Alan’s Book

Chapter 15 Parasites, their Life Cycle and treatment


The first parasite to be discussed is “Whitespot”. This parasite is frequently seen after sudden drops in water temperature.

One of the most common of all fish diseases is known as “Whitespot” or “Ich”. The latter name is a shortened version of the name of the parasite “ichthyophthirius multifiliis”.

The common name “Whitespot” was given by aquarium keepers due to the fact that badly infected tropical fish could be seen with white spots on their bodies. These white spots are rarely seen on koi unless the infection is severe and well advanced: suspicion of the disease is by characteristic changes in the koi’s behaviour and diagnosis is confirmed by microscopic examination of skin/gills scrapes

Life Cycle

Ich has an interesting life cycle, one stage of which is illustrated below in Fig 1.

This shows a typical view of a mature specimen of Ich at a magnification of 100X through the microscope, however at this stage of development it can readily be seen at a setting of 30X.

For simplicity we will take the stage shown in Fig.1 as the first stage of the life cycle of Ich.

At this mature stage the parasite leaves the fish and drops to the pond floor where it covers itself with a protective coating and continues to develop.

Utilizing energy, which it has gained from feeding on the fish’s tissue, it divides into numerous individual “tomites” or baby “Ichs”. From 500/1000 individual tomites can emerge from this one encapsulated individual. These are frightening numbers, but as this parasite has evolved to meet natural stocking levels, it produces these huge quantities to increase the chance of one individual finding a host and continuing the life cycle, much the same as trees and flowers produce large quantities of seed on the off chance that one will find a suitable place to grow and perpetuate the tree or flower species.

On leaving the capsule, the tomite (Fig 2) becomes free swimming and searches for a host fish. At this time it is extremely small and being an entirely different shape to the mature specimen it is not often recognised.

If a fish is not found within a short time the tomite will die. If it finds a fish it can use this as a food source and will grow once again into the relatively large, easily recognizable form of Ich, and so the cycle continues.

If the number of fish available for the tomites to colonize is large, there is obviously a much greater chance of them being successful in their attempts at finding a new host fish. In koi ponds we stock at much higher densities than would normally be found in natural environments, hence the risk of re infection is much greater A few parasites would cause little harm to a fish, but once numbers increase significantly the fish soon becomes distressed and if treatment is not applied quickly, serious skin and gill damage will occur which can kill the fish.


The usual treatment for Ich is Malachite Green at 0.1 grams per ton (0.1 gram /1000 litre i.e. 0.1 ppm) together with formalin at 15 ml/ ton (15 ml/ 1000 litre i.e. 15 ppm). Commercial preparations of these chemicals are available and the instructions should be followed according to pond size.

This treatment will kill the parasite whilst it is in the free-swimming “tomite” stage in the water and on the fish, but when the parasite is in the mature form or encapsulated stage on the pond floor, it is protected from any know treatment that would kill it without affecting the fish.

The temperature of the pond governs the rate at which Ich develops. Sometimes instructions for treatment, give application times, which are based upon aquarium temperatures of 21C. At pond temperatures the cycle can be much slower.

In order to successfully eradicate Ich, it is essential that the life cycle is broken. Treatments are available which are added to the pond daily for five days and this is quite effective in eliminating the free swimming sages of this parasite. Treatments such as Eradich consist of low levels of malachite and formalin which are sufficient to control the parasite without undue damage to the filters. This is important when low temperatures can prevent filter bacteria from increasing their colonies. I have not seen any adverse effects even when as a precautionary measure at low temperatures of 8C I have extended the treatment to seven days. At temperatures above 14C I have found that the five day treatment eradicates Ich very efficiently.

It would be a wise precaution to continue to examine any fish showing signs of unusual behaviour incase late developing cysts release tomites after the above stated times.


Costia is a tiny parasite of fish and very difficult to photograph without specialist equipment. The picture shown below is of a skin fluke (Gyrodactylus) surrounded by dozens of tiny individuals of costia.

Costia is found on both the skin/fins and in the gills where it can cause serious damage. Like several other parasitic infections it can cause the infected fish to rub their gill plates on the pond floor and sides, together with any other protrusions in the pond. Fish, which are infested with costia often produce extra mucus and have a grey “bloom” on their skins. However this can be caused by other parasites too so it cannot be taken as a precise diagnosis for costia.

It is a very small parasite but can often be present in such high numbers that the typical flickering motion can be seen when viewed through the microscope at 100X

A magnification of at least 100X is needed to detect this parasite: 400X is recommended for positive identification once the very tiny flickering motion is detected. Careful preparation of the slide is essential. Avoid thick mucus preparations and add a little pond water to the slide if necessary such that a thin mucus sample is observed with plenty of “coastlines”.

