Protozoan & Mycotic Diseases of Fish |
by Zeeshan anjum department of veterinary & Animal sciences Faculty of Agriculture Rawalakot |
External Protozoan Diseases of Fish
Ichthyophthirius multifiliis ("Ich" or White Spot Disease)
A. The largest protozoan parasite of fish. The trophozoite are up to 100 microns diameter, ciliated and contain an oval horseshoe shaped nucleus.
B. This is a disease of aquarium and hatchery reared fish.
C. Clinically fish become hyperactive with fish flashing and cutting against rocks or sides of aquariums. As the trophozoites enlarge they cause hyperplasia of the epidermis with white spots forming on the skin and gills. Severely infected fish may have respiratory problems and die. Histologically there is epidermal hyperplasia with the encysted trophozoite present in the epidermis.
D. The life cycle is direct. Encysted trophozoites (trophonts) leave the fish and settle to the bottom of the tank. The trophozoites (tomonts)divide into numerous tomites (theronts) that are released to infect the skin of the fish. The life cycle takes approximately 4 days to complete. However, it can be sped up by increasing the water temperature.
E. The only way to treat the disease is by interrupting the life cycle of the parasite. Removal of fish from the infected water for 3 days (25°C) will usually interrupt the life cycle (Tomites live only 48 hours at 26°C). One must treat the water to kill the tomites to prevent spread of the disease (Malachite green, formalin, methylene blue, or KMnO4). Remember, these treatments only kill the tomites and not the trophozoites that are encysted in the fish.
F. Cryptocaryon irritans is the salt water equivalent to Ichthyophthirius.
Ichthyobodo necator (Costiasis)
A. Piriform shaped protozoa 6-12 microns long with two short and two long flagella. These are stalked protozoa that attach to the skin or gills.
B. This disease is observed in most aquariums and hatchery raised fish. This disease occurs primarily in cold waters (10°C) and affects very young fish when they are just beginning to eat food.
C. Clinically the fish may flash, produce abundant mucus over the skin (blue slime disease) and/or show respiratory distress (flaring of gills). Histologically the parasites are attached to the epithelial surface of the skin or gills.
D. Transmission of the parasite is by direct contact with the protozoa. This protozoon is a free swimmer so it can swim and then attach to the host where it undergoes binary fusion for reproduction.
Trichodina sp. (Trichodiniasis)
A. This disease is caused by a group of peritrichal ciliated protozoans. The organisms are saucer shaped, 50 microns diameter, with rows of cilia at both ends and a macro and micronucleus. When viewed dorsoventrally, the parasite appears as an ornate disk with a characteristic ring of interlocking denticles forming a circle in the middle of the organism. (Trichodina truttae is considered to be a specific pathogen for salmonids).
B. These are observed on most fresh and saltwater fish. This protozoon is relatively common on many fish and is not always associated with disease.
C. Clinically fish usually exhibit flashing and become lethargic. There is an increase in mucus production causing a white to bluish haze on the skin. The skin may develop ulcers and the fins may fray. If the gills are involved, the fish may have severe respiratory distress. Histologically, masses of organisms are attached by adhesive discs and denticles of exoskeleton to the epidermis. The underlying epithelial cells undergo necrosis. There is secondary hyperplasia and hypertrophy of the gill epithelium.
D. Transmission is by direct contact with infected fish and or contaminated water.
Tetrahymena corlissi and Tetrahymena pyriformis
A. Normally a free living oval ciliated 50-70 micron long protozoa.
B. The organism has been known to affect the fry of various cultured fish (Guppy "Guppie killer" and Northern pike).
C. Clinically, one may observe necrosis and hemorrhage of the skin. In severe cases the fish have rupture of the body walls and the fish eviscerate. Histologically one observes massive invasion of the musculature by this organism. (The ventral abdominal wall is severely affected.)
D. This is a free living protozoan that only becomes a problem at times of overcrowding and poor water quality.(water having a high organic matter content)
Dinoflagellates (Velvet disease, Coral fish disease)
A. Dinoflagellate 100 microns diameter containing chromatophores and a single eccentric nucleus. When free swimming they are 20 microns diameter contain a transverse flagellum in the transverse furrow and a longitudinal flagellum in the longitudinal sulcus. Several species of dinoflagellate are involved:
1. Oodinium (Velvet disease)
2. Amyloodinium (Coral fish disease)
B. Problem in aquarium and cultured fish.
C. Clinically, fish flash in the water and become depressed with lateral opercular movement. A shimmering heavy yellow colored mucus secretion over the skin and gills is observed. Histologically, large oval organism (80 microns diameter) with multiple chromatophores and a single eccentric nucleus are attached to epithelial cells by pseudopodia.
