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«The specific identify of many parasites, bacterial and viral diseases, and tumors can only be accomplished through complex laboratory techniques. Any ...»

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Fisheries Stream Survey Manual Appendix 7.

Appendix 7. Introduction to Common Fish Diseases

Fishing is a very popular outdoor activity in Minnesota and many fishermen are

interested in the welfare of the fish. Consequently, the fish they catch are often

scrutinized for anything unusual, and fishery biologists and fish pathologists are

questioned for identification of these conditions. Because fish, like humans are

attacked by a wide variety of parasites, bacteria, viruses and anomalies, many inquiries are received every year.

The specific identify of many parasites, bacterial and viral diseases, and tumors can only be accomplished through complex laboratory techniques. Any unusual specimens not described should be forwarded to the pathology laboratory. Live specimens are most desirable, but fresh refrigerated, or iced material is good, although material preserved in 10% neutral buffered formalin is also suitable. If there is any question about the method of preservation, the pathology lab should be contacted.

Fish are no different than other animals where disease is concerned. A healthy animal is more resistant to disease than a weak animal, or an animal under stress. Fish respond physiologically to environmental change. A fish’s body temperature changes with water temperature. Metabolism also changes with water temperature. If the temperature changes very rapidly, physiological processes are drastically altered, sometimes causing death. Such things as low oxygen, excess silting, lightning, excessive current, super saturation of water by gases and all kinds of pollutants exert stresses on fish. If fish are not damaged directly by these things, they may be weakened, their resistance lowered, and their vulnerability to disease caused by parasites, bacteria, and viruses increase.

Common terminology used to describe various stages in the life cycle of parasites

and pathological conditions are defined below:

1 Fisheries Stream Survey Manual Appendix 7.

1. Parasite: An organism which lives in or on another organism (host) and which depends on the host for its food, has a higher reproductive potential than the host, and is suspected of harming the host when present in large numbers.

2. Host: An animal or plant which harbors or nourishes another organism.

3. Accidental host: The host in which a parasite of another animal will live for a variable length of time.

4. Definitive host: The host in which a parasite passes its adult or sexual existence.

5. Intermediate host: A host in which a parasite passes a larval or non-sexual existence.

6. Cercaria: Larval trematode form, which produces rediae or sporocysts in infected snails.

7. Metacercaria: Larval stage of trematodes between cercarial and adult stage, a more or less quiescent stage. The larval stage of flukes, which follows the cercarial stage.

8. Pleurocercoid: Larval stage of cestodes.

9. Cestode: Girdle-form. Tapeworms.

10. Trematode: The flukes. Monogenea: ectoparasitic in general, one host;

Digenea: endoparasitic in general, two hosts or more.

11. Nematode: A diverse phylum of round worms, many of which are plant or animal parasites.

12. Lesion: Any visible alteration in the normal structure of organs, tissues, or cells.

13. Stress: A state manifested by a syndrome or bodily change caused by some force, condition, or circumstance in or on an organism or on one of its physiological or anatomical systems. Any condition which forces an organism to expend more energy to maintain homeostasis.

–  –  –

14. Infection: The introduction or reentry of a pathogen or parasite into a host, resulting in the presence of the pathogen or parasite within the body, tissues, or cells of the host, whether or not this results in overt disease.

15. Disease: A morbid process or condition of the body or its parts, having a characteristic train of signs that distinguishes it from other morbid processor conditions and from the normal state. Any state which results in a gradual degeneration of homeostasis.

–  –  –

Asian Tapeworm - Bothriocephalus acheilognathi (SA) Bothriocephalosis is an intestinal infection of certain fish by the cestode Bothriocephalus acheilognathi, a Pseudophyllidian tapeworm. This tapeworm has been reported in Asia, Europe, Australia, South Africa and North America. In North America it has been reported in Mexico, British Columbia throughout the southern US and in New Hampshire, New York and Hawaii. Fish become infected after ingesting infected copepods and development of the worm occurs in the anterior intestinal tract. Bothriocephalus is a thermophile that has an optimal temperature for growth and maturation above 25C.

