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Koi Parasites

Why do Koi get parasites?

Aquatic organisms are part of nature although only some are actually parasitic to Koi, and others are

termed free livers. Parasites such as, costia, chilodonella, and free living species such as trichodina, are introduced by new Koi , as there is no other route for their transmission into a pond. This does not mean the dealer is at fault it is normal. However, fish lice and leeches for example can be introduced by water birds and animals that can access infected ponds, streams, or rivers and then visit a Koi pond. This route has to be seen in perspective, as the infected water must be fairly near a healthy pond for this to happen. Aquatic organisms cannot survive without water and birds and animals are not hosts to fish parasites and their natural defences soon destroy them. This means that the bird or animal has to leave infected waters and then visit a koi pond fairly quickly to be the culprit that introduces a parasite. Pond plants can also be a source of certain parasites if taken from an infected source. Some aquatic centres may stock fish and plants in the same tanks and plants from a friends pond is yet another possible source.

Why is it that treating ponds in spring and autumn is recommended by some and advised against by others.

Some Koi keepers test pond water regularly, vacuum their ponds daily, and quarantine all new Koi. Such hobbyists will not need to use chemicals on regular basis, if ever. Other Koi enthusiasts enjoy shopping around for Koi, do not quarantine, and accept their type of pond might need treatment that is more chemical. Koi keepers vary greatly in their approach to keeping Koi and that is why dealers and others have to advise certain customers differently.


Why is it some hobbyists simply treat their ponds with chemicals regularly and others need a microscope yet parasitology is a highly specialised field?

To avoid exposing their Koi needlessly to chemicals many Koi keepers accept the challenge of using a microscope. They soon discover parasitology is a huge subject although they only need to identify the common Koi parasites and need no expertise. Soon by taking smears from several Koi not just one they are also able to assess the level of parasites and if treatment is required. Using a

microscope can save money on chemicals and it is far safer for the Koi


What safety measures are often overlooked when treating parasites?

Use a microscope, or consult a dealer or pond or aquatic consultant as inappropriate treatments can

make a problem worse.  It is safer to use VMD approved products purchased from a Koi dealer rather than generic or illegal substances.  Many pond treatments are not fully effective under 13-14C.  Never treat the pond on a hot day, in extreme sunlight, or when the air pressure is high. Air pumps are vital as all products lower dissolved oxygen in ponds and this causes fish losses.  Never mix products or use them in quick succession.  All pond water parameters should be tested before and after treatment to ensure safe use and filters might need boosting with filter bacteria after chemical treatment if there has been any dieback.  

How can parasites be avoided?

Buy Koi from a dealer who quarantines to a high standard, allows the mucus layer to re-establish

after treatment, and the Koi to be in excellent condition before sale. Avoid taking in another hobbyists Koi even short-term, and home quarantine is yet another option. A pond designed to stop water birds etc accessing it, such as a raised pond will also help.

Taking a Mucus Smear.

An easy to use microscope is binocular with two lenses as one lens can be harder to use. It should also have an inbuilt light, and a mechanical sub-stage to move the slide, and magnification capacity of times 500 or over, although there are simple and cheaper models that will do the job. Glass slides and a blunt implement to take the mucus smears are also basic requirements. Using a glass slide to take smears is for experts, hobbyists are safer using a thin blunt edged plastic item such as a plant tag or a small plastic spatula that can be washed after every use. Select several healthy Koi as well as those irritated or with a misty appearance for examination. One Koi alone will not represent the

condition of all the fish, as when run down they are more vulnerable to parasitic infection.  Also examining the mucus in one koi may not reveal every parasite species that is present.  


Once the fish is in the net at the pond side raise the net out of the water and if possible expose the body while keeping the gills in the water to keep the fish calmer. Gently take a small amount of mucus from various locations such as the head, the sides of the body, and the peduncle area near the tail. Mucus is easy to remove and the implement should only be used in the direction of the scale layout so as not to damage them. Taking spots of mucus avoids the scraping action that removes more mucus than necessary, and sampling several areas overcomes the fact that parasites favour different locations. The axilla is the indent the pectoral muscle rests in when the fins are folded against the body and it can be a haven for parasites. To sample in this area gentle restraint in a wet towel may be needed. It is unsafe to take smears inside the gill without sedation, even if sedated mucus should be sampled from the underside of the gill cover not the filament. Smears from the skin around the gill cavities often find gill flukes. Sedating Koi should not be carried frequently and with

experience smears can be taken in a few seconds without the need to sedate.



