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Fish Lice

Fish lice (Argulus japonicus) are the commonest of the larger visible parasites found on Koi. It is a saucer shaped parasite about 1 cm in diameter when adult. The young are miniature versions of the adults and all stages of fish lice will feed on Koi. They are usually spread by purchasing already infected Koi or aquatic plants. Fish lice lay sticky eggs on objects in the pond. The life cycle in comparison with the anchor worm is fairly simple as many of the metamorphic stages take place inside the egg itself. Fish lice moult numerous times and mature individuals cover a range of sizes. The complete life cycle can take up to 100 days but this will be influenced by the water temperature and other environmental factors. At a temperature of 16 degrees Celsius, it takes approximately six weeks to hatch. It is unlikely that the number of fish lice will diminish in the pond without the aid of chemicals to eradicate them, although in lower temperatures the rate of reproduction will decrease.

Last Updated on Saturday, 10 January 2009 22:54


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.

Last Updated on Tuesday, 08 March 2011 08:19


Gill Maggot (Ergasilus)

Gill maggots (Ergasilus) are parasitic crustaceans, named after the egg clumps that resemble maggots. The incidence of gill maggots has been greatly reduced, and this crustacean is seldom experienced by the hobbyist. If untreated it can cause major gill damage and result in large fish losses. The gill maggot is closely related to anchor worm (Lernaea) but is mainly found on the gills, gill covers and the mouths of infected koi. The gill maggot is a blood sucker and anaemia can result from an infestation. The most damage however is done to the gill filaments which bring about the most mortalities. When spotted, steps should be taken not only to eradicate the adults that are seen on the koi, but also any juvenile stages that are present in the water.

Last Updated on Friday, 05 November 2010 17:33



Leeches can be a problem in a pond, especially if it a planted one. Although not frequently encountered, leeches do appear in ponds. It is normally introduced through new plants, new fish, live food and visiting wildlife. Observing a leech making a meal of the blood supply of a favourite Koi is not pleasant. It can be seen firmly attached to the fish and its body contracting and expanding as it feeds. Worst is the fact that when a leech is spotted on a fish, chances are that many more is already in the pond, waiting to attach themselves to the other Koi in your collection. Leeches attach themselves to a host Koi with their sucking mouth parts. These mouth parts are then used to draw blood as a food source from the host Koi. The health risks that pertain to all parasites with the ability to damage the body or gills of fish apply to the leech. There are always the potential secondary infections and the actual transmission of disease. The fish leech is known to carry and transmit organisms that in turn parasitize the blood and major organs of fish. There is also the added risk of anaemia.

Last Updated on Monday, 12 January 2009 12:49



Trichodina is an external parasite which is commonly found on a koi, especially at times of temperature change. It is also found more profusely in poorly maintained systems where mulm and sediment have been allowed to build up in the pond and in the filter. Trichodina can not be seen with the naked eye and can only be accurately identified through a microscope. It looks like a small circle and it may be spinning and moving at quite a high speed. Inside the larger circle a number of hooks are seen. These hooks are used for attaching themselves to a host fish. It is said that Trichodina is perhaps overrated in its ability to kill fish. The secondary infections will however finish the job if allowed to continue without treatment. The acute irritation is normally the cause of self mutilation as the fish scratches and flicks. Trichodina will kill young fish. Large numbers can damage the skin and make it vulnerable to attack by bacteria. These bacteria may aid the reproduction and spread of Trichodina because the parasite will use the bacteria as a food source. Trichodina multiply by division and will swim from host to host. There are unconfirmed reports that Trichodina can form cysts (Encysting Protozoan).

Last Updated on Sunday, 11 January 2009 10:27

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