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Parasitic Diseases of Fish


All of the major groups of animal parasites are found in fish, and apparently healthy wild fish often carry heavy parasite burdens. Parasites with direct life cycles can be important pathogens of cultured fish; parasites with indirect life cycles frequently use fish as intermediate hosts. Knowledge of specific fish hosts greatly facilitates identification of parasites with marked host and tissue specificity, while others are recognized because of their common occurrence and lack of host specificity. Examination of fresh smears that contain living parasites is often diagnostic.

The most common parasites of fish are protozoa (see Table: Protozoan Parasites of FishTables). These include species found on external surfaces and species found in specific organs. Most protozoa have direct life cycles, but the myxosporidia require an invertebrate intermediate host.

Table 3

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Protozoa Infecting Gills and Skin

Ciliated protozoa are among the most common external parasites of fish. Most ciliates have a simple life cycle and divide by binary fission. Ciliates can be motile, attached, or found within the epithelium. The most well-known organism in the latter group is Ichthyophthirius multifiliis, which has a more complex life cycle than the other ciliates.

The infection caused by I multifiliis is referred to as “ich” or “white spot disease.” The parasite is an obligate pathogen that cannot survive without the presence of living fish. All fish are susceptible, and a similar appearing parasite, Crytocaryon irritans, occurs in marine species. The parasite is readily transmitted horizontally via direct exposure to infected fish or via fomites (nets, etc). Fish that survive an outbreak may be refractory in future outbreaks, but may also serve as a source of infection to previously unexposed individuals. The parasite invades epithelial tissue of gills, skin, or fins, leaving a small wound and visible white spot or nodule where each parasite encysts. The organism causes substantial damage because of its unique life cycle (see below), which allows a rapid intensification of infection. Infected fish are extremely lethargic and covered with visible white dots. Mortality can be rapid and catastrophic. Infections that are confined to gill tissue may not be recognized by nonprofessionals (because white spots are not grossly visible), but are easily diagnosed using gill biopsy techniques. The organism is identified using a light microscope at magnification of 40× or 100×. It is large (0.5–1 mm), round, covered with cilia, and has a characteristic horseshoe-shaped macronucleus. Its characteristic movement varies from constantly rotating to ameboid-like.

Ich infections require immediate and thorough medical treatment. Formalin or copper are often drugs of choice. Over-the-counter medications for pet fish often contain formalin and malachite green and are effective, but due to regulatory concerns regarding the use of malachite green, should not be dispensed by the veterinarian. Multiple chemical treatments (with intervals determined by water temperature) are required for successful treatment of I multifiliis. At warm temperatures typical of home aquaria (eg, >26°C), infected fish should be treated every 2–3 days. Constant chemical exposure for at least 3 wk is generally recommended to control Cryptocaryon in marine systems; lowering salinity to 16–18 ppt is often helpful.

I multifiliis has a direct life cycle, but has massive reproductive potential from each adult parasite. Adults leave the fish host and encyst in the environment, releasing hundreds of immature parasites (tomites) that must find a host within a specific time frame (days for warm water fish, weeks for cold water fish), determined by temperature. For this reason, leaving a system fallow is one means of preventing reinfection. While encysted, parasitic life stages are refractory to chemical treatment, but cysts can be removed by thorough cleaning and removal of debris from gravel substrates.

There are two important groups of ciliates that are motile and move on the surface of skin and gills of fish. They include Chilodonella spp (which has a marine counterpart, Brooklynella spp) and the trichodinids, which are found on both freshwater and marine fish. Fish with chilodonelliasis typically lose condition, and copious mucous secretions may be noticed in areas where infestation is most severe. If gills are heavily infested, the fish may show signs of respiratory distress, including rapid breathing and coughing. The gills may be visibly swollen and mucoid. Infected fish may be irritated as evidenced by flashing (scratching) and decreased appetite. Chilodonella can be easily identified from fresh biopsies of infected tissues. They are 0.5–0.7 mm, are somewhat heart-shaped with parallel bands of cilia, and move in a characteristic slow spiral. See Protozoan Parasites of FishTables for treatment.