D. Transmission is by direct contact with infected fish, and contaminated water.
Epistylis (Heteropolaria sp.; Red sore disease)
A. Branched stalked ciliated protozoan (Heteropolaria colisarum).
B. Found primarily in wild populations of scaled fish.
C. Clinically, one observes ulcers or cotton-like growth on the skin, scales and spine resulting in a red colored lesion. In catfish the lesion involves the spines and bones that underlie the skin of the head and pectoral girdle. This protozoan parasite has also been observed on eggs.
D. This ciliated protozoan is primarily a free-living protozoan that lives on aquatic plants and is believed to be an opportunist. Outbreaks have occurred in catfish and salmon that have been maintained in water high in organic content.
Glossatella
A. This disease is caused by the ciliated protozoan Apiosoma that has a barrel shaped body with cilia at the distal end and a large rounded macronucleus.
B. This organism usually is not a problem but can affect many species of fish.
C. The organism can appear on the gills or skin causing increased mucus production and hyperplasia. Severe infections of the gills will cause respiratory problems.
This disease is a problem when fish are exposed to poor water quality.
Internal Protozoal Diseases of Fish
Henneguya (Blister disease, Myxosporidiosis)
A. Myxosporidean parasite (6 Henneguya spp.) with two polar capsules and a long tail like extension of the spore shell. This parasite is believed to be a Myxosporidean in the fish and an Aurantiactinomyxoa in the mud worm.
B. Problem in many cultured freshwater fish; channel catfish can be heavily infected.
C. Clinically, fish are presented with numerous white cysts on the skin and gills. Cyst can become very large. Cysts may lead to gill epithelial hyperplasia leading to anoxia. Interlamellar forms may cause some necrosis of gills and occasional death. Treating affected fish with chemotherapeutic agents is usually ineffective and may cause more deaths.
D. The life cycle is unknown. It is felt that a mud worm (Oligochaete sp.) is involved in an indirect life cycle with asexual and sexual stages in the mud worm (Aurantiactinomyxoa sp.) and catfish (Myxosporidean).
E. Henneguya exilis kudo was once believed to be the cause of Proliferative Gill Disease. However, the evidence suggests that the interlamellar form of the parasite that evokes a serious inflammatory response is probably due to another Myxosporidean (Aurantiactinomyxoa sp. or the extrasporogenic stage of the myxozoan Sphaerospora ictaluri).
Proliferative gill disease (Hamburger gill disease)
A. Myxosporidean parasite; most likely an Aurantiactinomyxoa sp. (Triactinomyxid myxozoan). Note: some feel that this may represent the extrasporogenic stage of the myxozoan Sphaerospora ictaluri.
B. Problem in many cultured freshwater fish (primarily catfish) and usually involves new ponds.
C. Clinically there is rapid onset with the disease killing 10% to 95% of the fish. Water temperatures between 16 and 20 degrees centigrade favor optimal growth of the organism. Fish are presented in severe respiratory distress. Grossly there is intense granulomatous inflammation and swelling of the gills with epithelial hyperplasia and gill necrosis. Histologically, the cyst observed in the gill lamella cause necrosis of the cartilage, distortion of the gill lamella and an intense inflammatory response with numerous macrophages infiltrating the gill lamella around the cysts. Cyst have been observed in other organs (brain, spleen, liver, kidney).
D. The life cycle is unknown. The parasite is believed to maintain mild subclinical infections in some fish host or has an indirect life cycle involving a mud worm (Oligochaete of the Duro sp. (Duro digitata)). Infected oligochaetes release Aurantiactinomyxoa spores that infect more oligochaete and the channel catfish. Transmission of the spores from the fish to the oligochaete have not been observed. This suggests that the catfish may be an abnormal host for this parasite.
