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Koi Parasites In Your Pond

Updated: Sun, 14 Jun 2015

Problems with parasites are usually associated with either new introductions to the pond or some environmental problem such as low oxygen levels, nitrite or high nitrate levels. Low oxygen levels can severely stress fish and can be caused in several ways.
Koi Parasites In Your Pond

Parasites in Koi ponds can often cause serious problems for the Koi, and hobbyist alike. By this I mean, that the Koi have the parasite problem, and the hobbyist has the problem of curing it! Knowing which parasite or parasites are present is essential for the correct remedy.

In order to be able to control levels of Koi parasites in ponds, it is necessary to understand something of their life cycles. Understanding how they reproduce, various life stages are, can allow us an opportunity to seriously reduce their numbers.

Let's look at a few things:

  • Environmental Conditions
  • Signs of Disease
  • Life Cycle
  • Treatments

Problems with parasites are usually associated with either new introductions to the pond or some environmental problem such as low oxygen levels, nitrite or high nitrate levels. Low oxygen levels can severely stress fish and can be caused in several ways.

Low oxygen levels are more likely to prevail in summertime, when temperatures are higher, although some treatments for parasites can target algae and pond debris as part of the chemical reaction aimed at the parasites, and in so doing, create oxygen shortages.

It is wise to increase aeration in the pond before, during and after any medications are used. As fish consume approximately four times more oxygen after feeding, it is best that food is withheld for the duration of any treatment.

Simply overfeeding the fish can cause nitrite levels to rise above normal levels, or it can happen when the stocking level is in excess of that which the biological filter can cope with. Low oxygen levels and low KH frequently lead to nitrite levels rising. The whole pond system relies upon oxygen to function and adequate resources of calcium carbonate are essential to the bacterial activity of the filter.

Nitrate is the end product of the biological filter function and can be controlled by regular water changes. Test kits are available for measuring all the water parameters mentioned.

Frequent parasite problems would tend to suggest that the environmental factors within the pond are less than ideal. Fish living in a pond with good water parameters are able to cope with the small levels of parasites, which are often present in small numbers in perfectly healthy ponds.

At low temperatures very little activity occurs in a pond, bacteria, parasites and fish are all relatively inactive. However once temperatures begin to rise all activity increases. This can cause problems for a Koi because the increased bacteria and parasite activity are occurring before the Koi immune system is able to react to this threat. Bearing this in mind all Koi hobbyists should be particularly vigilant in the springtime.
Some keepers take the prophylactic approach: they rid the pond of any undue organic waste, and rid the Koi of as many parasites as possible in the late autumn when temperatures are still above 59*. Signs of Disease or Parasites

Most parasites will cause fish to react in a similar fashion. Points to watch for would include the following:

  • Fish remains alone and ceases to be a sociable shoal fish.
  • Fish jumping or scraping against pond side and floor.
  • Fish refuses to feed.
  • Fish breathes heavily, opening and closing of the mouth and gills.

It is possible to have parasites in a pond and only one, or a few of the fish seem to be affected. As individuals they can differ in personality as well as their ability to resist disease and parasites.

If this is seen to happen, it is possible to treat an individual by way of a salt bath. Seawater contains about 3% salt. The bodies of freshwater parasites contain about 0.5% salt solution. Osmosis is a simple physics fact which guaranties that water will always tend to go towards an area of high salt concentration. In a salt bath of water of 3% solution, the parasites, being simpler creatures than the fish, can have the fluids withdrawn from their bodies by this difference in salt concentration. The same laws of osmosis exist for the fish so care and continuous observation of the fish during this treatment is essential. Once the fish begins to lie over on its side in a similar manner to that which occurs when a fish is anesthetized, it should be removed from this salt bath and placed into another container to recuperate. A maximum of two minutes is advised for this treatment which can be repeated if necessary a short time later.

An improvement in the disposition of a fish after this treatment can suggest that parasites may be the problem.

Let's look at some of the most common bugs..............

Crustacean parasite, Lernaea - Anchor worm is a common parasite on our Koi which is clearly visible to the naked eye and can reach 10 to 12mm. The parasite burrows its head into the Koi's tissue, under a scale and only the body and tail are normally visible. The juvenile stages settle in the gills of Koi, when they mature they mate and the male leaves the Koi, the fertilized female settles on the body of the Koi and continues to grow, becoming the familiar worm shape. The female buries into the skin and underlying tissue to hold on. The damage caused can become a target for bacterial or fungal infection which can spread. Lernaea lay eggs which can lay undetected in the pond and can hatch when conditions and water temperatures are right. Treatment is by manual removal of the parasite with tweezers under anesthetic, ensuring that the whole parasite is removed. To be sure of complete removal, dip a cotton bud in strong potassium permanganate solution and dab the worm with this solution where upon it will release its grip immediately.