Several genera of peritrichous ciliates have been grouped together and are collectively referred to as the trichodinids. These include Trichodina, Trichodinella, Tripartiella, and Vauchomia spp. Clinical signs associated with trichodinid infestation are similar to those of chilodonelliasis, although secretion of mucus is not usually as noticeable. Trichodinids are easily identified from biopsies of infected gill or skin tissue. They are readily visible using a light microscope at 40–100×. Trichodinids move along the surface of infested tissue and appear as little saucers or, from a lateral view, as little bubbles. The body of the organism may be cylindrical, hemispherical, or discoid. Trichodinids are characterized by an attaching disk with a corona of denticles on the adoral sucker surface. For treatment of trichodinids, see Protozoan Parasites of FishTables. Infestations of Trichodina often indicate poor sanitation and/or overcrowding, so chemical treatment alone may not be adequate for complete control.

Tetrahymena corlissi, another ciliate, may be motile and surface-dwelling but is also occasionally found within tissue, including skeletal muscle and ocular fluids. Similar protozoa, Uronema spp, are found on marine fish. Tetrahymena spp are pear-shaped and 10–20 μm long, with longitudinal rows of cilia and inconspicuous cytostomes. External infestations of Tetrahymena spp are not uncommon on moribund fish removed from the bottom of a tank or aquarium and are often associated with an environment rich in organic material. As long as Tetrahymena spp are restricted to the external surface of the fish, they are easily eliminated with chemical treatment and sanitation. When they become established internally, they are not treatable and can cause significant mortality. Fish with intraocular infections of Tetrahymena spp develop extreme exophthalmos. The parasite is readily identified by examining ocular fluids with a light microscope.

Ambiphyra and Apiosoma are sessile ciliates that can be found on the skin, gills, and fins of fish. These seem to be more common in pond fish than tank-reared fish and have a predilection for organically rich environments. They are not generally found on marine fish. When examined from a lateral view, Ambiphyra is the shape of a tin can with a ciliated band around the middle and at the cytostome, which is distal to the attachment site. Apiosoma spp are vase-shaped. Neither Ambiphyra spp nor Apiosoma spp are particularly pathogenic if present in low numbers (no more than 1–2/low-power field); however, when present in high numbers, these parasites can cause significant epithelial damage, predisposing fish to opportunistic pathogens in the environment and compromising respiration and osmoregulation. Infested fish demonstrate flashing, decreased appetite, loss of condition, and hyperplasia of infested epithelial surfaces. Severe infestation of the gills is particularly damaging. The organisms can be controlled with a single treatment of formalin, copper sulfate, potassium permanganate, or a salt dip. Excessive crowding and poor sanitation are frequently associated with heavy infestations and should be corrected.

Heteropolaria spp are stalked, colonial ciliates that most frequently attach to bony surfaces of fish, particularly the tips of fin rays and opercula. It is most common in freshwater gamefish, particularly centrarchids, (eg, largemouth bass, bluegill, and sunfish) and is frequently associated with development of “red sore disease.” In the earliest stages of infection, bony protuberances appear slightly raised and erythematous; as the colony grows, they appear cottony. Examination of material with a light microscope is required to differentiate Heteropolaria spp from fungal hyphae, and mixed infections are common. Further progression of the disease is typified by development of shallow ulcers on the lateral surface of the fish. A wet mount of fresh tissue from the margin of the lesion is required to differentiate between Heteropolaria spp, fungal hyphae, and columnaris bacteria. Coinfection with the bacterium Aeromonas hydrophila is typical of red sore disease. If deaths occur, a single treatment of potassium permanganate or copper sulfate should be administered. If systemic bacterial infection is a component of the epizootic, antibiotics should be provided in medicated feed, if affected fish will accept a pelleted diet.

Ichthyobodo spp, Costia spp are some of the most common and smallest (∼15 × 5 μm) flagellated protozoan parasites of the skin and gills. They are flattened, pear-shaped organisms with 2 flagellae of unequal lengths. These parasites can be found on freshwater or marine fish from a broad geographic range. Ichthyobodo move in a jerky, spiral pattern, and free-swimming organisms are fairly easy to identify in direct smear preparations. Once attached, the organism can be difficult to see, but movement typical of a flickering flame may be seen under 400× and is characteristic. Affected skin often has a steel-gray discoloration due to copious mucus production (“blue slime disease”), and gills may appear swollen. Behavioral signs of infestation include lethargy, anorexia, piping, and flashing. Ichthyobodo is readily controlled with salt, formalin, copper sulfate, or potassium permanganate baths. Because the parasite has a direct life cycle, a single treatment should be adequate. If reinfestation occurs, sanitation and quarantine practices should be evaluated.

