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Parasitic Diseases in Aquaculture

By Roy P. E. Yanong, VMD, University of Florida ; Ruth Francis-Floyd, DVM, MS, DACZM, Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Florida

Protists Infecting Gills and Skin:

For information on protists infecting the gills and skin of fish, see Protistans Infecting Gills and Skin. Information presented here is relevant to fish reared in intensively cultured systems. As previously mentioned, trichodinids are indicators of poor sanitation and/or overcrowding. These organisms are commonly found in cultured fish, especially in systems with high stocking densities, high organic loads, and minimal (or no) in-line sanitation (eg, UV or ozone). When fish are heavily infested, losses will occur. In these cases, chemical treatment alone may not be adequate for complete control. Addressing management problems that favor infestation should be incorporated into the treatment plan. Decreasing organic material is essential for treatment efficacy, because these organisms can be protected from chemical treatment by “hiding” within organic debris.

Ambiphyra and Apiosoma are sessile ciliates found on the skin, gills, and fins of fish. These are common in pond-reared fish and also have a predilection for organically rich environments. They are not generally found on marine fish. 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. These 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.

For identification of protists affecting external surfaces of cultured fish, see Protistans Infecting Gills and Skin. Control strategies are determined by the species affected and their intended use, the type of system the fish is housed in, cost of treatment, and environmental considerations.

Internal Protistan Parasites:

For information on flagellated parasites (diplomonads such as Spironucleus, and kinetoplastids such as Cryptobia iubilans), see Internal Protistan Parasites. These parasites are also common in the GI tract of susceptible aquaculture species. The use of chemical treatment to control these organisms is not feasible in food fish; however, improvements in environmental conditions and husbandry will decrease morbidity and mortality.

Opalinids, larger “cilate-like” parasites commonly found in the GI tract, may be found occasionally in kissing gourami, discus, some catfishes, and other species. Considered commensal by some, these may or may not be associated with disease.

Internal Metazoan Parasites:

Myxosporean (Myxozoa) diseases significant in aquaculture include whirling disease and proliferative kidney disease (PKD) 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.

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 four 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 myxosporean infection of channel catfish caused by Aurantiactinomyxon ictaluri. The organism has a complex life cycle, with the oligochaete worm Dero digitata serving as the intermediate host. Channel catfish may be aberrant hosts 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. Quantitative PCR has also been used diagnostically.

Platyhelminthes (Flatworms):

Flatworms (Platyhelminthes: monogenea, digenea, leeches, turbellaria, see Monogeneans) are also of concern in aquaculture.

Monogeneans, including gyrodactylids, dactylogyrids, ancyrocephalids, and capsalids, have all been identified in numerous aquacultured species.

Gyrodactylus salaris is a reportable disease of salmonids but has not been reported in the USA. Any gyrodactylid found on a salmonid species should be identified well enough to determine whether it is G salaris. Neobenedenia and Benedenia are important monogeneans in marine aquaculture.

Although treatments will vary depending on the aquaculture setting and species of parasite and fish host, formalin is approved for use in food fish. Hyposalinity for marine species, including freshwater dips, can help reduce loading on fish, but environmental reservoirs (eg, eggs) will need to be controlled as well. Neobenedenia was controlled in one study when salt levels were reduced to ≤15 g/L for 5 days and ≤18 g/L for 7 days. Hydrogen peroxide is not approved for use against monogenean infestations in food fish but can be used extra-label under the supervision of a veterinarian. Doses are experimental (see Use of Hydrogen Peroxide in Finfish Aquaculture [] for some reported doses). Freshwater monogenea can be treated with formalin or saltwater dips to remove a majority of parasites from fish, but environmental stages (eg, eggs) will also require control.

Digeneans have complicated life cycles, with several larval stages that infect one or more hosts. As discussed above, fish raised in outdoor aquaculture systems are more likely to become infested by this group of parasites. With rare exceptions, the first intermediate host is a mollusc, without which the life cycle generally cannot be completed. The final host for many common freshwater aquacultured fish digenean parasites is a wading bird. 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. In many cases, low level infestations may not result in significant morbidity but will prevent sale of fish for aesthetic reasons. Treatment is not recommended.

Pond-reared, juvenile, tropical fish may develop severe gill disease from metacercarial cysts of the digenean Centrocestus formosanus 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. If only a small percentage of gill tissue is infested, then fish may be clinically normal. Treatment of infected fish has not been successful; however, prevention of the disease by elimination of the intermediate host, the freshwater snail Melanoides tuberculata (common name, the red-rimmed melania), has been effective.

Bolbophorus confusus is a digenean trematode that causes morbidity and 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 most 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 that of 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.

Other Vermiform Internal Parasites:

Acanthocephalids (thorny-headed worms) are common in wild fish as both larval tissue stages and adult intestinal parasites. They are most 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.

Pentastomids, which may present as either “white grub”–like lesions externally or within the body cavity in numerous organs, have been identified in aquacultured aquarium fish species, including swordtails. Fish are infected with the larval or “nymph” stage, and the final hosts are aquatic reptiles (turtles, snakes, alligators). Under the microscope, pentastome nymphs appear as thick, stout, curved, and “segmented” worms (see Pentastomid Infections in Fish []). Fish often show no behavioral changes and may have minimal morbidity or mortality, but fish cannot be sold for aesthetic reasons. Treatment is not recommended, but management involves removal of final hosts from the system.

Crustacean Parasites:

Crustacean parasites, including Argulus, Lernaea, and Lepeophtheirus, can become significant problems in aquaculture (see Crustaceans). Treatment of these organisms is not feasible for food fish species.