Signs: Koi will flick or flash o objects and can develop excess mucus, a microscope is required to see skin fluke.  Numerous fluke species can live internally as well as externally in Koi. Gyrodactylus known as skin fluke by Koi keepers is a common irritant although it is a free-living organism not a true fish parasite.  It lives in many ponds at a low level without being a cause for concern. Skin fluke feed on the waste  matter held in the mucus layer to obtain the nutrients essential to their survival. This organism does not directly parasitise fish and any skin damage is due to the Koi relieving the irritation a high-level infection can cause. Skin fluke give live birth into the mucus layer and several future generations can be observed inside the body of the adult fluke when viewed microscopically. At birth, some are mature and this creates a potential level of reproduction that can become seriously excessive. The skin fluke is most active at summer water temperatures and mucus smears from several Koi rather than only one will reveal the scale of the outbreak. Finding a few skin fluke does not suggest a heavy infection, however, changes in the mucus layer that create a white film and flicking to relieve irritation mean a pond treatment is necessary. If a high fluke level is found only on one Koi that fish may be unwell or vulnerable for an unknown reason. 


Treatment: Products based on praziquantel or flubendazole target skin flukes



Signs: There are no signs of infection at a low level some Koi may me listless if level rises, becoming lethargic if the eggs hatch. At an advanced stage Koi are unable to close gill fully. A microscope is needed to see the gill fluke. Dactylogyrus or gill fluke is a true fish parasite and by using the gill for feeding and reproduction, the

parasite can damage the gill filaments if the infection is severe. The gill fluke has one or two pairs of black eyespots depending on its species making it easy to tell apart from skin fluke on a microscope slide. Unlike skin fluke, gill fluke lay eggs so there is no offspring visible inside the adult. Gill fluke adheres to the gill filaments and this can prevent the uptake of oxygen in a high-level infestation. If damage occurs to the filaments bacteria and fungi can then invade to cause secondary disease. In the early stages, there are no signs of gill fluke activity, even if the infection progresses Koi rarely appear irritated, although they often become lethargic as the gill becomes congested. In advanced outbreaks, the gill may be unable to close completely due to tissue inflammation. If the infection progresses and viable eggs are held in the gill tissue, they will hatch at around 18-19C. However, many are washed out of the gill and as they have hours to find a new host, only a few survive.  A few gill fluke do not constitute a major problem as many die off naturally. However, when many Koi are found to carry them pond treatment is appropriate. If the gill is severely damaged survival is unlikely.

Treatment: Products sold to treat skin fluke based on praziquantel or flubendazole are also effective for gill fluke.



Signs: During a minor infection Koi may become listless and as it advances irritation and excess mucus develops .microscope required Much of the time costia lives in harmony with fish until it is triggered into a level of reproduction that causes irritation. In turn, this can lead to skin damage and studying the costia life cycle is not a reliable control method. Whilst Costia is easy to treat, it seldom leaves a pond and it only takes a change in the conditions such as decline in the standard of the water quality to trigger it again. Costia recognises the weakness in fish that are not in good health and it is a very common secondary infection when fish are suffering from disease or other parasitic infestations. Costia is most active at spring summer temperatures and can still be found in smears in autumn when it is more likely to be living in harmony again depending on the pond conditions. It is common for only one Koi in a pond to be infected if that fish alone is unwell. In many cases, fish need to be in good health and excellent pond conditions in order to exert their natural control measures against costia and other organisms. 


Treatment: Malachite green and formalin or a branded product suggested by a Koi dealer.


Signs - Irritation and excess mucus a microscope is required for identification.  Trichodina is a free-living organism not a fish parasite. The organism is rounded in shape although from one viewpoint it can resemble a flying saucer and from another a Mexican hat. There are other trichonid family members found in Koi ponds although their morphology or body shape varies.  Trichodina are seen in ponds at low level needing no treatment, although when present at a higher level it is likely that greater attention is required to the removal of waste matter from the pond. When the fish are irritated by the level of trichodina whilst this is not a serious threat, the pond needs treating to avoid the skin damage that relieving irritation can trigger. Trichodina is most active at spring and summer temperatures and can be a stubborn organism that is rarely completely eradicated.

Treatment: There are several products that can control trichodina - potassium permanganate is the most popular. Care must be taken with this substance to follow the advice on the chosen brand not the Internet. This substance can make skin vulnerable avoid repeated treatment.


Signs: Food refusal - lethargy – motionless near oxygen supply - irritation - microscope required White spot is carried harmlessly by healthy Koi until triggered into activity by changes in living conditions primarily water temperature. Consequently, utbreaks are common after Koi are moved for any reason such as importation, when purchased from a dealer, taken to and from a Koi show and even heavy rainfall can chill the pond. It is wise to monitor any Koi that have been through temperature changes. With a microscope at X100 magnification, white spot is visible although clarity is greater at X 400 and daughter cells inside the encysted white spot cell can be seen. When the cell ruptures, the daughter cells invade the pond and go through many stages that cannot be observed until returning to the fish as tomites that resemble a coffee bean. They then go through a maturing phase and form a new cyst to complete the life cycle. The powerhouse of the parasite is its white horseshoe shaped nucleus, and the cyst spins clockwise by means of cilia that take up nutrients in the debris on the body. When spinning stops the cell is about to rupture. The body of fish suffering from white spot appear to be sprinkled with salt. The spots are minute and hard to see until an

outbreak is advanced. However, Koi can be listless, irritated, and mortalities occurring long before the actual spots are visible. Water temperature greatly influences the life cycle and at 20C, a full cycle will take around 8 days in most ponds. Higher water temperatures can kill off the parasite but heat is an unreliable control method as fish losses still occur until the upper temperature is reached. Confusingly for those who keep tropical fish as well as Koi there are other species also termed white spot that live at higher temperatures. 


