Monogenean trematodes, also called flatworms or flukes, commonly invade the gills, skin, and fins of fish. Monogeneans have a direct life cycle (no intermediate host) and are host- and site-specific. In fact, some adults will 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”, gulping air at the water surface, may be observed in severe respiratory distress.
Large numbers (>10 organisms per low power field) of monogeneans on either the skin or gills may result in significant damage and mortality. Secondary infection by bacteria and fungus is common on tissue with monogenean damage.
Gyrodactylus and Dactylogyrus are the two most common genera of monogeneans that infect freshwater fish. They differ in their reproductive strategies and their 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) in which the adult parasite can be seen with 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 gills. They have two to four eyespots, one pair of large anchor hooks, and are egg layers. The 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 treatment, and multiple applications of a treatment are usually recommended to control this group of organisms. Freshwater and marine fish.
In Africa, Dactylosoma has been found in cichlids (species of Oreochromis, Astatorheochromis and Haplochromis) and grey mullets (Mugilidae -Mugilcephalus, Lizadummerelli and L.richardsoni). The latter two species of grey mullets are thus far the only known African hosts for Hemogregarina.
Treatment of monogeneans is usually not satisfactory unless the primary cause of increased fluke infestations is found and alleviated. The treatment of choice for freshwater fish is formalin, administered as a short-term or prolonged bath.
Fish that are sick do not tolerate formalin well, so they need to be carefully monitored during treatment. Potassium permanganate can also be effective in controlling monogeneans.
Digenean Trematodes
Digenean trematodes have a complex life cycle involving a series of hosts (Figure 18). Fish can be the primary or intermediate host depending on the digenean species. They are found externally or internally, in any organ. For the majority of digenean trematodes, pathogenicity to the host is limited.
The life stage most commonly observed in fish is the metacercaria, which encysts in fish tissues (Figure 19). Again, metacercaria that live in fish rarely cause major problems.
However, in the ornamental fish industry, digenetic trematodes from the family Heterophyida, have been responsible for substantial mortalities in pond-raised fish. These digeneans become encysted into gill tissue and respiratory distress is eminent.
Another example of a metacercaria that could cause problems in cultured fish is the genus Posthodiplostonum or the white grub. This has caused mortalities in baitfish, but usually the only negative effect is reduced growth rate, even when the infection rate is high.
In cases where mortalities occur, there are unusually high numbers in the eye, head, and throughout the visceral organs.
Another fluke is Clinostonum, often called yellow grub. It is a large trematode and although it does not cause any major problems for fish, it is readily seen and will make fish unmarketable for aesthetic reasons.
The best control of digenean trematodes is to break the life cycle of the parasite. Elimination of the first intermediate host, the freshwater snail is often recommended.
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Copper sulfate in ponds has been used with limited success and is most effective against snails when applied at night, due to their nocturnal feeding activity
Nematodes
Nematodes, also called roundworms, occur worldwide in all animals. Nematodes are very common parasites of fish. The larvae may be found in cysts or coiled in or on the internal organs. Adults are usually found in the intestines.
Some are found coiled under the skin. They can infect all organs of the host, causing loss of function of the damaged area.
Signs of nematodiasis include anemia, emaciation, unthriftiness and reduced vitality. Three common nematodes affecting fish are described.
Camillanus
Camillanus is easily recognized as a small thread-like worm protruding from the anus of the fish. Control of this nematode in non-food fish is with fenbendazole, a common antihelminthic.
Fenbendazole can be mixed with fish food (using gelatin as a binder) at a rate of 0.25% for treatment. It should be fed for three days, and repeated in three weeks.
Capillaria
Capillaria is a large roundworm commonly found in the gut of angelfish (Figure 20), often recognized by its double operculated eggs in the female worm (Figure 21). Heavy infestations are associated with debilitated fish, but a few worms per fish may be benign.
Fenbendazole is recommended for treatment.
Eustrongylides
Eustrongylides is a nematode that uses fish as its intermediate host. The definitive host is a wading bird, a common visitor to ponds.
The worm encysts in the peritoneum or muscle of the fish and appears to cause little damage. Because of the large size of the worms (Figure 22), infected fish may appear unsuitable for retail sales.
Protecting fish from wading birds and eliminating the intermediate host, the oligocheate or Tubifex (soft-bodied worms), are the best means to prevent infection.
Cestodes
Cestodes, also called tapeworms, are found in a wide variety of animals, including fish (Figure 23). The life cycle of cestodes is extremely varied with fish used as the primary or intermediate host.
Cestodes infect the alimentary tract, muscle or other internal organs. Larval cestodes called plerocercoids are some of the most damaging parasites to freshwater fish.
Plerocercoids decrease carcass value if present in muscle, and impair reproduction when they infect gonadal tissue. Problems also occur when the cestode damages vital organs such as the brain, eye or heart.
One of the most serious adult cestodes that affect fish is the Asian tapeworm, Bothriocephalus acheilognathi. Praziquantel at 2 — 10 mg/L for 1 to 3 hours in a bath is effective in treating adult cestode infections in ornamental fish.
At this time, there is no treatment that can be used for food fish. Also, there is no successful treatment for plerocercoids. Ponds can be disinfected to eradicate the intermediate host, the copepod.
In summary, 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).
Worms are common in both wild and cultured fish. Fish frequently serve as intermediate or transport hosts for larval parasites of many animals, including humans.
Worms with direct life cycles are most important in dense populations, and heavy parasite burdens are sometimes found. In general, heavy parasite burdens seem to be more common in fish originating from wild sources.
Cultured fish are subject to a rapid buildup of parasites by continuous infection and worm transfer to other fish in the tank or pond.
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