▷ Digenetic flukes in fish | Details, photos and treatment

symptoms

external symptoms

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Fish breathe heavily

Fish swim around frantically

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internal symptoms

Bumps or nodules on organs

With

The symptoms described only occur while the fish are being attacked by cercaria and they are looking for their target. After the formation of the metacercaria, these symptoms disappear again.

photos

Clicking on a photo shows the whole picture, if available.

Photos: Jörg / Shaky

Pundamilia nyererei with black spots possibly caused by metacercaria

Photos: Christopher

Scalar with black spots on pectoral fin. The spots may have been caused by metacercaria. At the same time, a scalar had white dots on its skin. These fell off after a few days. It is possible that these spots were also caused by metacercaria. Possibly the white spots also had a bacterial cause and it was an abscess or something similar.

Photos: Moni Lasota

Red neons with metacercaria. The animals became very pale, got black spots and separated from the group. To treat the suspected worms, a clove of garlic was placed in the tank for a few days and the temperature increased to 30° for 2 days. After that they regained their color and appeared healthy.

causes

Digenic flukes are also called trematodes for short. Digenic trematodes would be the more correct term, as there are also monogenic trematodes or flukes. As long as trematodes only occur sporadically, they are harmless to infested fish. In large numbers, they can kill infested fish. In their development cycle, trematodes require one or more intermediate hosts. A fish can be an intermediate or final host. The final host is the living being in which the adult (sexually mature) worms live. Intermediate hosts are the creatures in which the non-adult stages of development live. If the fish is the intermediate host, trematode larvae can appear in the skin, gills and organs of infected fish. Since the larva is also called metacercaria, the symptoms of the disease are also called metacercaria(s). Strictly speaking, larvae are visible, which are encapsulated by the connective tissue of the fish and can be recognized as white or black dots on the fish surface.

There are thousands of trematode species that live in fish as adult worms. There are also many species that only need fish as intermediate hosts and in which only the larval stages infest fish. Except for one family of trematodes, all species have a suction cup at their front end and on the ventral side in the middle of their body. The suction cup at the front end encloses the mouth. The worm attaches itself to the fish with both suction cups. Although the worms have mouths and intestines, they can also ingest food through the skin. Only the bloodworms (Sanguinicolidae) that live in the blood do not need suction cups due to their way of life in the blood.

Trematodes have a complicated life cycle that varies from species to species. Due to this development cycle and the additional specialization of the various trematode species on specific intermediate and definitive hosts, trematodes only rarely occur in the aquarium and then usually do not spread.

Symptoms can resemble an infestation with certain sporozoans. For a clear diagnosis, a microscope is used to check whether the worms have two sucking mouths.

In many trematode species, the adult worms are hermaphrodites, meaning they have both male and female reproductive organs. However, there are also species with separate sexes and numerous transitional forms.

Adult trematodes lay numerous eggs, which end up in the water with the faeces of the final host. So if the respective trematode species lives as an adult (sexually mature) worm within fish, the eggs get into the water with the fish excrement. The eggs are either of molluscs, z. B. snails, eaten or so-called eyelash larvae (ciliated larvae, miracidia) hatch, which in turn actively penetrate molluscs themselves. Eyelash larvae also develop in the host from eaten eggs. Each trematode species prefers certain mollusks as intermediate hosts.

In the first generation, the (mother) sporocysts develop from the ciliated larvae in the intermediate host. These are germ tubes that have almost no organs. This results in the so-called redia (daughter sporocysts) as the 2nd generation. These have the fauces, intestines, birth opening and central nervous system. The third generation, the cercaria (fork-tailed larvae), develops from the redia. The cercaria have fauces, intestines, a two-part rudder tail and two suction cups. They have no sex organs. The cercaria actively leave their host animals and swim freely in the water with the help of their rudder tail.

The cercaria either actively enter a fish or are ingested by the fish with the food. Once the cercaria is in the fish, it becomes the metacercaria by shedding the rudder tail. The metacercaria migrate to their target organ in the fish. Some species stay just below the scales. There they can be seen as 0.5 to 1 mm black or white dots.

Scale worms, Transversotrema patialense, slide under the scales of the fish.

A species of the genus Diplostomum, called worm starfish, migrates into the eyes and can thus lead to the blindness of infested fish. Still other species penetrate further through the skin to internal organs. As a defense reaction, the affected organ encapsulates the metacercaria of most trematode species with connective tissue and pigments.

The so-called black spot disease (Black Spot Disease) is z. B. an infestation with metacercaria of the genus Posthodiplostonum. The disease occurs primarily in wild-caught South American tetras and chichides. The defense reaction coats the white or yellow worm larvae with black pigments. The disease does not spread due to the lack of intermediate hosts in the aquarium and is harmless if the infestation is weak. The adult black spot worms live in the internal organs of fish-eating birds, as do the adult worms of the starworm.

The pathogen Clinostomum causes yellow spot disease. This disease occurs in wild-caught catfish, but also in bred armored catfish, rainbow fish and livebearing toothcarps. As the name suggests, the nodules are white to yellow due to the enveloping pigments. The adult yellow spot worms live in the mouths and throats of fish-eating birds.

Only cercaria of bloodworms and the family Azygiidae directly develop into adult worms in fish. All other species develop into metacercaria in the fish. As metacercariae, the larvae can survive for a long time. Only when it is ingested by a fish-eating creature (human, predatory fish, bird, etc.) does the metacercaria develop into an adult worm. Depending on the species, the worm is then in the intestines, other organs, muscles, etc.

Sexually mature worms can therefore only be transmitted from fish to fish if the fish infected first is eaten by a second fish and both fish species are definitive hosts for this trematode species.

Individual metacercariae hardly damage infested fish and the fish can live with the metacercariae for a long time. A mass infestation can destroy so much tissue or muscle that infested fish become weak and even unable to move. Infection of the eyes can lead to cloudy lenses and blindness. Infestation of the brain can lead to balance disorders and paralysis. The weakened fish become susceptible to other diseases, especially bacterial infections. Infected fish often lose weight. In severely debilitated fish, the immune system may be compromised to such an extent that the encapsulation is weakened and the metacercaria are able to exit the capsule. The metacercaria, now freely roaming about in the fish, can cause fatal damage.

The color of light spots is given differently. Some authors speak of white spots, others of silvery ones.

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