Monogenean trematodes, also called flatworms or flukes, commonly invade the gills, skin, and fins of fish. Monogeneans have a direct life cycle, meaning they do not require an intermediate host, and they are highly host- and site-specific. Some adults remain permanently attached to a single site on the host.
Freshwater fish infested with skin-inhabiting flukes become lethargic, swim near the surface, seek the sides of the pool or pond, and their appetite dwindles. They may be seen rubbing the bottom or sides of the holding facility (flashing).
The skin where the flukes are attached shows areas of scale loss and may ooze a pinkish fluid. Gills may be swollen and pale, respiration rate may be increased, and fish will be less tolerant of low oxygen conditions. “Piping” or gulping air at the water surface may be observed in severe respiratory distress.
Large numbers of monogeneans on either the skin or gills can cause significant damage and mortality. Secondary infections by bacteria and fungi are common on tissue damaged by monogeneans.
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Gyrodactylus and Dactylogyrus
The two most common genera of monogeneans that infect freshwater fish are Gyrodactylus and Dactylogyrus. These genera differ in reproductive strategies and in the method of attachment to the host fish. Gyrodactylus have no eyespots, two pairs of anchor hooks, and are generally found on the skin and fins of fish.
They are live bearers (viviparous), where the adult parasite contains a fully developed embryo inside the adult’s reproductive tract. This reproductive strategy allows populations of Gyrodactylus to multiply quickly, particularly in closed systems where water exchange is minimal.
Dactylogyrus prefers to attach to the gills. They have two to four eyespots, one pair of large anchor hooks, and are egg layers. Their eggs hatch into free-swimming larvae and are carried to a new host by water currents and their own ciliated movement.
The eggs can be resilient to chemical treatments, and multiple applications are usually recommended to control this group of organisms. In Africa, Dactylosoma has been found in cichlids (species of Oreochromis, Astatorheochromis, and Haplochromis) and grey mullets (Mugilidae—Mugil cephalus, Liza dummerelli, and L. richardsoni). The latter two species of grey mullets are the only known African hosts for Hemogregarina.
Treatment for Monogeneans
The treatment of monogeneans is usually not satisfactory unless the primary cause of increased fluke infestations is found and alleviated. Formalin is the treatment of choice for freshwater fish, administered as a short-term or prolonged bath.
Sick fish do not tolerate formalin well, so careful monitoring during treatment is necessary. Potassium permanganate can also be effective in controlling monogeneans.
Digenean Trematodes in Fish
Digenean trematodes have a complex life cycle involving multiple hosts. Fish can be either the primary or intermediate host, depending on the digenean species. These parasites may be found externally or internally, in any organ. For most digenean trematodes, pathogenicity to the host is limited.
The life stage most commonly observed in fish is the metacercaria, which encysts in fish tissues. Metacercaria rarely cause significant problems in fish. However, in the ornamental fish industry, digenean trematodes from the family Heterophyidae have caused substantial mortalities in pond-raised fish. These digeneans become encysted in gill tissue, leading to respiratory distress.
Another example of a metacercaria that can cause problems in cultured fish is the genus Posthodiplostonum, or white grub. While it can cause mortalities in baitfish, the more common effect is a reduced growth rate, even when infection rates are high. When mortalities occur, high numbers of metacercaria are found in the eye, head, and throughout the visceral organs.
Clinostonum, often called yellow grub, is another trematode. It is large and does not cause major problems for the fish itself, but its presence makes fish unmarketable for aesthetic reasons.
Control of Digenean Trematodes
The best control for digenean trematodes is breaking the life cycle of the parasite. Eliminating the first intermediate host, the freshwater snail, is often recommended. Copper sulfate has been used in ponds with limited success and is most effective when applied at night, as snails are nocturnal feeders.
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Nematodes In Fish
Nematodes, also called roundworms, are widespread parasites of fish. Larval nematodes may be found in cysts or coiled within internal organs, while adults usually reside in the intestines. Some nematodes may be found coiled under the skin. They can infect any organ of the host, causing loss of function in the damaged area. Signs of nematode infections include anemia, emaciation, unthriftiness, and reduced vitality.
Camillanus, Capillaria, and Eustrongylides are three common nematodes that affect fish.
1. Camillanus: This nematode is easily recognized as a small, thread-like worm protruding from the fish’s anus. In non-food fish, it can be controlled with fenbendazole, an antihelminthic drug. Fenbendazole should be mixed with fish food (using gelatin as a binder) and fed for three days, with a repeat dose in three weeks.
2. Capillaria: This large roundworm is commonly found in the gut of angelfish. Heavy infestations can debilitate fish, although a few worms per fish may not cause significant harm. Fenbendazole is also recommended for treatment.
3. Eustrongylides: This nematode uses fish as its intermediate host, with wading birds as the definitive host. The worm encysts in the peritoneum or muscle of the fish and appears to cause little damage. However, the large size of the worm can make infected fish unsuitable for retail sale. Preventing infection involves protecting fish from wading birds and eliminating the intermediate host, such as the Tubifex worm.
Cestodes in Fish
Cestodes, or tapeworms, are parasites found in a wide variety of animals, including fish. The life cycle of cestodes is varied, with fish serving as either the primary or intermediate host. Cestodes infect the alimentary tract, muscle, or other internal organs.
Plerocercoids, the larval stage of cestodes, are particularly harmful to freshwater fish, as they decrease the carcass value if present in muscle tissue and impair reproduction when they infect gonadal tissue.
One of the most serious adult cestodes affecting fish is the Asian tapeworm (Bothriocephalus acheilognathi). Praziquantel, administered at 2–10 mg/L for 1 to 3 hours in a bath, is effective in treating adult cestode infections in ornamental fish.
At present, no treatment exists for plerocercoids or for cestode infections in food fish. Disinfecting ponds to eradicate the intermediate host (copepod) is currently the best preventive measure.
In this article, the elements of various parasites affecting fish, including monogenean and digenean trematodes, nematodes, and cestodes, have been presented.
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