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FAQs on: Flukes/Trematodes 1, Flukes/Trematodes 2, Flukes/Trematodes 3, Flukes/Trematodes 4,
FAQs on Flukes/Trematodes: Diagnosis/Symptomology, Etiology/Prevention, Treatments That Don't Work, Cures That Do Work, Products/Manufacturers; Case Histories

Related FAQs; Flatworms/Planaria 1, Flatworms 2Flatworms 3, & FAQs on: Flatworm Identification, Flatworm ID 2, Flatworm ID 3, Flatworm Behavior, Flatworm Compatibility, Flatworm Control, Predator Control, Chemical Control, Flatworm Selection, Flatworm Systems, Flatworm Feeding, Flatworm Disease, Flatworm Reproduction,

Related Articles: Pest Flatworm Control by Anthony Calfo, Worm Diversity

Pathologically Speaking Series:

The Flatworms That Are Flukes,
the Trematoda


By Bob Fenner


More common than most folks are aware; there are times when Fluke presence results in captive fish losses. These small to tiny flatworms are very prevalent on several families of popular aquarium fishes; and often survive being collected along with their host, causing debilitation and mortality.

And this is a great shame; as the external Flukes are usually easily excluded from introduction by using simple dip/bath techniques. We’ll discuss these preventative measures, as well as the less desirable chemical poison remedies that are best avoided.


Flukes are worms; Vermes… amongst many extant phyla. More particularly, they are Flatworms; making up a subdivision of the phylum Platyhelminthes. This branch of zoological classification includes the mostly free-living Turbellaria, and parasitic Tapeworms, the Cestoda. There are other schemes of classifying Flukes, but for our purposes here, I’ll stick with them all comprising the Trematoda. The origin of the common name "Fluke" is related to many species overall rhomboid body shape; reminiscent of the flounders folks call flukes; though others are irregular and tube-worm shaped.

A bit more taxonomy is called for in dividing Flukes; into digenes (Digenea) and monogenes (Monogenea):

Digenea means "two generations" and refers to these 11,000 species more complex life histories. Digenes go through two or more intermediate hosts… as an example infesting an aquatic snail or crustacean, this being consumed by a fish… and possibly the raw fish being consumed by a land mammal… where the worms metamorphose into a stage where sexual reproduction takes place (in the definitive host by definition) and make their way back into the water to infest the intermediate host/s where they reproduce asexually.

Digenes almost always have two holdfasts, mechanisms to attach to their host; a ring around their mouth that acts like a sucker, allowing the worm to feed and hold on, and one about mid-body on their underside.

Monogenea as you might assume, have direct life cycles; the more than 1,100 species not requiring intermediate hosts to re-infest their hosts; trouble in closed-system aquariums. Monogeneans are mostly external parasites on fishes; though some are parasitic on amphibians and aquatic reptiles.

Monogenes differ from Digenes in possessing a single set of attachment apparatus; called haptors, these are large hooks and numerous configurations of suckers on their distal (from the head) end.

A public domain graphic from the 1911 edition of Encyclopedia Brittanica showing range examples in body shapes of Flukes. Most found as external fish parasites are decidedly worm-like in appearance.




Fluke parasites of fishes span a large range of sizes; from 0.2 mm (0.0079 in) and 6 mm (0.24 in) in length. The smallish ones require a microscope to make out; the larger species are macroscopically appreciable as black, white dots on fishes eyes, gills and skin. However, most times Fluke infestation is detected secondarily. Behavior of host fish that have flukes often expresses itself in cessation of feeding, lethargy, erratic swimming and glancing, rubbing their face, bodies on the substrate, against other materials in the system; labored rapid breathing. In advanced cases body lesions may develop; showing as red sores. These areas, the gills and fin origins are good areas for sampling for microscopic examination.




Get thee to a cleaning station!

Here a Parrotfish in Curacao can be seen to be infested with external Flukes. External Trematodes (oh yes; there are internal species) are often found on gills, eyes, around the mouth and cloaca, fins and fin origins on the body, flukes are usually white or black in appearance where large, and transparent to translucent when small.

These parasitic flatworms feed on blood and soft tissue; including the softer integument of fishes. Too many flukes populating a host fish may result in irritation, loss of blood and other tissues, ionic constitution imbalance… and death if not dealt with promptly.