Search along this “coastline” for tiny flickering movements, which are a typical indication of the presence of costia.

Two types of costia are believed to infest fish; one prefers the gills as a site of infection whereas the other prefers the skin. Both are described as “kidney” to” bean” shaped and 10-20 microns long, having flagella from 9-18 microns long. (Twenty millionths of a meter long) (Flagella are whip like hairs, which are used to propel this parasite.)

Life Cycle

Both types have simple life cycles in that they divide themselves into two individuals to multiply their numbers: this is known as “binary fission”. Costia does not have any intermediate stages but can infect other fish by contact. It can only live for a short while away from a host fish.


The best recognized treatment for costia is potassium permanganate added to the pond. Potassium Permanganate can be a harsh treatment for fish so it is essential that the correct dose be administered. One and a half grams/ton (1000 litre) i.e. 1.5 ppm, is considered to be a safe effective treatment, which will do a minimum of damage to the biological filters of re circulating pond systems. In order to minimize damage to the filter bacteria (especially that which deals with nitrite) the water should not be circulated through the filters until some of its energy has been expended. If possible re-circulate the filters for first two hours of treatment.

After mixing the chemical with a bucketful of pond water it should be left for zinc approximately half an hour before it is added to the pond. Once mixed in the bucket it will turn the water to a deep purple colour. When added to the pond it will make this water purple too. If a pond contains relatively high amounts of organic matter (algae, fish waste etc) the effectiveness of the treatment may be reduced due to the oxidizing effect of the chemical being used up on this matter instead of the costia. A reasonable indication of the effectiveness is the length of time it takes for the pond water to change from purple to pink and then to a brownish tea colour. If the pond water becomes “tea coloured” in less than four hours after the treatment is added to the pond, it may be necessary to re-treat after 24 hours, or after substantial water changes. Care must be taken not to excessively change the water temperature when this is being done.

Careful use of the microscope will confirm the effectiveness of the treatment.


This is another parasite, which can do serious harm to the gills of a fish; it can also cause severe irritation to the skin

This parasite has a typical way of moving which helps identification under the microscope. It tends to stop moving in any particular direction then turns around in a full circle as if pivoted from one end. It is roughly heart shaped and propels itself using cilia that are located at the thinner end of the parasite.

Life Cycle and Treatment

It is believed to multiply by binary fission and is fairly easy to eradicate by the application of malachite and formalin as described for “Whitespot”.

It is one of the few parasites, which are eradicated by salt at 0.5%, and I have not seen a parasitic problem with koi carp, in re-circulated filter systems where chilodonella alone is involved. On the rare occasions that it has been seen on slides, it has not been present in large numbers. However, it is reported to cause problems in young fish, and may cause problems in ponds with high organic matter.

Flukes – Gyrodactylus and Dactylogyrus

Skin Flukes (Gyrodactylus)

Skin flukes are a common parasite of koi and can cause severe skin irritation, which encourages the fish to rub itself aggressively on any protrusion within the pond. This can lead to damaged skin, and bacterial and fungal infections may follow.

They are readily recognized on skin scrapes at a magnification of 30X. (See Photo)

At 100X the large hooks with which the fluke anchors itself to the fish’s skin can be clearly seen. Inside the adult fluke, the young gyrodactylus can usually be seen together with its attachment mechanism, which may be mistaken for another attachment mechanism in the mid section of the adult fluke.

Life Cycle

The life cycle of Gyrodactylus is particularly interesting in that each adult parasite carries within it another smaller Dactylogyrus to which it gives birth. This young parasite has yet another embryo within it. The rate at which the parasite reproduces increases with temperature within the normal temperature range as found in koi ponds.

Being a livebearer, once the adult is killed the cycle is broken.

Gill Flukes (Dactylogyrus)

Gill flukes are also a common parasite of koi carp. They irritate the gills and can cause the fish to aggressively rub its gill covers against any suitable surface in an effort to relieve the irritation. Severe gill damage and death can be caused if remedial action is not taken.

These flukes are rarely seen on skin scrapes as adults. However the small juvenile stage can sometimes be seen. (See Photograph). Having hatched from the egg this stage becomes free swimming and searches for a new host. If a host fish is not found within a 24 hr period, it dies.

Once it finds a host, it crawls along the skin in a similar manner to a caterpillar until it reaches the gills where it attaches to a gill filament using small attachment hooks located at one end of its body. Utilizing the fish’s tissue as food, the young fluke grows quickly to reproductive age and re commences the cycle. Comparing the photographs of the juvenile and adult gill flukes clearly shows the vast growth achieved from the egg to adult stage.