Carp pox. A virus that produces solid waxy lumps on Koi. It will not kill Koi and is generally harmless, but can look unsightly. It is most often present in small Koi and in cold weather, clearing up disappearing when Koi grow and in the spring when water temperatures rise.


Columnaris (Flexibacter Columnaris) or Cotton Wool Disease is another bacterial infection. The common name comes from the white tufts that develop around the mouth and spread to the body and fins, often leading to ulcers and a thin appearance. Often mistaken for a fungal infection because of its mold-like lesions, Columnaris is a common bacterial infection in cultured fish, particularly live bearing fish and catfish. Its name is derived from columnar shaped bacteria, which are present in virtually all pond environments. The bacteria are most likely to infect fish that have been stressed by such conditions as poor water quality, inadequate diet, or handling and shipping. Columnaris can enter the fish through the gills, mouth, or via small wounds on the skin. The disease is highly contagious and may be spread through contaminated nets, specimen containers, and even food. Treatment with anti bacterial medicine is usually effective.

A number of bacteria are associated with finrot, lesions and internal hemorrhaging, notably Aeromonas and Pseudomonas. Ulcers usually start at the site of an injury, the bacteria then infect it causing further damage, and fungal infection can also occur. Such holes result in osmoregulatory problems, leading to damaged kidneys and death if not treated. It is worth adding a weak salt solution to the pond as well as anti bacterial remedy, a concentration of 3gm per litter will help to restore the osmotic balance and reduce strain on the kidneys (make sure that the salt is fully dissolved before you add it to the pond).Finrot is easily noticeable, the fins and/or tail look chewed and are red at the edges. Secondary bacterial and fungal infections can develop. Treatment: ProForm-C.

Argulus another crustacean parasite, round and up to 1cm wide. They have a sucker to hold on to the Koi with needle-like mouth parts which they stick into the Koi and inject a toxin. This causes intense irritation to the Koi and they scratch and jump and can cause bacterial infection. If they infect the gills they cause severe damage and often death.

Gill maggots are the mature females of the parasitic crustacean Ergasilus. Ergasilus (gill maggots) will appear as grayish black and white parasites several millimeters long infesting the gills. Heavy infestations can cause severe damage, eroding the gill filaments and allowing secondary infections to develop. Recommended treatment: ProForm-C follow up with BGDX

KHV Is a very contagious, damaging and deadly disease for koi, many times causing up to 90% loss in just a few days. Symptoms can include respiratory distress, hyperactivity, loss of coordination, and severe gill necrosis. It's usually seen when the water temperature is between 70 and 80 degrees. As with most viral diseases there is no known cure. Some people have had success with heat treatment to 86F, but the carrier state of the survivors is currently unknown. Goldfish are unaffected.

Monogenea are small parasitic flatworms mainly found on skin or gills of fish. They are rarely longer than about 2 cm. A few species infecting certain marine fish are larger and marine forms are generally larger than those found on fresh water hosts. Monogynies lack respiratory, skeletal and circulatory systems and have no or weakly-developed oral suckers. Monogenea attach to hosts using hooks, clamps and a variety of other specialized structures. They are often capable of dramatically elongating and shortening as they move. Biologists need to ensure that specimens are completely relaxed before measurements are taken. Like all ectoparasites monogeneans have well developed attachment structures. The anterior structures are collectively termed theprohaptor, while the posterior ones are collectively termed the opisthaptor. The posterior opishaptor with its hooks, anchors, clamps etc. is typically the major attachment organ.
Like other flatworms, Monogenea have no true body cavity (coelom). They have a simple digestive system consisting of a mouth opening with a muscular pharynx and an intestine with no terminal opening (anus). Generally, they also are hermaphroditic with functional reproductive organs of both sexes occurring in one individual. Most species are oviparous but a few are viviparous. Monogenea are Platyhelminthes and therefore are among the lowest invertebrates to possess three embryonic germ layers—endoderm, mesoderm, and ectoderm. In addition, they have a head region that contains concentrated sense organs and nervous tissue (brain). Treatment: Praziquantel as Prolonged immersion 10 g to each 100 gal.

