernaea, or Anchor Worm, is a crustacean parasite more commonly found in coldwater fish such as Koi. It is so called because the head of the parasite develops a shape similar to an anchor under the skin of its victim. They anchor themselves to the host will eat away at the tissues, this can cause secondary infections and fungus as well as causing immense tissue damage.
This parasite is large and easy to spot with the naked eye, however, it is quite a tough parasite and will prove challenging to eliminate. Leaving it untreated will result in the parasites successfully reproducing and gradually killing off the fish around them. It is only the adult females that will be attached to the fish.
Prevention & Cure
Quarantine all new fish before adding to the general population and if the source is suspect, be careful to not add any of the source's water to your own as young anchor worms are free swimming.
The parasites will die if not exposed to water from dehydration, if a pond or tank is infected, complete draining and leaving for 24 hours dry will kill off any of the parasites. They are also intolerant of salt, salt dips will kill off the parasite also.
They cannot survive harsh winters and will die if the temperature drops below 15°C (59°F) .
This is the challenging part. It will involve physical removal with a pair of tweezers and the fish isolated away from all others. While removing it must be done very carefully to ensure that the head of the parasite is removed as well as the body.
It has been reported that dipping the parasite in potassium permanganate will cause the parasite to release from its victim and be easier to remove.
Once the parasite has been removed, a salt dip and careful antibacterial treatment of the effected area should be done.