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Strictly speaking, a parasitic infection does not constitute a disease, but through its actions act as a disease causing agent. There are thousands of various types of parasites infecting ornamental fishes. Parasites are always present on fish, much like fleas on a dog, and pose no real threat to fish in a collection. The threat becomes real if some factor triggers the parasites to multiply, like stress caused by water quality problems. It is not possible to medicate a fish until every single parasite is killed, because by that time, the fish will also be dead. Protozoan ectoparasites are the most common parasite encountered by koi keepers. This group is a diverse array of mainly ciliates and flagellates that feed on the most superficial skin layer, known as the epithelium. Most parasites feed only on the epithelium’s surface, while some parasites like Ich (Ichtyophthirius multifillis), penetrate into the epithelium. The feeding activities cause serious visual damage to the host. When parasites pierce the epithelium, it breaches the final outer defence of the fish and secondary infections will take hold. Another common damage caused by the parasites is a reactive hyperplasia of the epithelium and an increase in mucus production. When the hyperplasia is severe, it appears as a white cyst, or cloudiness on the skin. This may also happen on the gills and can lead to insufficient oxygen intake. All protozoan parasites have a direct life cycle and reproduce faster at higher temperatures. Under favourable conditions they will quickly overwhelm a host population.

Effective treatment (getting rid of most of them) of parasites depends on an understanding of the two major types of life cycles namely encysting and non-encysting. The worm family (flukes) also have two different life cycles namely, egg producing and live bearing reproduction.

Encysting protozoan
During the feeding stage of these parasites, a feeding nodule is formed in the skin or gill epithelium. When mature, they break through the epithelium, fall off the host and form an encapsulated dividing stage. This is known as a tomont which secretes a sticky substance that sticks to any object in the pond including nets and plants. It then multiplies by binary fusion and within eight to 24 hours, large numbers off new swarmer cells are formed. They then break free of the capsule and actively search for a new host. The only way this type of parasite can be treated effectively, is to effect treatment during the free swimming stage.

Non encysting Protozoan
These parasites attach themselves to the gills, fins, in the muscle tissue and on the skin. They complete their life cycle on the host and are relatively easy to treat. Normally a single short term drug application is enough except when the infections are found in the internal organs. This makes it more complicated for the average koi keeper.

Many fluke varieties exist. The difference between most of them to us as koi keepers are merely academical, but we must distinguish between the two main varieties, namely egg producing and live bearing. The distinction should be made because live bearers can be treated once and the egg layers need consecutive treatments. The reason for this is the fact that the eggs cannot be eradicated through treatment.  

Unfortunately most symptoms related to microscopic parasites may appear very similar and it can therefore not be accurately identified through observation only.  

IMPORTANT: Because of the encysting and non-encysting forms of parasites, microscopic identification is important when deciding on a treatment protocol. Also, after identification and treatment, mortalities may still occur. This leads to hobbyists jumping from treatment to treatment in the belief that the ones employed did not work. If you have made an informed decision to use a specific treatment, perseverance is called for. Sometimes a fish will be infected by more that one type of parasite and the treatment regimen should be aimed at all the species simultaneously.

See also www.fish-helpline.co.uk

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



Chilodinella is one of the fastest fish killers there is. It attaches to the skin and gills of koi and severe infections can result in rapid fish losses. Chilodinella is a ciliated protozoan that cannot be seen with the naked eye, and can only be identified with a microscope. It means that a skin scrape must be taken. Chilodinella will appear as a heart shaped organism with rows of tiny hair-like (cilia) extensions along its longer side. It may also appear as a round organism full of tiny bubbles.Staining the slide with Methylene Blue may help if you have never seen this parasite before.

Last Updated on Friday, 11 March 2011 09:15



Costia (Ichthyobodo necatrix), also known as “Killer Don” is a ciliated protozoan that has the capability to kill fish in great numbers. It can only be identified through a skin scrape and a microscope and is active and proliferates in water temperatures from as low as 2 degrees Celsius to 25 degrees Celsius. The parasite cannot survive temperatures of 28 degrees Celsius and above. Below 10 degrees Celcius, Costia may form into a cyst for protection. Costia can survive as a free swimming organism or attached to a host. Costia has been described as misshapen circles, bean or comma shaped with two hair-like flaggela of unequal length that enable it to swim. It cannot survive long without a host. Costia usually inhabits the skin as well as the gills. Once attached to the host, it actually destroys tissue at that site. Koi suffering from a heavy infestation of Costia may not make it through the treatment as is the case with many parasitic infections.

Last Updated on Friday, 23 January 2009 19:37


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


Gill and Skin Flukes

The most common types of worm infections that will be encountered by the koi keeper are Gill Flukes (Dactylogyrus) and Skin Flukes (Gyrodactylus). Although it is commonly known as Skin and Gill Flukes, these parasites are not confined to these specific areas. The skin and gill can be infected by either of the two types. If identified early, they can normally be easily eradicated.

Gill flukes (Dactylogyrus) attack the gills of the koi but can also be found on the skin. They have hooks at the rear end of the bodies that are used to attach to the host and these hooks are arranged in a circular form. Gill flukes are hermaphrodites, so each worm can produce and fertilize a single egg at a time. The egg will develop over a number of days into free-swimming larvae that must find a host within hours. Overcrowding will assist the parasite to find a host more readily and to start reproducing. The adult parasite can produce up to 20 eggs per hour in water with a temperature of 24 degrees Celsius. It is estimated that under ideal circumstances, one adult Gill fluke can produce more than two thousand individuals in only thirty days. Treatment for gill flukes should be aimed at the adult fluke and the free-swimming larvae. The eggs are not sensitive to treatment and consecutive treatments should be done to eradicate the infestation.

Last Updated on Friday, 05 November 2010 17:32


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




Ich (Ichtyophthirius multifillis), also known as white spot, is a very common protozoan parasite that most koi keepers will encounter at some time. Ich encysts under the epidermis of the fish and in this form causes small white spots all over the fish’s body, fins and gills. In the encyst form it feeds on blood and skin tissue by continuously moving in a circular manner. Once the parasite reaches maturity, it will fall from the host and attach itself to plants, rocks or against the side and bottom of the pond where it will develop into a cyst. This is the reproductive stage where the parasite multiplies by division (splitting) and many hundreds of “swarmers” will form. These swarmers or tomites will then break free of the cyst and actively look for a new host to start the cycle over again. The life cycle is temperature dependant and may take 20 days or more during colder months. In warmer water, the life cycle may be as short as 4 days. If the free swimming parasite cannot find a host within 24 hours, it will perish. This explains why overstocking of a pond is so dangerous.

Last Updated on Thursday, 22 October 2009 12:35



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