Most members of the Family Cyprinidae are considered potential hosts, with the exception of goldfish. Infections have also been reported in species in the following families: Siluridae, Poeciliidae, Percidae, Centrachidae, Gobiidae, and Cyprinodontidae.

Bass Tapeworm - Proteocephalus (SB)

–  –  –

Several species of Proteocephalus may be found in a wide variety of fresh water fish species. This tapeworm has been given the common name of the bass tapeworm as Proteocephalus ambloplitis is commonly found in the adult stage in the intestine of both largemouth and smallmouth black bass. The plerocercoid larvae, however, are found in the body cavity and internal organs of many species of fish, especially rock, large and smallmouth bass in many lakes and streams. It is the larval plerocercoid stage which is most often seen, and which causes damage to fish. The Plerocercoids develop in the body cavity and internal organs, especially the liver and ovaries. Because they do not encyst, but continue to move around, they destroy tissue and cause multiple tiny hemorrhages. This may produce adhesions and a brown color in the body cavity. Heavy ovarian infestation may cause sterility.





The life cycle of this tapeworm involves a larger bass eating a smaller fish (intermediate host) infested with plerocercoids. Research has shown that the pleurocercoid may also migrate from the body cavity directly into the gut, thus omitting the intermediate host stage. These larval tapeworms attach to the intestinal wall of the larger fish and grow to maturity. Eggs produced by the adult worm pass into the water where they are fed upon by copepods and amphipods. Inside these invertebrate hosts a larval form emerges from the egg, penetrates into the crustacean’s body cavity, and develops into a pleurocercoid. When an infested crustacean is ingested by a small fish, the pleurocercoid emerges, burrows through the intestinal wall of the fish, and migrates into the visceral organs where it may cause extensive damage as a plerocercoid. The pleurocercoid may live several months in the internal organs of a fish.

The bass tapeworm will not infest humans.

Ceratomyxa shasta (SC)

–  –  –

Ceratomyxosis is a disease of salmonid fishes caused by the myxosporidean Ceratomyxa shasta. The parasite has a tropism for the intestinal tissue of the fish and causes high mortalities in susceptible strains of salmonids.

Ceratomyxosis is an important parasite in the Pacific Northwest because it not only causes losses in hatchery reared and wild juvenile salmonids but also contributes significantly to prespawning mortality in adult salmon.

Clinical signs vary among salmonid species. Infected juvenile rainbow trout and steelhead become anorexic, lethargic, and darken. Ascites may distend the abdomen, the vent may be swollen and hemorrhaged and exopthalmia is common. Internally, the intestinal tract becomes swollen and hemorrhaged with the intestinal contents mucoid and caseous material lines the intestine and cecca. The entire digestive tract may become hemorrhaged and necrotic.

This lifecycle also involves an intermediate host and is not present in the state.

Chilodonella (SD) These ciliated protozoans are most frequently found on warmwater fish such as pike and carp, although infestation of trout fry in hatcheries is not uncommon.

The parasites are tiny, 50 to 70 microns long, and cannot be seen without magnification, although heavily parasitized fish may show blotchy gray areas

–  –  –

on the surface of the skin. Under magnification the parasites may be seen as tiny, motile, oval bodies covered with fine cilia.

When Chilodonella occurs in very great numbers on a fish, particularly on the gills, it causes the fish to produce great quantities of mucus, which impair respiration. Affected fish may become lazy, lie on their sides, rise to the surface, and eventually die. The parasite shows a preference for debilitated or undernourished fish. It is frequently observed on northern pike in the spring of the year as they enter a marsh for spawning.

The parasites are not harmful to man.

Diplostomum spathaceum (SE) Diplostomum spathaceum utilizes many species of fishes as a second intermediate host, the metacercariae localizing in eye tissue. Several fisheating aquatic bird species, especially gulls, are the primary hosts of this fluke.

The life cycle of D. spathaceum begins as an adult trematode in the intestine of gulls or other piscivorous birds. The body is 0.3 to 0.5 mm in length and distinctly divided into a flattened anterior fore body and a more cylindrical and narrower hind body. Eggs are shed and passed in feces to the water.

