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Gill Flukes

Flukes or Monogeneans are tiny, often microscopic and are one of the main causes of pond fish ulcers.

Flukes bite through the koi’s protective “slime coat” and into the skin of the Koi causing deadly bacterial forming ulcers which can be fatal to your Koi. Gill flukes will eat away healthy gills to a point that your Koi will not be able to breathe.

Because the Koi is unable to breathe properly it gasps at the surface for air. The gills become swollen.

The flukes lay eggs which hatch within 4 days onto free swimming larvae that infect other fish. Each fluke depending on the temperature of your water, can produce between 12-20 eggs an hour. They are monogenetic which means they reproduce asexually and spend their whole life within a single host.

Flukes are one of the most common and dangerous parasites. They actually carry Aeromonas and Pseudomonas bacteria which are responsible for causing sometimes incurable ulcers in fish. And -- flukes are very fertile. Once you get rid of the adults you must re-treat to get the babies that have hatched.

Salt is not a treatment for flukes.

Flukes are a type of flatworm commonly found on Koi and other ornamental fish. There are two major types of freshwater flukes that impact Koi: Gill Flukes (Dactylogyrus) and Skin Flukes (Gyrodactylus) – but hundreds of sub-species exist for each.


Aqua Prazi would be the best choice for treating your koi pond for gill flukes and body flukes. Aqua Prazi will not harm the “good bacteria” in your filter or stress your pond fish in any way. Plus, it will not kill your plants and is safe for your pets.

Prazi Pond Plus is another treatment for Flukes.

Gill Flukes

Skin Flukes

Skin Flukes, although similar looking to gill flukes can be distinguished by the lack of eye spots and produce live young. These youngsters immediately attach to the host. The skin fluke has a similar proliferation for reproduction – and it is believed that the embryo actually produces a fertilized egg while still in the mother!

Skin Flukes are worm shaped and have a set of hooks for fastening on to the rear of the Koi body. When viewing the skin flukes through a microscope up to four developing flukes can be seen within the adult, each one located within the next. Skin flukes give birth to live young, but only one at a time, and the reproduction rate is normally low, unless the Koi are stressed which is when an outbreak will occur. Once a skin fluke is attached to the host Koi, it will live on mucus, skin and blood. However, even if a skin fluke loses its host, it can still survive for up to five days.


Diagnosis of flukes is relatively easy via a microscopic examination of skin scrapes and gill snips. Under 40x magnification, if you see dancing worms, you’ve probably got flukes! Flukes, like most other parasites cause flashing of the host fish, which in turn can open skin lesions and in the presence of certain bacteria (e.g. aeromonas) may result in ulcers.

Remember, flukes live on the host, feeding on blood or mucus, but they also swim freely and lay eggs on plants, etc. – thus treatment of the entire pond system is appropriate.

Recommendation for treating with Praziquantel calls for adding 2.5mg/L to the pond environment (proper volume calculation is essential). Praziquantel is listed as safe for plants and the pond’s biological filter.

Remember any diagnosis or treatment should be under the direction of a licensed state veterinarian.

Flukes: An Update

Commonly encountered Flukes belong to one of two classes, either Gyrodactylus or Dactylogyrus. They are distinguishable by virtue of the presence, number, or absence of eye spots, and whether they are oviparous or viviparous. They have been shown to live on the gill, or on the body, hence the names Gill and Body Flukes, but there is considerable overlap. Flukes have been regarded, at least by this author, as one of the easier parasites to diagnose, but harder to treat.

Medications, in order of preference include:
  1. The organophosphates, which imply some risk to the fish being treated.
  2. The formalin containing compounds, which are famed for their effectiveness, but also their ability to burn fish and kill filters.
  3. Mixtures of organophospates and insecticides, a group most prominently represented by "Fluke Tabs". This compound contains carbonate insecticide.
  4. Praziquantel, which is too expensive for use in ponds, but works nicely in tanks.
  5. Last choice compounds would include potassium permanganate, copper, and maybe others. This last choice class is dangerous, to say the least and should be reserved for professional fisheries personnel.
Impediments to treatment with Organophosphates include:
  1. The minimum dose is 0.25 PPM but this should be increased in harder water or water with a lot of carbonate alkalinity or a correspondingly higher pH. Organophophates are bound by carbonates. Used in systems with a pH over 7.4, a hardess over 30 PPM or a total alkalinity over 80 PPM, the compounds need to be used at double and in some cases, based only on serial microscopy, quadrupled strength, to achieve a level that can influence the fluke population. Increases involve two different modifications of either amount or interval, including possible double or triple dosing at .5 PPM or .75 PPM or using the drug twice as frequently.
  2. Cold water make organophosphates less safe because the fish do not metabolize the compounds as well as in warmer water. I have treated fish in 55F (springtime) water and seen them go corkscrewing. This is a reversible sign, simply by either changing out some of the water, warming the fish in a holding facility or simply suspending treatment and allowing the fish to revive.
  3. Very warm water, over 80F permits the drug to be absorbed far too fast, and at regular dosing intervals, the fish become intoxicated as well.
The Formalin Compounds may Fail:
  1. Formalin compounds sink in cooler water and are very hard to disperse for good effect.
  2. Formalin is bound by organics so in dirty systems, levels of formalin may not remain high enough to kill adult flukes and intercept emerging larvae.
  3. Oxygen consumption by the formalin compounds leaves caustic areas on the surface of the fish and destruction of precious filter bacteria are also hazards of its use.

Formalin is basically Formaldehyde in water. The most common is 37%. Unfortunately, most commonly available preparations contain some Methanol, which contributes heavily to the compound's toxicity to smaller fish. An important point to consider when using Formalin in fresh water systems is that the compound uses or binds free oxygen in systems and the following rule applies: for every 5mg/l (PPM) of Formaldehyde, 1 PPM free oxygen will be used. Formalin's primary use would be only in the treatment of Saprolegnia, (fungus) or Gill Flukes that had not responded to salt. The only other time to use Formalin is if the plants, which might be harmed by salt are more important than the fish. Formalin is used most effectively as a continuous treatment by adding 1cc per 10 gallons water, directly to the system. The most effective way to add it is to drop the water level to half of the pond's volume, then add the amount of Formalin that was calculated for the entire volume. Dump in the calculated amount, wait 2 hours and then top off the pond. You could do a 30 - 40% water change 2 days later and re-apply at full dose using the same drain, treat and top off method. Then repeat 2 - 3 days again, after a 30-40 % water change. A study done in 1976 suggests that Formalin will kill off a substantial portion of your nitrifying bacteria, causing water quality deterioration, in addition to the losses of Oxygen.

Fluke Tabs must be dosed high enough and long enough. Here again, in cool water, the parasites move through their life cycles slowly, so it is important to blanket the system and leave in medication long enough to intercept the emerging larvae. Praziquantel is expensive, but it is also effective.

Aqua-Prazi is 100% Pure Praziquantel. Praziquantel used to only be available for dogs and cats but now it is available in Aqua Prazi.

by DR Eric Johnson D.V.M.

Photo used with permission of Stars n Sites

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