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Fungal Diseases of Fish
by Robert B. Moeller Jr., DVM

California Animal Health and Food Safety Laboratory System
University of California


  1. Caused by various groups of aquatic fungi; primarily Saprolegnia, Achlya, and Aphanomyces.
  2. Saprolegniasis affects all species and ages of freshwater and estuarine fish.
  3. 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.
  4. 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.
  5. Most fish die due to osmotic or respiratory problems if the affected area of skin or gills is large.
  6. 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)

  1. Caused by two species Branchiomyces sanguinis and B. demigrans.
  2. Primarily a problem in carp, rainbow and brown trout, and eels.
  3. 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.
  4. 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.


  1. Ichthyophonus hoferi; large 10 250 micron spores which may germinate to form large hyphae (similar to the hyphae of Saprolegnia).
  2. This fungus infects all species of fish.
  3. 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.
  4. Transmission is unknown, but believed to be due to ingestion of contaminated feed.

Exophiala sp.

  1. Exophiala salmonis and E. psychrophila; these fungal organisms have hyphae that are septated, irregular in width and branched.
  2. 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.
  3. 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.
  4. Transmission is unknown.
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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.

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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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