E. Survivors are believed to be resistant to reinfection.
Myxobolus cerebralis (Myxosoma cerebralis or Whirling Disease)
A. Myxosporidean parasite with a 10-micron oval spore with 2 piriform polar capsules.
B. Parasite affects primarily young salmonids (rainbow trout most susceptible; Brown trout and Coho salmon resistant).
C. Clinically, fish develop blackened tails and become deformed about the head and spine (scoliosis) with the fish swimming erratically (whirling). Histologically, there is necrosis of the cartilage, particularly of the head and spine, with numerous spores present in the area of inflammation. The necrosis of the cartilage is the cause of the deformation.
D. Transmission is believed to be by ingestion of spores or spore attachment and penetration. The life cycle of this organism is not completely known. A tubificid oligochaetes (tubifex mud worm, Tubifex tubifex) is an important intermediate or transport host. It is believed that the parasite undergoes sporulation in the tubifex worm were the organism takes on the form of a Triactinomyxon sp. It is believed that this parasite is then released from the tubifex worm and infects the trout. Tubifex worms are infected for life. Trout are believed to become infected by the ingestion of Triactinomyxon spores by eating the mud worms, by the ingestion of spores free in the water or by free spores penetrating the epithelial surface of the fish. Released spores may attach and penetrate the epithelial surface of the fish (body, tail, gills, causal fin, or mouth). Spores develop into sporoplasms and invade epidermal cells (goblet or mucosal cells). These parasites then multiply and progressively migrate to the peripheral nerves by day 4-post infection. Later they migrate to the bone and cartilage. In the cartilage, the sporoplasms develop into trophozoites that undergo asexual mitosis forming numerous spores that infect the cartilage. Spore development is substantially influenced by temperature with lower temperatures causing spore development to take longer.
E. Spores are very resistant to environmental conditions and can with stand freezing and thawing, temperatures as high as 66°C, passing through the gut of birds and fish, and survive in sediment for up to 30 years. Control is done by removal of all dead or infected fish and disinfecting the pond with Calcium Cyananide, lime, or chlorine. Decreasing the Oligocheate in the water can also be accomplished by concrete lining of ponds and raceways. Spores can be reduced in water by ultraviolet treatment of the water. Infected fish can be treated with Fumagillin in feed at 0.5g/kg of feed for two weeks.
Microsporidians (Glugea, Pleistophora, Loma)
A. Microsporidian parasites form cysts in various organs. The cysts are filled with small 1 to 2 micron spores. Parasitic cyst may induce hypertrophy of the infected cell (Glugea, Loma, Spraguea, and Ichthyosporidium) or does not cause hypertrophy of infected cells (Pleistophora).
B. Microsporidian parasites are found in numerous fresh and saltwater fish.
C. Clinically microsporidian present themselves as individual or multiple cysts that can become quite large and may give the appearance of neoplasms (xenomas). These cysts are filled with numerous refractile spores.
1. Glugea and Loma: Infect macrophages and other mesenchymal tissues which then undergo massive hypertrophy causing deformity of visceral organs (liver, gut, ovaries) as well as infections in the muscle and subcutis.
2. Pleistophora hyphessobryconis (Neon tetra disease): This microsporidian infect the sarcoplasm of muscle fibers causing these fibers to be filled with these organisms. There is no inflammatory reaction around the cyst.
D. Transmission of the disease is most likely direct.
Coccidiosis
A. Primarily of the genus Eimeria. Various species of Eimeria are observed in the different fish.
- Affects both fresh and saltwater fish. The coccidia not only infects the epithelium but also many other organs including the gonads. This is a very important problem in the carp and goldfish culture.
- Eimeria subepithelialis; carp: Nodular white raised areas in the middle and anterior gut.
- Eimeria carpelli; carp: Ulcerative, hemorrhagic enteritis.
- Eimeria sardinae; marine fish: Granulomatous reaction in the liver and testicles.
Hexamita salmonis
A. Binucleated piriform protozoan with 6 anterior and 2 posterior flagella.
B. Infects young salmonids.
C. Clinically the young fish have anorexia, and become debilitated with reduced growth. The fish develop acute enteritis with numerous organisms present in the feces.
D. In farmed Chinook and Atlantic salmon the disease can become systemic with fish becoming anemic with swollen kidneys and exophthalmus. Boils on the dorsal skin and numerous granulomas with organisms present have been observed.
