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Taking a Mucus Scrape

article by Dr Paula Reynolds Aquatic Patho-biologist

Why is it that whilst most Koi keepers simply   treat their ponds with remedies for eradicating parasites others go to the expense of buying a microscope and try to make sense of such a specialised field as parasitology? The answer is that some Koi-keepers see the wisdom of protecting their Koi from needless exposure to chemicals and enjoy the challenge of adding a new dimension to their hobby.




In the confines of a pond, it is not surprising that many organisms find it easy to attack fish not just to seek nourishment for survival but also to reproduce. Each species various in physiology and life style and it is this that complicates chemical control methods. With a microscope, there is no need to expose Koi to undesirable chemicals and their side effects unless there is a genuine need to treat the pond for parasites. In the long term, the money saved on chemicals is nothing compared to Koi living a natural lifestyle free of risk posed by chemically based pond remedies. In addition, the pond and filter biology is preserved which is also important for the support and well being of Koi.


White spot

Ichthyophthirius multifiliis is the best-known ecto – parasite (external) found affecting Koi. Commonly known as White spot this organism thrives in the same environments as most coldwater fish. The parasite is carried by healthy Koi and easily triggered by water temperature changes and consequently is often seen infecting Koi soon after transportation   although the supplier of the Koi   does not have an outbreak. The benefits of using the microscope are evident during out breaks as each stage of the life cycle of the parasite can be fully observed. White spot can be seen with the naked eye if there are many mature cysts on the skin and then it resembles salt sprinkled over the body. At a magnification of X 100, the parasite is visible but at X 400, the clarity improves so that the daughter cells inside the encysted white spot cell can be seen. When this organism ruptures these daughter cells, invade the water and go through various stages before taking up residence on the skin again as tomites then they mature to form the encysted and final stage of the life cycle. The powerhouse of the cell is the white horseshoe shaped nucleus although the teeming life in the cell often obscures this. The encysted white cell spins clockwise by means of cilia as it feeds from debris on the body. When spinning stops the cell is about to rupture. Water temperature influences the rate at which the parasite replicates and commonly two or three chemical treatments can be needed to eradicate it depending on the treatment in use unless of course a microscope is available to monitor progress.


The microscope

A binocular microscope with a light and mechanical substage capable of viewing at 400- 500 times magnification or above will be easier to use but there is a lot of choice on the market. Glass slides and a blunt implement to take the smears are basic requirements. Glass slides can be used to actually take smears directly but this needs a steady hand. Koi are prone to flip and I have seen accidents as the glass slides are very sharp so I think hobbyists are safer with a thin plastic plant tag or similar item. Whatever is used should be kept strictly for fish use and washed before and after use. There are other optional items available when a microscope is purchased form a specialist centre such as kits with dyes for greater definition and gels that slow down the movement of organisms when being viewed microscopically so that they can be better observed.


Selecting the Koi

If the behaviour of the Koi has changed or something is not right in the pond always test the water first. The next step when you own a microscope is to select Koi that appear to have extra mucus or that have been flicking or flashing or behaving oddly. Also choose Koi for examination that are in good health and behaving normally or the sample will not represent the condition of all the fish in the pond. It will take two people to catch the fish and then one can hold the net and the other take the smear. This works if the pond is at ground level. Alternatively, the Koi can be transferred into a Koi bowl if this makes handling safer and easier. The person who is to hold the Koi should wet their hands so they are cool and damp or the Koi will not like being handled.


Taking the mucus smear

Catch the first Koi selected and gently bring the net to the side of the pond. Raise the net very slightly out of the water to expose the top of the head, the sides of the body and the tail region. The handler gently restrains the fish against the net while the other person takes the smear. If a bowl is being used to transfer the Koi from the net with the bowl underneath it, so the fish is never struggling in the net. Ensure the water level in the bowl is   low but covers the gills to ensure the fish will be more controllable. Taking a small amount of mucus from several areas makes for a better examination. The various parasitic species found in Koi mucus have their own favourite locations on the skin and it is not representative to take a smear from one place. Only a spot of mucus is needed and a scraping action will remove more of the mucus layer then is necessary so simply take a small amount from each area.  


It may useful to sample the axilla  which is what I  term the Koi “armpit” the pectoral muscle rests in this area when the fins are folded back against the body and this is a parasite haven. The Koi will have to be held gently over the net or restrained on one side in a bowl to take mucus from this area. Around the gills is another important area to sample but it is not safe to go into the gill cavity unless the fish is sedated. Even then, mucus should be taken only from the underside of the gill cover not the filament. Sedating Koi is something that should not be carried out without good cause as   this is chemical exposure and taking smear can be carried out in a few seconds without any need to use a sedative as long as the handler is gentle.


Preparing the slide and viewing with the microscope


Place the mucus on a glass slide with a tiny drop of water unless it is fairly wet already and then put another glass slide or a cover slip over the mucus. Place the slide on the mechanical sub-stage of the microscope, secure in place and switch on the light. At a low magnification, nothing will be clear and it will take time to set up the instrument. Disregard black stationary objects as air and debris will be trapped in the mucus. Remember water will flow in one direction and can confuse the viewer that this is a parasitic army on the march. Look for movement in the mucus that is spinning or multidirectional and   bear in mind that the heat of the light under the slide can kill off some parasites quickly. Using the controls to move the slide the viewer can see the edges of the mucus, this a good place to see costia for example. With higher magnification, the internal apparatus of each parasite allows for comparison. Gill Fluke for example have black eye dots at the rear of the body and are found only in or around the gill whereas the next generation of Skin Fluke can be seen inside the body cavity to aid identification. Whit Spot rotates to the right and has a   horseshoe shaped nucleaus and Trichodina   resembles a flying saucer when fully mobile and Mexican hat when it is at rest.                                                                                



It is important to appreciate that there are millions or aquatic organisms that can take up residence in any pond. Very few pose any risk to Koi although they may well be parasitic to other life forms. Patience is the “armchair” or in the case of Koi-keepers hopefully the “deckchair” parasitologists’ way to success as the microscope will take a little time to master. Most organisms have a complex lifestyle and the lower stages of Koi parasites are rarely photographed so the viewer has to look for the mature better-known stages when unable to identify the lower stages that may be observed microscopically. It is only with experience that the benefits become obvious and the cost of the purchase equals the savings on pond treatments and the fish really thrive from being well cared for.


The most common Koi Ecto - parasites (Ecto means external)


Visible without a microscope    

  • Argulus or  Fish Louse -

  • Piscola Geometra a type of fish leech

  • Lernea or Anchor worm

Requiring a microscope

  • Chilodonella  

  • Costia  

  • Dactylogyrus or Gill fluke  

  • Gyrodactylus or Skin fluke - is not true fish parasite

  • Trichodina - is not a true fish parasite

  • Ichthyophthirius multifiliis or White spot

Learning in a good cause  


Those Koi-keepers that opt for a microscope appreciate they are not going to become parasitologists. All they will be indentifying are some of the more common Koi parasites. Knowing which parasites pose harm and always require treatment  from those that are often present in the mucus layer of healthy Koi and can be ignored comes with experience. It is the numbers of some organisms found in mucus that is the factor as to whether chemical treatment is needed. In other situations, why re-infection by a specific parasite is occurring is the issue. For example, trichodina is carried by most Koi at low level and it is pointless and harmful to the Koi to continuously treat the pond. Trichodina is not a true parasite it lives in ponds where it can harbour so good hygiene often helps to keep the level low and the organism then poses no threat and only requires treatment if  out of control.  



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