Treatment: Malachite and formalin or a specific white spot remedy



Signs: Listlessness, gasping at the surface, irritation, clamped fins, excess mucus, and a microscope is required for identification.

When viewed microscopically chilodonella is slightly larger than white spot for example. It is pear shaped although its body repositions as it moves and the upper part can form a heart shape. One side of the organism has short cilia or hairs and the ventral or underside has longer cilia where the body is pointed. Cilia are fine hairs that create the gliding motion that enables the organism to move through the mucus seeking nourishment. Inside the parasite is a nucleus that powers its activity and two vacuoles that can be seen contracting. Chilodonella reproduces either by conjugation involving two organisms or by mitotic division, which is more common. It is an opportunistic parasite that remains inactivate awaiting any weakness in the fish before attacking. It is the gills in Koi that are vulnerable when the level of the parasite is high. Chilodonella can survive temperatures as low as 5C although it will not be creating the level of infestation that it is capable of at summer water temperatures . It is highly infectious and harmful so treatment should be carried out as soon as it is identified.


Treatment: Malachite green and formalin 



Signs: The Anchor worm can easily be seen as it protrudes from the body and resembles a small fish bone. Outbreaks of anchor worm have decreased particularly in newly imported Koi due to better controls in Koi farms .The life cycle has many stages that cannot be observed in the pond. The females become capable of breeding while still going through metamorphosis and they outlive the males to complete the life cycle. Once the anchor end of the body is embedded in the skin of Koi, usually under a scale, the female lays her eggs and soon the first stage of the cycle known as nauplii hatch out. Anchor worm can be removed with tweezers trying not to break the delicate body. If the anchor end is left in the skin this can create an entry portal for bacteria, fungus, and other diseases. Dab the area with an iodine based product after removal. When the anchor worm is removed look for the yellow egg sac, if this is missing, the eggs may have been laid and in the case of a new Koi possibly before purchase. There is no need to treat the pond if only one or two anchor worm are observed and safely removed. Only when there is an obvious outbreak and the parasite is reproducing is pond treatment required. 

Treatment: A product that specifically targets Lernea.



Signs: Argulus resemble black spots on skin or fins and can be without a microscope.  Fish lice are easy to identify once one or two have been closely examined. Juvenile lice are identical to adults they originate like black dots and gradually get larger as they mature. Lice can hide in the skin pattern on Koi and the trick is to keep watching the same black spot to see if it moves. When lice are suspected, filling a glass with pond water and letting any debris in the water sink will reveal the juveniles as fast moving black dots. Treating at this stage could rid the pond of a whole lice generation but once they reproduce several treatments will be needed. Reproduction at spring and summer water temperatures is normal although this ceases at approximately 12C. Apart from irritation and skin damage, lice spread diseases and if their numbers are high enough per fish, they can cause anaemia as they inject a stylet into the fish and excrete an anticoagulant to ensure the blood of the host fish keeps flowing. They live primarily on blood as well as nutrients on the skin like other parasites. Koi differ in their response to lice and can be listless and sit on the pond floor or irritated and jumping out of the water. Some hide a low-level infestation for weeks and even learn to cope with them whilst others react immediately either way the parasite needs treating as soon as possible.

Treatment: A product that specifically targets fish lice general pond treatment will fail.



Signs: Resemble grey or brown worms and can be seen clearly on Koi The fish leech resembles a worm and firmly attaches to the host Koi by means of a sucker at one end of its body while the rest of the body is seen to expand and contract. They live for several months depending on the pond conditions and can grow to 50mm in length. The leech can be grey or brown as their colour changes when they ingest fish blood and Koi mortalities are possible in severe infestations due to blood loss. Koi can be cross-infected by any disease that leeches potentially carry. In addition, bacteria, fungi, and other harmful agents that live in all pond water can enter the body through the entry portal that the leech creates. Leeches do not remain on their victims continuously and will swim off in order to digest the blood and later seek another host .There are several species of leech seen in UK ponds and not all parasitise fish. The fish leech is often mistaken for the grey pond leech that is slightly pointed to the rear of its body and is ever seen on fish. Leeches are active during the warmer months of the year and the major problem in their control is the cocoons they lay to harbour and mature the next leech generation. The cocoons are not always permeated by chemicals and this means that the juvenile leeches have to hatch in order to be treated successfully in some infestations. Treat the pond as soon as the fish leech is identified, as several treatments could be required.


Treatment: A product that specifically targets the fish leech a general pond treatment will fail.

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