Definitive identification requires sampling and observation under a microscope. Gently remove the subject fish from the water and hold it in a wet net and damp, clean (no soap, detergent residue) cloth, and using either a scalpel or other scraping tool, remove some body mucus by gliding the tool from the head to tail in direction. Sampling live fish gills is a bit trickier. I use a flat tipped forceps to carefully grab and remove spots and mucus from the lower area of the gills, gingerly prising up the gill membrane.


Immediate/Short Term Help:

If your fish/es situation is dire, some short term relief can be had by lowering the system water temperature and/or specific gravity. Cooler, less dense water holds more oxygen and improves conditions for the host fish/es over the Flukes.


By far the best treatment is none at all. Healthy specimens placed in suitable environments, kept nitrified by foods and supplements; whose systems are well-maintained may well have appreciable parasite fauna. However; more advantageous still is providing all the above and doing what you can to exclude the introduction of these and other parasites.

Dips (short duration) and Baths (minutes) of simple pH adjusted, aerated freshwater can remove many if not most external Flukes. Adding a modicum of formalin to the dip/bath is even more efficacious. A stock solution (37%) is diluted by adding five milliliters to a gallon of bath solution; the fish immersed for about five-ten minutes, WITH aeration provided. Stronger (more concentrated) baths can be used, but these are more dangerous; toxic to the fish being treated. Formalin is a biocide; it kills all life; and is carcinogenic. Take care in its use. Also; BE present for the duration of dipping, bathing your fishes; lest they show signs of too much stress and have to be removed.

Do look at the bottom of the dip bucket; you may find little curled up grains which are shed flukes. Oh, and do take care to poor the dip/bath water down the sanitary sewer, NOT back in a tank.


There are chemical Anthelminthics ("against worms") and Vermifuges ("flee worms") on the market. The best currently is Praziquantel. The active ingredient can be administered to the water (with marines) at a dose of 20 mg/liter. Alternatively or concurrently it can be mixed into foods at 40 mg/ 10 gm.s of food, and offered for a week. Feeding this compound better insures a physiological dose and may act to remove lumenal (gut) worm parasites as well.

Cleaner Organisms:

Why not go au naturale? There is a wide selection of non-obligate cleaner organisms that help remove Trematodes in the wild; and they can do the same for your fishes in captivity. For peaceful systems that lack shrimp eaters, there are Hippolytid and Stenopid shrimps that will do double duty as parasite removers and interesting specimens in their own right.

For set ups with shrimp eaters the small gobies of the genera Gobiosoma and Elacatinus are superb choices. Even though they are little, even large predaceous fishes that hail from other areas recognize these fishes as helping friends and generally leave them alone.

A quick note here re the use of obligate (have to) cleaners like the Wrasses of the genus Labroides. Not only are these touchy aquarium species that ship poorly, but should they survive, they often starve to death in our small captive systems for lack of sufficient food. Leave them in the oceans.





There are quite a few purposeful cleaning organisms that are known to remove and ingest Flukes. Some faves here: At right, a Cleaner Goby, Gobiosoma oceanops.



IF your livestock won’t treat them as food organisms, there are several species of cleaner shrimps to employ. Here a couple of Lysmata amboinensis are giving the author a once over in Bali, Indonesia.




If you’ve lost fish specimens for "anomalous", make that unknown reasons, you may well have had a Fluke issue. These Platyhelminths are ubiquitous on fishes; and many other aquatic animals; and generally "kept in balance" in the wild due to currents, much more space between hosts, and cleaner organisms. A conscientious aquarist will not only know the signs of Trematode appearance and symptomology, but take steps to prudently exclude these parasites; treat them should they "show up"; and utilize useful cleaners to limit their numbers, offer surcease to their captive livestock.

Be vigilant in screening all new purchases; isolating them to avoid contaminating your main/display system; and doing a standard dip/bath to further prevent their introduction. Should your principal set up become infested, consider your choices of treatment carefully. There are definite downsides to trying to selectively poison your tanks to eradicate Trematodes. Other worm life kill-off, the resultant pollution and its ill-effects can cause major trouble in established systems.

Bibliography/Further Reading:

Flatworms; Wiki: https://en.wikipedia.org/wiki/Flatworm#Major_subgroups

Noga, Ed. J. 1996. Fish Disease. Diagnosis & Treatment. Mosby. 367 pg.s This is the first edition; there is a second.

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