The larval and juvenile stage is sometimes eradicated by malachite and formalin but this is not a reliable way of terminating an infection of this parasite.

Identification of the juvenile stage on the skin is reasonably easy. They can be seen as small moving objects at a magnification of 30X. Once suspected, they can then be examined at 100X and are confirmed as gill flukes at this stage by the four “eye spots” at one end of the body. (See Photo)
The adult gill flukes are readily seen in sections of gill removed from a dead fish. (See Photo) Dactylogyrus is clearly differentiated from Gyrodactylus by comparing their heads. The first one has a “four point anterior” and the latter has a” two point anterior”, i.e. one looks very much like a paw with four pads, the other like a paw with two pads. It is not uncommon to find both of these fluke species in the gills.

Life Cycle

The life cycle of the gill fluke differs from that of a skin fluke in that it lays eggs, which are released into the pond environment where they develop and hatch at a later date. The rate of development depends on temperature and it is believed that eggs can survive for long periods in cooler water temperatures.


An effective treatment is now available which will eradicate both types of flukes at the same time. Used correctly this treatment is considered safe when applied at the precise dose to a pond.

Flubenol (flubendazole) 5% is a wormer used for pigs; other strengths of Flubenol are used for poultry. I have found flubenol to be extremely effective for the control of both skin and gill flukes. Being ovicidal it kills off the eggs as well as the adult and young stages; this is particularly important where gill flukes are being treated as live eggs deposited into the environment before and during treatment can re infect koi when they hatch after the treatment has degraded. It is mixed with hot water and added to the pond at the rate of 2.6 grams/tonne (1000 litre). The water is initially cloudy due to the suspension of the flubenol powder but this clears in a few days.

Because it kills the eggs of gill fluke one treatment is all that is normally required however re dosing if further juveniles are seen does not appear to cause any problems to the koi or any other pond fish or the filtration system.


This chemical has been used for several years to control flukes in koi. It is not considered safe to use and re infection occurs when eggs hatch and continue the life cycle of the gill fluke. CAUTION Goldfish are reported to be killed by this treatment whereas Orfe and Sturgeon have been seen to survive.


Trichodina is yet another parasite which can cause serious skin and gill damage. It is usually associated with poor environmental conditions such as high organic loads and unclean filter systems. This parasite lives within the pond water where it feeds principally upon bacteria and as such it does not need koi or other fish in order to survive. It is an opportunist parasite, which will infect fish when their immune systems are not functioning at optimum level.
It is readily detected on skin scrapes at a magnification of 30X. (See photo) When alive, this parasite dashes about resembling a “flying saucer”, when dead it resembles a circle with hairs or teeth around its circumference. The cilia, which are positioned around the circumference, are used to propel the parasite.

Life Cycle

Like Costia, Trichodina multiplies by binary fission.


The same treatment is used as for Costia. That is Potassium Permanganate added to the pond water at 1.5 ppm.

This will eradicate Trichodina from the fish and the water column.

The above mentioned parasites are those that are normally encountered in koi ponds; other less common parasites can affect koi and these are briefly listed below.

Anchor Worm (Lernea)

Anchor worm is a small parasite, visible without the use of the microscope, which attaches by its head under the scale of a koi; the body of the anchor worm protrudes from the scale pocket which can easily become infected when these parasites are present. Virtually any scale can accommodate this parasite

If anchor worm is detected on a koi the recognised treatment is Dimilin which should be available from your vet.

Dimilin does not kill the adult form of these parasites but it prevents the eggs they produce from hatching and continuing the life cycle. It is used at the rate of 1 gram per tonne of pond water at water temperatures over 8C.

To remove the parasite the koi should be sedated; the parasite is easily seen protruding from under the scale. Care is needed such that the whole body of the parasite is removed; the anchor shaped attachment cannot be seen when it is attached to the koi but if the points of tweezers are dipped in a strong solution of potassium permanganate and some of the potassium permanganate is transferred to the body of the parasite it will release its hold from the flesh under the scale and it can be gently removed by pulling with the tweezers. The site of attachment should be cleaned and a suitable topical treatment used to seal the area.

At second treatment with Dimilin after 10 days is necessary to ensure that the life cycle of this parasite is broken. Careful examination of the scales will confirm the success or otherwise of the treatment.

ARGULUS (Fish Lice).

This parsite is also easily visible without using the microscope. Strong salt baths as descibed above will encourage this parasite to release its hold on the koi and tweezers can be used to remove it. A suitable topical treatment should be used at the site of attachment.

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