One of the most serious health problems of captive marine fish is the parasitic dino-flagellate Amyloodinium spp. Its freshwater counterpart, Oodinium spp, is less common but can also result in high mortality. These parasites produce a disease that has been called “velvet,” “rust,” “gold-dust,” and “coral disease” because of the brownish gold color they impart to infected fish. The pathogenic stages of the organism are pigmented, photosynthetic, nonflagellated, nonmotile algae that attach to and invade the skin and gills during their parasitic existence. When mature, these parasites give rise to cysts that contain numerous flagellated, small, free-swimming stages that can initiate new infections. Control of Amyloodinium is challenging, and the prognosis is guarded. Copper sulfate is the only therapeutic option for food animals in the USA, and repeated treatments are necessary to break the life cycle. The disease is particularly problematic in clown fish. The treatment of choice in ornamental fish is chloroquine, delivered at 10 mg/L as an indefinite bath.

Internal Protozoan Parasites

Hexamita and Spironucleus spp are common, small (∼9 μm), bilaterally symmetric, flagellated (4 pairs) protozoa most frequently found in the intestinal tract of finfish. Among ornamental fish, the cichlids are highly susceptible. Pathogenicity of these organisms is variable and correlated with the number present. If there is a loss of condition, or more than 15 organisms are seen per low-power field on wet mounts of intestinal tissue or contents, then treatment is strongly recommended. The only treatment available for hexamitiasis is metronidazole (use only in ornamental species), which should be given orally but can be administered as a bath if fish are anorectic. Chronic infections have been seen in fish maintained in unsanitary or crowded conditions.

Cryptobia and Trypanosoma spp are slender, elongated (6–20 μm), actively motile, biflagellated protozoa that are easily detected in fresh blood and tissue smears of both marine and freshwater finfish. Hematogenous forms are generally described as Trypanosoma and have a well-developed undulating membrane. Trypanosomes may be transmitted by leeches and have been associated with anemia in blue-eyed plecostomus imported from South America. Cryptobia iubilans has been associated with granulomatous disease in African cichlids and discus. Clinical disease is manifest by severe weight loss and cachexia. Clinically affected fish should be culled. Presumptive diagnosis can be made from microscopic examination of fresh tissue. Typically, granulomas will be found in the stomach, which may be visibly thickened. Acid-fast material will not be found in granulomas caused by Cryptobia. Motile flagellates may be visible using magnification of 400× or greater.

Coccidiosis, while common in freshwater or marine finfish, is rarely diagnosed in live fish. Many species of finfish are affected. The life cycles of many fish coccidia are unknown, and some involve >1 host to complete their development. In addition to intestinal infection, the internal organs may be affected; sporulated Eimeria-like oocysts and sexual and asexual stages can be found in direct smears and histologic sections of the internal organs. Sulfamethazine, at 22–24 g/100 kg of fish wt/day in the feed for 50 days at 50°F (10°C), is used to treat food fish (21-day withdrawal time) in some countries. An FDA-approved form of this drug is not currently available in the USA. For aquarium fish, 10 ppm sulfamethazine in the aquarium water once a week for 2–3 wk has been reported to be preventive, but safety and efficacy data are sparse.

Myxosporidians are common fish parasites. Myxosporidia have indirect life cycles and use other aquatic organisms (eg, annelids) as intermediate hosts. Hence, myxosporidian infections are more common in, and more pathogenic for, wild fish or fish reared intensively in outdoor fish ponds. The organisms tend to be host- and tissue-specific. Accordingly, expression of the disease is related to the specific pathogen and host. Myxosporidian-infected fish in captive display aquaria are not able to transmit the infection unless the necessary intermediate hosts are present.

There are two important myxosporidian infections of ornamental fish. “Renal dropsy of goldfish” is caused by the myxosporidian Sphaerospora auratus. The disease is characterized by renal degeneration and ascites and is usually diagnosed by identification of spores in histologic sections of the kidney. Affected fish present with extreme abdominal distention but may have few other clinical signs. Radiographs may reveal a mass in the area of the posterior kidney; definitive diagnosis is made at necropsy and confirmed histologically. No practical treatment is available. Henneguya, a myxosporidian occasionally found in ornamental fish, causes white nodular lesions that are usually found in gill tissue and may be grossly visible. Henneguya is easily identified by the forked-tail appendage of the spore, seen microscopically. If ponds are dried and limed heavily, infection can be eliminated, apparently by reduction of the intermediate hosts. Aquarium infection can be self-limiting in the absence of intermediate hosts. Although an occasional cyst may be considered an incidental finding, severe damage has been associated with diffuse distribution of interlamellar cysts.