E. Transmission is by ingestion of infective cyst.
Proliferative Kidney Disease (PKD, PKX, X Disease)
A. Believed to be caused by a myxosporan parasite (Sphaerospora sp), however, the taxonomy of the parasite is not completely worked out.
B. Parasite causes a serious problem in cultured salmonids (Rainbow trout and salmon) in Europe and North America. Infected ponds can see a mortality between 10% and 95%. Outbreaks tend to occur in fingerlings with rising water temperatures. Water temperatures of 16 degrees centigrade seem to favor growth of the organism.
C. Clinically infected fish have a darker body pigmentation, exophthalmos, ascites and pale gills. Internally, the kidneys are swollen and have numerous grey white area of granulomatous inflammation scattered throughout. Diseased fish also develop anemia and hypoproteinemia. Histologically, the kidney has a granulomatous interstitial nephritis with macrophages and lymphocytes surrounding the amoeboid parasites (15 diameter and usually with multiple daughter cells). There is usually prominent tubular and hematopoietic tissue loss. The parasite may also be identified in the spleen, liver, muscle, gills and intestines.
D. The life cycle of the parasite is unknown. The marked inflammatory response observed in the infected fish and the lack of mature spores suggests that the fish may be an aberrant host.
Cryptosporidiosis
A. Intercellular extracytoplasmic protozoan
B. Cryptosporidium infects the intestine of several species of fish. (Carp; Naso tang, Naso litatus; tropical freshwater catfish, Plecostomus sp.; and cichlids)
C. The importance of cryptosporidiosis as a pathogen in fish is unknown. May cause some debilitation; believed to be a secondary invader after the immune system is depressed. Infected fish usually are presented emaciated and not doing well.
The importance of this organism as a reservoir for infection in other animals and man is unknown.
Fungal Diseases of Fish
Saprolegniasis
A. Caused by various groups of aquatic fungi; primarily Saprolegnia, Achlya, and Aphanomyces.
- Saprolegniasis affects all species and ages of freshwater and estuarine fish.
- Clinically, affected fish develop white to brown cotton like growths on skin, fins, gills and dead eggs. This organism is an opportunist that will usually grow over previous ulcers or lesions. Diagnosis is by finding broad nonseptate branching hyphae that produce motile flagellated zoospores in the terminal sporangia.
- In the Atlantic menhaden, gizzard shad, and some other marine fishes, this fungus may present as an ulcerative mycosis that may progress to a deep necrotic lesion involving the muscle. Histologically there is an intense granulomatous inflammation with broad (7 to 14 micron), nonseptate hyphae.
- Most fish die due to osmotic or respiratory problems if the affected area of skin or gills is large.
- The fungi are normal water inhabitants that invade the traumatized epidermis. Improper handling, bacterial or viral skin diseases, and trauma are the major causes of the disease. It is interesting to note that temperature has a significant effect on the development of infections. Most epizootics occur when temperatures are below the optimal temperature range for that species of fish.
Branchiomycosis (Gill rot)
A. Caused by two species Branchiomyces sanguinis and B. demigrans.
B. Primarily a problem in carp, rainbow and brown trout, and eels.
C. Affected fish usually show respiratory distress. There is prominent gill necrosis caused by thrombosis of blood vessels in the gills. Histologically, the identification of nonseptate branching hyphae with an intrahyphal eosinophilic round body (apleospores) in and around blood vessels of the gill is diagnostic.
D. The disease occurs most commonly in overcrowded ponds with abundant organic matter and high ammonia levels. Usually warm water temperatures (20-25°C) bring about the disease.
Ichthyosporidiosis
A. Ichthyophonus hoferi; large 10 250 micron spores which may germinate to form large hyphae (similar to the hyphae of Saprolegnia).
- This fungus infects all species of fish.
- Clinically the fish are emaciated with small round occasionally ulcerated black granulomas in the skin. Scoliosis is occasionally observed. Internally, numerous granulomas are observed in many visceral organs. Microscopically, the lesion consists of granulomas with encysted large PAS positive spores. Occasionally large irregular shaped hyphae are observed.
- Transmission is unknown, but believed to be due to ingestion of contaminated feed.
Exophiala sp.
A. Exophiala salmonis and E. psychrophila; these fungal organisms have hyphae that are septated, irregular in width and branched.
- This disease is observed in many species of fresh and saltwater fish. E. salmonis has become an organism of increased importance in caged cultured salmonids.
