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Anchor Worm

Anchor Worm

One of the most notorious crustacean parasites that infest fish is the anchor worm, Lernaea. There are several species of anchor worm, all of which are parasitic on fish. The complete life cycle of the anchor worm takes 17-33 days, depending on the temperature. At 68°F (20°C), for example, the cycle takes 25 days; below 59°F (15°C) it may not complete at all.

The newly hatched anchor worm, or nauplius, is elliptical in shape and free swimming in the pond water. The nauplius moults into a second stage (he metanauplius), which is also free swimming. Subsequent juvenile stages are parasitic and must find a fish host, where they settle on the gills. When these juveniles mature into adults, the males mate with the females and then become nonparasitic and leave the host.

The fertilized females leave the gills and settle on the body of the Koi, where they continue to grow, gradually losing their crustacean features and becoming elongated and wormlike in shape. At the same time, they burrow deep into the skin and underlying tissues, developing anchor-like processes that hold them firmly in place. At this point, the mature anchor worms are clearly visible attached to the fish, being up to 0.8in (20mm) long, depending on species.

Digestive juices secreted by the anchor worms enable them to draw in and feed on dissolved body tissues. The nutrients received from feeding on the Koi are used to produce eggs, visible as twin white egg sacs at the posterior end of the mature female. A single female can produce several thousand eggs in her lifespan. As these eggs are released into the water, so the life cycle starts again.

Anchor worms are usually seen on Koi when they have been feeding for some time, producing characteristic bloody spots that are highly prone to secondary infection by bacteria or fungi. And if an anchor worm dies while still attached to the Koi, bacteria and/or fungi will readily invade the dead parasite's tissues and then spread to the Koi. In fact, anchor worms are probably one of the biggest culprits in causing holes on the body.


Lernaea will attack a Koi in many places but the most common sties for infection are the body (practically under the scales), the mouth, around the eyes, on the fins, the gills, and the joints of fins where they meet the body. In the very early stages of an anchor worm infection it is not possible to spot the typical anchor worm shape, and so infection may go unnoticed or be wrongly diagnosed, as it can look as if the Koi has another parasite infection, such as ich, because of the presence of white dots on the body. These are in fact the young anchor worms.

Infections can reach quite high levels before they cause any real threat to the Koi, except when the mouth or gills are being attacked as this may affect the Koi's ability to breathe or eat. Another problem is that the attachment site normally becomes infected either by bacteria, fungus or even a virus. This happens because the anchor worm actually punctures the skin to feed on tissue fluid and cells which leaves an area prone to infection. Characteristic symptoms are scales lifting and the area around the worm becoming red. In very extreme cases of infection you may notice your Koi hanging in the water, and weight loss may also be observed.


The first course of action should be to remove the adult worm physically from the Koi. Sedate the Koi then gently pull the worm from the Koi with a with a sterilized pair of tweezers. Then apply a suitable topical treatment, such as malachite green and propolis or just propolis, to the attachment site.

To get the full benefit from doing this, it is vital that all Koi in the pond are inspected at the same time, and unfortunately this will need to be repeated after ten days or so to ensure that no new worms have attached themselves or that others have been missed. While inspecting the Koi you should also check for any secondary bacterial infection that may have occurred and treat accordingly.

It is also vital to treat the pond to try and destroy the free-swimming stages of the Lernaea parasite and once again this will have to be done a number of times to ensure that complete eradication occurs as the adults and any eggs sacs are normally not affected by such treatment. Organophosphate-based treatments are effective in the eradication of anchor worm, but organophosphate-based medications have recently been banned in numerous countries. Manufacturers of pond treatments are actively developing new products which claim to be effective against anchor worm. As improvements are made to them, it is to be hoped that these medicines will increase in availability and effectiveness.

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Lice and Anchor Worm Treatment

~Price $23.00 32 oz size Treats up to 9,600 gallons


~Price $40.00 32 oz Treats up to 10,000 Gallons.

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