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Anchor worm

Anchor worm (Lernia cyprinacea) has a rather complex life cycle. Only the female is parasitic. The juveniles do not resemble the adult parasite and must go through a series of metamorphic changes before achieving adulthood. It is unclear on what they feed on during these juvenile stages. During the initial stage, a juvenile anchor worm is oval shaped and known as nauplii). Water temperature has an influence on the development but about 50 hours after emerging from the eggs the juveniles moult to become metanauplii. More stages of development follow in which the organism is termed a copepodid.

The juveniles reach maturity at the sixth copepodid stage. During all these stages of development, the organism is invisible to the naked eye. At the point of sexual maturity, males and females congregate on the gill tissue of a host where mating take place. The male fertilise the female and then dies. If a female is not fertilised at this stage, she will also perish, normally within 48 to 72 hours. The fertilised female burrows under a scale into the flesh of the fish. This can be anywhere but particularly on areas where it cannot be scraped off by the fish when flashing and rubbing. This is normally alongside the dorsal fin, leaving only the rear end of their bodies exposed. During this stage the female undergoes a most dramatic change. The four soft appendages on the head will harden to form the enlarged anchor while the body will become long and slender, resembling a thin white fish bone. The ovaries then move into position down her body. Just before the parasite is ready to produce the eggs, the yellow egg sacs can be clearly seen on the rear end of the female. It is inside these paired sacks that the eggs themselves mature. The egg sacs are shed and the new generation of anchor worms breaks free from them.



During the early stages of development the organism is invisible to the naked eye but once the female has lodged herself under the skin of a host, it is clearly visible and resembles a thin white fish bone. It is very common to find ulcers developing at the site of attachment.


Life cycle






If a newly purchased koi have anchor worms on them, it is fairly simple to remove it with a pair of tweezers. I must be done carefully to prevent the anchor of the parasite from remaining embedded in the fish. The easiest way to remove an anchor worm is to dip the tweezers into a strong solution of Potassium Permanganate. Normally a drop of this solution will remain on the tweezers and when the parasite is touched, the effect of the Potassium Permanganate will cause the anchor worm to release its hold on the fish immediately. It can then be safely removed. If a pair of tweezers is not available, the anchor worm can be pushed out with a fingernail. Make sure that it comes out whole and that no part is left still attached to the koi. Unfortunately, removing the worm is not the end of the story. The attachment site often requires topical treatment.


It is possible that disease can find access through the wound that is created by the anchor worm and this may seriously impact on the aesthetics of a valuable fish, especially as the parasite favours the most conspicuous sites on the fish. Bacterial and viral infections can also gain access via the wound. Quarantining new Koi can help to prevent introducing anchor worm into the pond, although it may happen that the parasite goes unnoticed during the quarantine period. If it is known that a newly purchased Koi has recently been hauled from a mud pond, it is worth-while to do the first treatment during quarantine with Dimmilin to get rid of the parasites before the eggs are shed into the water. In cases of heavy infestation, numerous parasites can be seen on a koi, and chemical treatment should be used. The following photos were taken by Christie of Cape Koi Aquarium. It is the worst case of anchor worm infestation that I have seen!



Last Updated on Tuesday, 08 March 2011 08:19