They hatch in about 21 days at summer water temperatures into freeswimming ciliated miracidia.

Miracidia seek aquatic snails for a first intermediate host; only lymnaeid snails are acceptable. The miracidia penetrate the hepatopancreas of the snail and metamorphose to a mother sporocyst, then to one or more daughter

–  –  –

sporocysts. Each produces many cercariae which are released into the water.

The free-swimming cercariae seek second intermediate hosts.

The usual route of transmission from the snail to the second intermediate host is through water and active penetration of the cercariae. However, much evidence points to the fact that transmission is possible by fishes feeding on snails containing cercariae. Some cercariae which enter the skin, fins, and gills enter the blood stream and are carried to the eyes within 30 minutes of the time of penetration.

The snails cause a parasitic blindness diagnosed by cataract and isolation of the parasite.

Myofibrogranuloma (SF) Myofibrogranuloma (MFG) is a muscular dystrophy-like anomaly of walleye in which the skeletal muscle has undergone profound structural changes. The myopathy is recognized by its swollen, coarsely fibrous, granular, and fatty characteristics. The lesion has an opaque yellow-brown color. Included in this pattern of striated muscle deformation is a consolidation and fusion of contiguous muscle fibers to form prominent aggregates of rough, cordlike strands, which eventually undergo a coagulation necrosis and calcification. A simple description is that it looks like the flesh has been freezer burned. The lesions are typically found along the vertebral column while filleting.

Myofibrogranuloma has been found exclusively in adult walleye whose ages range from 3 to 10 years. The sex frequency ratio of the disease is about equal. A higher frequency of this anomaly has been observed to occur in

–  –  –

walleye from comparatively small, fertile lakes and ponds in which the species is maintained exclusively by periodic stocking of hatchery-reared walleye.

Argulus, fish lice (SG) Argulus sp. have been given the common name fish lice as they have the ability to creep about over the surface of the fish. These are large copepods and consequently, they are conspicuous objects on the fish they inhabit. At first glance they look like a scale but on closer examination are seen to be saucer-shaped and flattened against the side of the fish. They have jointed legs and two large sucking discs for attachment, which may give them the appearance of having large eyes. Argulids penetrate the skin of the host fish, inject a cytolytic substance, and feed on blood. Lice prefer those parts of the skin best supplied with blood vessels like the mouth region, the operculum, and the base of the fins.

Heterosporis (SH)

A microsporidea recently detected in the muscle of Yellow perch in the Eagle Chain of lakes in Wisconsin, Leech Lake in Minnesota and Lake Ontario. This parasite has been found in Walleye, northern pike and tullibee.

Experimentally Fathead minnow, rainbow trout are susceptible and Largemouth bass and sunfish are somewhat refractory. Heterosporis has been reported in aquarium species from Germany, France and Thailand.

The parasite infects muscle cells which cause the cells to have a white appearance. The parasites life cycle is not completely known but research is underway to learn more.

–  –  –

Ichthyopthirius multifiliis, Ich (SI) A common disease of hatchery and aquaria fish is white spot, a condition caused by large ciliated protozoans. The adults of this parasite are up to 1 mm in diameter, and may be seen with the unaided eye as tiny white spots on affected fish. The parasites live under the epithelial layers of the skin, fins, and gills of many species of fish, especially young fish. They are found more frequently on warmwater fishes than on fish from coldwater because low temperatures inhibit their activity.

When the parasite has grown to maturity, it leaves the host and becomes enclosed in a cyst. Within this cyst multiplication occurs resulting in the production of from 400 to 2,000 young parasites. These are also ciliated, and when they leave the cyst, swim actively until contacting a fish. If a fish is not found within a few days, the parasites die. If they find a fish, they burrow into the skin, migrate for a time, and then grow to maturity. The entire life cycle takes from 4 days to 3 weeks, depending on the water temperature.

White spot can be very serious, causing high mortality, especially when fish are under crowded conditions and heavy infestation occurs.



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