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Trematodes and Nematodes in Fish (Flukes and Internal Round Worms)


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Trematodes and Nematodes in Fish (Flukes and Internal Round Worms); basics, Identification, and treatment in aquarium and pond fish.


The trematodes belong to the Animal Kingdom Phylum Platyhelminthes, the flatworms and are commonly referred to as flukes. Trematodes usually have flattened bodies, a primitive digestive system, suckers for attachment to their hosts, and are hermaphrodites (an organism that possesses both male and female sex organs). The Monogenea are the class ectoparasites that infect fish.

Internal Trematodes:

Internal Flukes generally use intermediate hosts such as snails or other mollusks. The eggs must get into water in order to hatch and be able to infect their first host, a freshwater snail. The fluke develops in the snail and then burrows out to seek the second host which is a freshwater fish. Many species of snail and fish may carry these internal flukes.

Gill Flukes:

Gill flukes (Dactylogyrus) will appear on the gill filaments as tiny dark spots 0.04-0.08in (1-2mm) long. Gill flukes can infect freshwater and saltwater fish and are found on fish from the wild as well as farmed fish. These parasites attach to the gills of the fish and feed on mucus, epithelial cells (cells that line the inside of gills and perform the tasks of secretion, absorption, protection, and transcellular transport), and blood from the host, much of this material being produced by the fish in response to the physical injury caused by the parasites. Damage is physical and inflammatory, with secondary bacterial infections such as septicemia. Small or weak fish may die from the stress of acute infestations.

Common carriers of gill flukes include: Plecostomus, Otocinclus, Corydoras, Koi, Discus, Characins, livebearers (Poeciliidae), some tetras (Characins) and Barbs (Cyprinidae), and Elephant Nose (Gnathonemus petersi).


Also known as Roundworms are a very common phyla of animals of which there are many parasitic forms. Nematodes are one of the simplest animal groups to have a complete digestive system, with a separate orifice for food intake and waste excretion unlike the Trematodes mentioned above. Reproduction is usually *** and males are usually smaller than females. Parasitic Nematodes can have quite complicated life cycles, moving between several different hosts or even locations in the host's body.

If the Nematode has a direct life cycle, then it does not need an intermediate host and infection can spread directly from one fish to another by means of a fish ingesting of eggs or larvae.

If the Nematode has an indirect life cycle the nematode eggs or larvae enter an invertebrate intermediate host (such as copepod, tubifex worm, or insect larva) or a fish intermediate host (these fish are then consumed by larger carnivorous fish) prior to being eaten by or entering the final host fish.

A method of diagnosing a nematode problem is generally just a guess. When a fish is eating well yet is still not putting on weight, an intestinal infestation may be suspected. This is particularly plausible when a fish is eating regularly yet continues to lose weight, metabolizing body musculature to stay alive. This is usually seen as thinning along the back on either side of the dorsal fin. This often results in a well-fed fish starving to death.

True identification begins with a microscope. Nematodes are smooth, cylindrical, relatively long worms, which distinguishes them from the flatter, segmented tapeworms and from the wider and shorter Monogenea Flukes.

A few common Nematodes:

Capillaria is a relatively common nematode that lives in the intestines of angelfish and discus (and other fish as well)

Eustrongylid nematodes are found in muscle within the body cavity or encapsulated on the liver and other organs. These nematodes can affect a number of different species such guppies, gar, danios, and angelfish. Affected fish typically have bloated abdomens (similar to dropsy of bacterial origin), as these nematodes often migrate into the body cavity and can be quite large. Unfortunately the treatment of these Nematodes usually fails (which is often misdiagnosed as bacterial Dropsy leading to the statements that “Dropsy is un-treatable”)

Camallanus Nematodes infect the gastrointestinal tract of live-bearers, cichlids and other species of freshwater fish. Usually, the first indication of infection is a red worm extending from the anus of a fish (sometimes mistaken for feces)

TREATMENTS for internal Trematodes and Nematodes:

*Jungle Internal Parasite Guard (ingredients: Metronidazole, Praziquantel)

Treatments for external Flukes:
*Potassium Permanganate

By Carl Strohmeyer


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