Myxosporidian diseases significant in aquaculture include whirling disease and proliferative kidney disease of salmonids and proliferative gill disease (“hamburger gill disease”) of channel catfish. Whirling disease is caused by Myxobolus cerebralis. Fish are infected as fingerlings when the parasite infects cartilage in the vertebral column and skull, resulting in visible skeletal deformities. Affected fingerlings typically show rapid tail-chasing behavior (whirling) when startled. The disease is also sometimes called “blacktail” because the peduncle and tail may darken significantly. Recovered fish remain carriers. Adults do not show behavioral signs, but skeletal deformities associated with infection do not resolve. The disease can be prevented by purchasing uninfected breeding stock and maintaining them in an environment free of the intermediate hosts (tubifex worms). A presumptive diagnosis of whirling disease is made by detection of spores from skulls of infected fish. Diagnosis may be confirmed histologically or serologically. Whirling disease is of regulatory concern in some states.

Proliferative kidney disease (PKD) is one of the most economically important diseases affecting salmonid industries of North America and Europe. Rainbow trout are particularly susceptible. PKD is caused by Tetracapsuloides bryosalmonae, a myxosporidian with 4 distinct polar capsules. It occurs most commonly in the summer when water temperatures are >12°C, and the parasite primarily infects yearling and younger fish. Clinical signs include lethargy, darkening, and fluid accumulation indicated by exophthalmos, ascites, and lateral body swelling. Infected fish are frequently anemic, resulting in gill pallor. Grossly, the posterior kidney appears gray, mottled, and significantly enlarged. Presumptive diagnosis can be based on observation of suspect organisms, 10–20 μm in diameter, in Giemsa-stained wet mounts of kidney tissue. Histologic examination of infected tissue, stained with H&E, and immunohistochemistry are required for confirmation. There is no treatment, but fish that recover from the infection are resistant to subsequent outbreaks. Infected stocks in nonendemic areas should be depopulated, the premises sanitized, and disease-free stock obtained for replacement. Avoidance is the best preventive measure.

Proliferative gill disease (“hamburger gill disease”) is a myxosporidian infection of channel catfish caused by Aurantiactinomyxon ictaluri. The organism has a complex life cycle, with the oligochete worm Dero digitata serving as the intermediate host. Channel catfish may be aberrant host for A ictaluri, and the disease usually occurs in new ponds or previously infected ponds that have been drained and refilled. Although proliferative gill disease can cause catastrophic mortality approaching 100%, losses may be as low as 1%. Disease occurs at water temperatures of 16–26°C, and mortality is exacerbated by poor water quality, particularly low dissolved oxygen or high levels of un-ionized ammonia. Gills of affected fish are severely swollen and bloody. A presumptive diagnosis can be made from a wet mount of infected tissue, in which filaments appear swollen, clubbed, and broken. Cartilaginous necrosis is strongly supportive of a diagnosis of proliferative gill disease; however, histology is required for confirmation.

Microsporidians are tiny, intracellular, spore-forming organisms with single polar filaments that are common parasites of finfish. They are host- and tissue-specific and can also infect helminth parasites of fish. The spores are extremely resistant, and microsporidian diseases are considered non-treatable. Microsporidians have a direct life cycle; therefore, horizontal transmission in an aquarium is likely. Depopulation and disinfection are recommended for elimination of microsporidian infections.

Pleistophora ovariae infects ovarian tissue of golden shiners (bait fish), resulting in sterility. The organism has no intermediate host and is transmitted horizontally (through ingestion of infective spores) or vertically (through infected ova). Fertility declines as fish age, eventually resulting in sterility. Grossly, infected ovarian tissue appears marbled. The diagnosis is confirmed by examination of a wet mount of suspect tissue, revealing the presence of microsporidian spores.

Neon tetra disease is caused by Pleistophora hyphessobryconis, which infects the skeletal musculature of a number of species of aquarium fish, including tetras, angelfish, rasporas, and barbs. Infected fish may exhibit abnormal locomotion caused by muscle damage, and muscle tissue may appear marbled or necrotic at necropsy. The parasitic spores are readily visualized in wet mounts of infected tissue.


Helminths are common in both wild and cultured fish (see Helminth Parasites of FishTables). Fish frequently serve as intermediate or transport hosts for larval parasites of many animals, including humans. Helminths 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.

