Aquarium Bible

Helminth Diseases in Aquarium Fishes

Helminth Diseases in Aquarium Fishes
Helminths are a collection of worms that consist of a large heterogeneous group of parasites, which includes the following: flukes (trematodes), roundworms (nematodes), tapeworms (cestodes), thorny-headed worms (acanthocephala), and tongue worms (linguatulids). In the natural fish world they are quite common. However, many of these parasites rely on secondary hosts such as snails, crustacea, birds, and other mammals, to complete their complicated life cycles, therefore they are relatively rare in the aquarium. Nevertheless, there are a number of monogenetic skin flukes that have no need for an intermediate host and could therefore cause problems if they are allowed to develop in the aquarium.


Skin Flukes


A live bearing parasite, which has the extraordinary ability to produce an embryo (child) that contains a secondary embryo (grand-child), and quite often that embryo contains yet another embryo (great-grand-child), thus pre-forming four generations in one process. After being born the young worms may remain on the same host or go in search of another. Lack of aquarium maintenance is usually associated with these flukes.

Gyrodactylus can be found on the body surface and/or gills of its host, and growing up to 1mm in length are visible with the naked eye. They attach themselves by using a cluster of hooks, which are located at the rear end of their body, and by producing a sticky secretion within the head.

An attack of Gyrodactylus will result in skin damage, causing skin colour to fade along with an excess production of mucus. Small spots of blood may be seen in areas where the fluke’s hooks have been active. If the gills are attacked respiration will be affected and the gill cover will be open wider than normal. Heavy infections will affect the fish’s mobility, younger fishes being more at risk; a diseased fish will become more and more weakened until it eventually dies.


Treatment has to be swift; an attack from this fluke can be harmful. Fishes already too weakened may not be saved. First of all infected fishes should be removed from the tank and placed in isolation, this way the larvae will fail to find a new host and will die within a few days. Whilst in isolation diseased fishes can be treated with a pre-mixed, ready-to-use medication from your aquarist store. Methylene blue, Formalin, and Chloramine are also effective treatments. See Method of Treatment



A monogenetic parasite of the gills. It is an egg-laying fluke and the eggs can lay dormant for a long time in cold water, however, in the warmer waters of the tropical aquarium they will hatch in a few days. When the parasite finds a host fish it will initially attach itself by means of a ring of small hooks, which are situated at its rear end. Once it is attached it will drive two larger hooks, which lie within the circle of small hooks, into its victims skin. The fluke also has gland openings at its front end; these glands secrete a thick, sticky liquid, which gives it a further purchase onto the surface of the skin or gills.

A heavy attack of Dactylogyrus can cause devastating damage in the aquarium as it attacks the gills of fishes, with younger fishes being more at risk. Infected fishes can be seen gaping for breath, their gill covers wide open and their pallid gills expanded as the infection destroys the gill tissues. Drooping fins, emaciation, and loss of colour will accompany the respiratory collapse as the fish eventually dies from asphyxiation.


Same as Gyrodactylus
First of all infected fishes should be removed from the tank and placed in isolation, this way the larvae will fail to find a new host and will die within a few days. Whilst in isolation diseased fish can be treated with a pre-mixed, ready-to-use medication from your aquarist store. Methylene blue, Formalin, and Chloramine are also effective treatments. See Method of Treatment



Diplozoon paradoxum is another monogenetic fluke or trematode, which can be seen with the naked eye, measuring between 1 and 5mm in length. A parasite of freshwater fish, the adult body is made up of two individuals fused together, neither of which can survive alone. The larva of this species is called a diporpa. There is no development unless two diporpa larvae come together, and then each grasps the dorsal button of the other by means of their ventral sucker. This triggers a metamorphosis that leaves the two larvae fused. The intestines spread and branch out through both individuals and the male and female reproductive ducts become reciprocally fused, so they are in a state of permanent copulation and cross-fertilisation is assured.

Rarely seen in the aquarium, although it can be brought in on newly imported species from open waters. This parasite attacks the gills of fishes; and can be seen as a greyish-brown worm-like creature between the filaments of the gills. As the attack increases it can cause the gills to become inflamed, whereby bleeding may follow, the fish will close its gills and show signs of breathing difficulties.

Other Diplozoon species include D. barbi , which is found mainly in Cyprinids, and D. tetragonopterini that is restricted to the Characin family.


Initially you could try a salt solution bath, 20 minutes, in a 15gm per litre solution. Whilst in isolation diseased fish can be treated with a pre-mixed, ready-to-use medication from your aquarist store. Harder to treat than Gyrodactylus or Dactylogyrus I haven’t come across anything specific to treat Diplozoon paradoxum. Longer immersions in Trypaflavine or Mepacrine (Atabrine) could be tried. See Method of Treatment.


Other helminths

Diplostomum and Clinostomum, Black-Spot disease

Two other parasitic worms in this category are worth a mention these are Diplostomum and Clinostomum. It is the larvae of these worms that cause the disease, by attacking the fish internally, for which there is no known cure. They may be found in body tissues, muscles, gills, eyes, blood, and internal organs. They may also be identified as cysts on the head, the cysts are often black, and hence the common name Black-Spot disease. The spots may appear to be brownish in colour on fishes with little pigment, and it is the accumulation of pigment cells that surround the cyst that make it visible from the outside, the slowly moving worm is curled up inside the cyst.

Although very rare in the aquarium, the disease has been found occasionally in recently introduced imported fishes, or after infected snails has been accidentally brought into the aquarium. The worms also retain natures most complex and longest life cycles, relying on several different hosts to complete their metamorphosis.

The adult parasite lives in the intestines of water birds. The eggs of the parasite leave the body of the bird in its faeces, and if they fall into a body of water will hatch and release ciliated larvae, called miracidia. Each miracidium swims in the water until it finds its first host, a water snail. The miracidium burrows its way into the liver tissue of the snail and develops into its second larval form, a sporocyst or spore-form.

Within the sporocyst a process known as Parthenogenesis (reproduction without fertilisation) occurs, which develops into the third larval stage called rediae, these rediae then produce new larval forms called cercariae. The cercaria escapes from the snail and one of two things happen.

1.) The cercaria will attach itself to aquatic vegetation, where it encysts.

It will remain encysted until an animal of one sort or another swallows it and the cyst wall breaks down, allowing the larva to migrate to the liver of the host, where it develops into an adult fluke, or

2.) The cercaria attaches itself to a host fish, where it encysts, the larvae are now known as metacercaria. If a bird consumes the infected fish the encysted metacercaria will dissolve in its stomach and become an adult worm. The life cycle of this fluke has been completed and is typical of the evolution of many members of the group.

Infected fishes can suffer muscle damage, which could lead to paralysis, and blindness may occur if the eyes are affected, although the disease is not infectious, heavy attacks can prove fatal.


The life cycle of this fluke is dependant on aquatic snails; therefore, if snails are not present in the aquarium this removes the possibility of the disease ever occurring. This is of course providing that it is not introduced into the aquarium with recently imported fishes, and this is a good reason why new stock should be quarantined for at least two weeks prior to being introduced into the main aquarium.

If anyone really must have snails in their aquarium, they should be aquarium-bred specimens, and quarantined, just as you do with fishes, plants etc. Snails collected from the pond must not be introduced into the aquarium. Many others and I avoid snails altogether.

1.) On principle that they are carriers of Redia

2.) I once had an almost impossible task of trying to rid my aquarium of snails when they were accidentally introduced on live plants; they are extremely prolific breeders and will take over your aquarium in a very short time.
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