- Clinically the fish become darker and lethargic, with erratic and whirling swimming behavior. Occasionally dermal nodules are present. Numerous round yellow to white granulomas are present in visceral organs (liver, kidney, spleen) with prominent enlargement of the posterior kidney common. Histologically, branched, irregular width, septated hyphae are present in the lesions.
Transmission is unknown.
REFERENCES
1. Roberts R.J: Fish Pathology, Bailliere Tindall, London, Second edition, 1989.
2. Ferguson H.W.: Systemic Pathology of Fish, Iowa State Press, Ames, Iowa, 1989.
3. Anderson B.G.: Atlas of Trout Histology, Wyoming Department of Fish and Game, 1974.
4. Fox J.C.: Laboratory Animal Medicine, Academic Press, 1984.
5. Magaki G., Rebelin W.E.: The Pathology of Fishes, The University of Wisconsin Press, 1975.
6. Wolf K.: Fish Viruses and Fish Viral Diseases, Cornell University Press, London 1988.
7. Tucker C.S.: Channel Catfish Culture, Elsevier Science Publishers, Amsterdam, 1985.
8. Principal Diseases of Farm Raised Catfish, Southern Cooperative Series Bulletin No 225, 1985.
9. Wales J.H.: Microscopic Anatomy of Salmonids. An Atlas, United States Department of the Interior, Resource Publication 150, 1983.
10. Grizzle J.M.: Anatomy and Histology of the Channel Catfish, Auburn Printing Co, 1976.
11. Reichenbach-Klinke H. H.: Fish Pathology, T.F.H. Publications, Inc. Neptune City, NJ. 1973.
12. Stoskopf, M.K.: Fish Medicine, W.B. Saunders Co. 1993.
13. DeTolla, L.J., Srinivas, S.: "Guidelines for the Care and Use of Fish in Research". Institute of Laboratory Animal Resourses Journal. Vol 37:4(1995), pp 159-173.
14. Kane, A.J., Gonzalez, J. F., Reimschuessel, R: "Fish and Amphibian Models Used in Laboratory Research". Laboratory Animal. Vol 25:6(1996), pp 33-38.
15. Lewbart G.A. Self-Assesment Color Review of Ornamental Fish, Iowa State Press,1998.
16. Bruno D.W., Poppe T.T., A color atlas of Salmonid Diseases. Academic press, 1996.
2. Ferguson H.W.: Systemic Pathology of Fish, Iowa State Press, Ames, Iowa, 1989.
3. Anderson B.G.: Atlas of Trout Histology, Wyoming Department of Fish and Game, 1974.
4. Fox J.C.: Laboratory Animal Medicine, Academic Press, 1984.
5. Magaki G., Rebelin W.E.: The Pathology of Fishes, The University of Wisconsin Press, 1975.
6. Wolf K.: Fish Viruses and Fish Viral Diseases, Cornell University Press, London 1988.
7. Tucker C.S.: Channel Catfish Culture, Elsevier Science Publishers, Amsterdam, 1985.
8. Principal Diseases of Farm Raised Catfish, Southern Cooperative Series Bulletin No 225, 1985.
9. Wales J.H.: Microscopic Anatomy of Salmonids. An Atlas, United States Department of the Interior, Resource Publication 150, 1983.
10. Grizzle J.M.: Anatomy and Histology of the Channel Catfish, Auburn Printing Co, 1976.
11. Reichenbach-Klinke H. H.: Fish Pathology, T.F.H. Publications, Inc. Neptune City, NJ. 1973.
12. Stoskopf, M.K.: Fish Medicine, W.B. Saunders Co. 1993.
13. DeTolla, L.J., Srinivas, S.: "Guidelines for the Care and Use of Fish in Research". Institute of Laboratory Animal Resourses Journal. Vol 37:4(1995), pp 159-173.
14. Kane, A.J., Gonzalez, J. F., Reimschuessel, R: "Fish and Amphibian Models Used in Laboratory Research". Laboratory Animal. Vol 25:6(1996), pp 33-38.
15. Lewbart G.A. Self-Assesment Color Review of Ornamental Fish, Iowa State Press,1998.
16. Bruno D.W., Poppe T.T., A color atlas of Salmonid Diseases. Academic press, 1996.
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