Table 4

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Monogeneans have direct life cycles and are common, highly pathogenic, and obligatory parasites of the skin and gills. Freshwater parasites tend to be ∼0.1–0.8 mm long and are best seen microscopically; however, several important species parasitizing marine fish are significantly larger and may be visible grossly. The worms can be identified by their characteristic hold-fast organ, the haptor, which is armed with large and small hooks. Aquarium and cultured fish are subject to a rapid buildup of parasites by continuous infection and worm transfer to other fish in the tank or pond. Although many species are host-specific, the more common types seen in aquaria are less selective. The 2 most common genera in freshwater aquaria are Gyrodactylus and Dactylogyrus. Gyrodactylus, a common parasite of goldfish, gives birth to live young and is usually found on skin; Dactylogyrus lays eggs and is principally a parasite of the gills. G salaris is a reportable disease of salmonids but has not been reported in the USA (see Helminth Parasites of FishTables). Any gyrodactylid found on a salmonid species should be identified well enough to determine whether or not it is G salaris. Neobenedenia and Benedenia are important monogeneans in marine fish. They also attach to skin and gill tissue, although Neobenedenia may also be found on the cornea. Both of these species lay sticky eggs that are easily transmitted via fomite. Monogenean-infected fish may show behavioral signs of irritation, including flashing and rubbing the sides of their bodies against objects in the aquarium. Fish become pale as colors fade. They breathe rapidly and distend their gill covers, exposing swollen, pale gills. Localized skin lesions appear with scattered hemorrhages and ulcerations. Ulceration of the cornea may become evident if the eyes are involved. Mortality may be high or chronic.

Praziquantel (2 mg/L, prolonged bath) is the treatment of choice for monogenean infection in freshwater and marine ornamental fish. Formalin is the only treatment option for food fish. Multiple treatments at weekly intervals are recommended for Dactylogyrus because eggs may be resistant to chemical treatment. Organophosphates (0.25 mg/L, prolonged bath) have been used successfully in ornamental fish in the past, but treatment with praziquantel is considered more effective. Organophosphates should be avoided in systems containing elasmobranchs. Monogenes on marine fish can be removed using freshwater dips for 1–5 min, depending on the tolerance of the species; however, eggs will not be damaged or removed. To prevent the disease, introduction of infected fish should be avoided.

Digeneans have complicated life cycles, with several larval stages that infect one or more hosts. With rare exceptions, the first intermediate host is a mollusc, without which the life cycle generally cannot be completed. A diagnosis usually can be established by gross or microscopic examinations that reveal the cercarial, metacercarial, or adult worms in any of the tissues or body cavities of the fish. Fish tend to form pigmented tissue encapsulations that encyst the parasites. Depending on the color of the cysts in the skin, the condition is called black, white, or yellow grub disease. Heavily parasitized fish often are weak, thin, inactive, and feed poorly. Treatment is not recommended.

Pond-reared, juvenile, tropical fish may develop severe gill disease from metacercarial cysts in gill tissue. Although acute death is occasionally seen, infected fish more commonly die during harvest or shipping when they may be exposed to suboptimal dissolved oxygen concentrations. Treatment of infected fish has not been successful; however, prevention of the disease by elimination of the intermediate host, a freshwater snail, has been effective.

Bolbophorus confusus is a digenean trematode that causes mortality in channel catfish fingerlings in production ponds in Mississippi, Louisiana, and Alabama. The definitive host of B confusus is the white pelican, and the first intermediate host is the ram's horn snail (Heliosoma spp). Cercariae released from snails encyst in fish tissue, forming metacercariae in any tissue, but the majority are found in skin and skeletal muscle of the peduncle of juvenile channel catfish. Severe disease (mortality up to 95%) occurs when metacercariae encyst in visceral organs, particularly the posterior kidney and liver. Involvement of these organs can result in a presentation similar to enteric septicemia or channel virus disease, characterized by fluid accumulation in the abdomen and exophthalmia. Skin and muscle lesions typically result in raised bumps that are white to reddish in color. The presence of digenea in skeletal muscle can result in condemnation of affected carcasses by processing plants.