Raised scales (rather like a pine cone) and eyes standing out from the head. Dropsy itself is not a disease, but rather a result of some other cause. Dropsy is a term given to the swelling that occurs internally in the fish. There are multiple possible causes. Sometimes it's not contagious, but sick fish should be isolated and treated since determining the actual cause may be impossible, and also because this will be easier on the fish. The fish's body will become swelled with fluid it is unable to expel. Eventually the swelling will cause the scales to rise, giving the fish what is called the "pine-cone" appearance.

Diagnosis, One of these situations may be the cause:
Sudden swelling: A bacterial infection will cause internal bleeding. Slow swelling: Growing tumors, or even parasites, in the fish may cause it to swell. Slow swelling: Mycobacterium tuberculosis. Highly contagious! Bacterial dropsy is infectious so treat with an anti bacterial remedy and if possible isolate affected Koi. One of the most common fungal infections of Koi. The fungal spores will grow anywhere on the Koi, including the gills, initially germinating on dead tissue. Their thread like hyphae release digestive juices which break down the tissue so the fungus can absorb it, as the fungus grows these juices start breaking down living tissue Fungus on the body appears as cotton wool like growths, it is hard to tell if a Koi has it in the gills, but if it hangs at the surface gulping for air it is likely. Treatment: or ProForm-C Gill and Skin flukes are two of the family of monogenetic trematode genera, all of which are characterized by the large grappling hooks which are used to attach themselves to their victims. Flukes are another common parasite affecting our koi are both egg layers and live bearers. They range from 0.05 to 3.00mm long and there are actually a huge number of species in the genus.
Affected Koi often exhibit classic signs of irritation and flash, jump or rub themselves against objects in the pond in an attempt to rid themselves of their attackers. Flukes are not visible with the naked eye. When viewed under a microscope, the parasites are clearly visible as nearly transparent and worm like, and the hooks are clearly visible. Flukes are a bit like fleas on dogs and cats and it is common to see one or two on a slide as a healthy Koi can control parasite numbers and their mucus helps prevent the parasite moving. Treatment is therefore only necessary if flukes are seen in numbers.
Recommended treatment: Praziquantel follow up with BGDX

Like the name suggests, SVC in seen only in the spring as pond temperatures are rising. It's usually seen between 40 and 60 degrees Fahrenheit.

This virus is much less damaging than KHV and currently is very rare in the . SVC produces symptoms typical of those seen with many infections and can include dropsy, hemorrhages and/ or darkening of the skin.

SVC is classified as a "reportable" disease, so a confirmed diagnosis can lead to heavy handed federal government involvement. Trichodina is one of the easiest protozoan parasites to detect under the microscope as it is almost perfectly round with hundreds of hooks which resemble cilia found its periphery and it constantly rotates as it moves through the mucus, causing tissues damage. It attacks both skin and gill tissues of our Koi, and can often cause more damage to gills than realized. Classed as a warm water parasite, it can survive for some time without a host. It causes vegetation of the skin giving rise to a grey white opaque appearance on the body of infected Koi which exhibit the classic symptoms of flashing, rubbing and lethargy.A magnification of 100 to 200 x is required to view this parasite.
Recommended treatments are Potassium Permanganate or ProForm-C. Follow up with BGDX Caused by Ichthyopthirius multifiliis. The white spots on the skin, gills and fins are individual protozoan cells that are under the skin and feed on the body fluids and cells. They then punch out of the skin and fall to the bottom of the pond, collect together and begin breeding, the offspring then re-invest the fish. As well as white spots symptoms are scratching and swimming into the water inlet, failure to feed and lethargy. It is fatal if untreated. Fortunately commercial white spot remedies are widely available. Recommended treatment. ProForm-C Costia is a minute Flagellate with 3-4 flagella. It affects both the skin and gills of Koi, and reproduces itself by binary fission. Infestations of this parasite can appear very rapidly indeed, and Koi suffering infestations exhibit the classic symptoms of lethargy, clamped fins, rubbing and flashing and the skin can take on a grey white opaqueness. Costia normally only affects fish that have already been debilitated by some other cause, and can often be seen on Koi as a secondary parasite. A high magnification must be used to view these parasites (300 x) and staining is recommended for positive identification. Recommended treatments include Potassium Permanganate, or ProForm-C. It should also be noted that disease and parasites are always present, even in the healthiest pond – they are part of the natural order of things.

A severe outbreak of parasites requires treatment. However, in any disease outbreak it is essential to identify the prime cause. I hate to think how many Koi deaths have occurred because of repeated and unsuccessful treatments for parasites without knowing cause of them, which is usually an environmental factor.

Description: Parasites in Koi ponds can often cause serious problems for the Koi, and hobbyist alike. By this I mean, that the Koi have the parasite problem, and the hobbyis