Both larval and adult cestodes are common in fish. Larval forms encyst in visceral organs and muscle, while adults usually are found in the intestinal tract. Aquatic Crustacea are the most common intermediate host for fish; accordingly, wild and cultured pond fish may be heavily infected. Diphyllobothrium latum, the broad fish tapeworm infection of humans, is acquired by eating larval tapeworms in the flesh of food fish. Aquarium fish may be purchased with heavy cestode infections but have limited exposure once in the aquarium (unless fed infected intermediate hosts). There is no safe, effective treatment for larval tapeworm infections. The Asian tapeworm, Bothriocephalus acheilognathus, is occasionally seen in carp and aquarium fish. It is usually found in the anterior intestine and may be associated with enteritis and degeneration of the intestinal wall. Praziquantel is the drug of choice for treatment of cestodes in ornamental fish, but it is not approved for any aquatic use and cannot be used in food animals.

Acanthocephalids (thorny-headed worms) are common in wild fish as both larval tissue stages and adult intestinal parasites. They are more common in salmonid and marine fish. Arthropods are the first intermediate host. Adult acanthocephala are easily recognized by their protrusible proboscis, armed with many recurrent hooks.

Nematodes are common in wild fish that are exposed to the intermediate hosts. Fish may be definitive hosts for adult nematodes, or they may act as transport or intermediate hosts for larval nematode forms (anisakids, eustrongylids, and others) that infect higher vertebrate predators, including humans. Encysted or free nematodes can be found in almost any tissue or body cavity of fish. Aquarium and cultured pond fish may be heavily infected if crustacean intermediate hosts are present. Cyclops and Daphnia spp are common intermediate hosts for Philometra sp, a nematode that is pathogenic for guppies and other aquarium fish. These blood-red worms can be seen in the swollen abdominal cavity and protruding from the anus of affected fish (red worm disease). Capillaria spp are commonly found in aquarium fish, particularly freshwater angelfish. Heavy infections in juvenile angelfish have been associated with poor growth rates and an inability to withstand shipping and handling. Treatment with fenbendazole (25 mg/kg for 3 days) is recommended, but efficacy has not been firmly established. Levamisole (10 mg/L) administered as a bath treatment for 3 days has also been recommended. Ivermectin is highly toxic to aquarium fish, particularly cichlids, and its use is not recommended.

Leeches are parasitic bloodsuckers of fish and also serve as vectors for blood parasites of fish (eg, Trypanosoma, Cryptobia, and haemogregarines). They can produce a debilitating anemia due to chronic blood loss and disease. Leech infestations are most common in wild fish, but aquarium and pond infestations can occur by introduction of infested fish, plants, etc. Organophosphates (0.25 mg/L, prolonged bath) are effective but not approved for use in food fish. Further, environmental regulations may restrict use in outdoor ponds. Multiple treatments may be required to control leeches because eggs are resilient and juveniles may continue to hatch. Preventive measures include avoiding leeches (ie, effective quarantine). Infestations in recreational fishing ponds are often self-limiting.


Some copepods, such as the anchor worm, are obligatory parasites of finfish during specific stages of their complicated life cycle. They lose their copepod form, including their appendages, and become rod- or sac-like structures specifically adapted for piercing, holding, feeding, and reproducing. Grossly, they appear as barb-like attachments to the skin or gills, where they feed on blood and tissue fluids. They can cause hemorrhage, anemia, and tissue destruction, as well as provide a portal of entry for other pathogens. Many different species of these parasites can be found on freshwater and marine fish. The anchor worms, Laernea spp, are commonly found in a wide variety of aquarium- and pond-reared fish, including goldfish and other cyprinids. Ergasilus spp infest the gills. Organophosphates are effective in controlling copepodid parasites, but legal restrictions constrain clinical use (see Parasiticides). Some success has been achieved in freshwater fish by giving infected fish a 3% (30 ppt = 30 g/L) salt dip (<10 min, remove fish when it rolls) followed by 5 ppt (5 g/L) salt added to the affected tank for 3 wk. The increased salinity kills immature forms as they hatch.

Lice (Branchiuria) are related to the parasitic copepods and have flattened bodies adapted for rapid movement over the skin surface. By means of hooks and suckers, they periodically attach for feeding by inserting the piercing mouth part (stylet) into the skin. Sea lice (Lepeophtheirus salmonis) are a significant disease problem of pen-reared salmonids. Consultation with a salmonid health specialist is suggested if these parasites are encountered, as treatment options are limited and environmental concerns are significant. Argulus spp are lice commonly found on aquarium, pond-reared, and wild freshwater fish. Organophosphates (0.25 ppm, prolonged bath) are used for treating infested aquarium fish but are not approved for use in food fish.

Last full review/revision July 2011 by Ruth Francis-Floyd, DVM, MS, DACZM

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