Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide


oomycete infections in fish



oomycete infections in fish


  • Last modified
  • 22 November 2019
  • Datasheet Type(s)
  • Animal Disease
  • Preferred Scientific Name
  • oomycete infections in fish
  • Pathogens
  • Achlya
  • Achlya ambisexualis
  • Achlya americana
  • Achlya apiculata
  • Achlya bisexualis
  • Overview
  • Fungal infections of fish by oomycetes, commonly known as water moulds, are widespread in fresh water and represent the most important fungal group affecting wild and cultured fish. Most fungi are multicellular and assi...

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Saprolegnia growth on the dorsal areas of an Atlantic salmon, Salmo salar, brood fish.
TitleSaprolegnia growth on an Atlantic salmon
CaptionSaprolegnia growth on the dorsal areas of an Atlantic salmon, Salmo salar, brood fish.
CopyrightD. W. Bruno & B. P. Wood
Saprolegnia growth on the dorsal areas of an Atlantic salmon, Salmo salar, brood fish.
Saprolegnia growth on an Atlantic salmonSaprolegnia growth on the dorsal areas of an Atlantic salmon, Salmo salar, brood fish.D. W. Bruno & B. P. Wood
Life cycle of Saprolegnia and other water moulds (based on Neish and Hughes, 1980).
TitleLife cycle of Saprolegnia
CaptionLife cycle of Saprolegnia and other water moulds (based on Neish and Hughes, 1980).
CopyrightD. W. Bruno & B. P. Wood
Life cycle of Saprolegnia and other water moulds (based on Neish and Hughes, 1980).
Life cycle of SaprolegniaLife cycle of Saprolegnia and other water moulds (based on Neish and Hughes, 1980).D. W. Bruno & B. P. Wood
Scanning electron micrograph (SEM) of primary zoospore of Saprolegnia parasitica isolate (TP41). Spore width = 5 mm. (A.W. Burr and G.W. Beakes, University of Newcastle, unpublished material.)
TitlePrimary zoospore of Saprolegnia parasitica isolate (TP41)
CaptionScanning electron micrograph (SEM) of primary zoospore of Saprolegnia parasitica isolate (TP41). Spore width = 5 mm. (A.W. Burr and G.W. Beakes, University of Newcastle, unpublished material.)
CopyrightD. W. Bruno & B. P. Wood
Scanning electron micrograph (SEM) of primary zoospore of Saprolegnia parasitica isolate (TP41). Spore width = 5 mm. (A.W. Burr and G.W. Beakes, University of Newcastle, unpublished material.)
Primary zoospore of Saprolegnia parasitica isolate (TP41)Scanning electron micrograph (SEM) of primary zoospore of Saprolegnia parasitica isolate (TP41). Spore width = 5 mm. (A.W. Burr and G.W. Beakes, University of Newcastle, unpublished material.)D. W. Bruno & B. P. Wood
Scanning electron micrograph (SEM) of secondary zoospore of isolate TP41. The mastigoneme hairs on the anterior flagellum appear as bobbles. (A.W. Burr and G.W. Beakes, unpublished material.)
TitleSecondary zoospore of isolate (TP41)
CaptionScanning electron micrograph (SEM) of secondary zoospore of isolate TP41. The mastigoneme hairs on the anterior flagellum appear as bobbles. (A.W. Burr and G.W. Beakes, unpublished material.)
CopyrightD. W. Bruno & B. P. Wood
Scanning electron micrograph (SEM) of secondary zoospore of isolate TP41. The mastigoneme hairs on the anterior flagellum appear as bobbles. (A.W. Burr and G.W. Beakes, unpublished material.)
Secondary zoospore of isolate (TP41)Scanning electron micrograph (SEM) of secondary zoospore of isolate TP41. The mastigoneme hairs on the anterior flagellum appear as bobbles. (A.W. Burr and G.W. Beakes, unpublished material.)D. W. Bruno & B. P. Wood
Cryofixed scanning electron micrograph (SEM) of a germinating encysted zoospore of Saprolegnia parasitica (A.W. Beakes, unpublished material).
TitleGerminating encysted zoospore of Saprolegnia parasitica
CaptionCryofixed scanning electron micrograph (SEM) of a germinating encysted zoospore of Saprolegnia parasitica (A.W. Beakes, unpublished material).
CopyrightD. W. Bruno & B. P. Wood
Cryofixed scanning electron micrograph (SEM) of a germinating encysted zoospore of Saprolegnia parasitica (A.W. Beakes, unpublished material).
Germinating encysted zoospore of Saprolegnia parasiticaCryofixed scanning electron micrograph (SEM) of a germinating encysted zoospore of Saprolegnia parasitica (A.W. Beakes, unpublished material).D. W. Bruno & B. P. Wood
Scanning electron micrograph (SEM) of germinating cyst, showing the appearance of long boat-hook bundles. The individual spines cannot be resolved but bundles (hoops) are clearly resolved. (A.W. Burr and G.W. Beakes, unpublished material.)
TitleGerminating cyst
CaptionScanning electron micrograph (SEM) of germinating cyst, showing the appearance of long boat-hook bundles. The individual spines cannot be resolved but bundles (hoops) are clearly resolved. (A.W. Burr and G.W. Beakes, unpublished material.)
CopyrightD. W. Bruno & B. P. Wood
Scanning electron micrograph (SEM) of germinating cyst, showing the appearance of long boat-hook bundles. The individual spines cannot be resolved but bundles (hoops) are clearly resolved. (A.W. Burr and G.W. Beakes, unpublished material.)
Germinating cystScanning electron micrograph (SEM) of germinating cyst, showing the appearance of long boat-hook bundles. The individual spines cannot be resolved but bundles (hoops) are clearly resolved. (A.W. Burr and G.W. Beakes, unpublished material.)D. W. Bruno & B. P. Wood


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Preferred Scientific Name

  • oomycete infections in fish

International Common Names

  • English: Achlya infections in fish; Aphanomyces infections in fish; fish mycoses; mycoses, fish; mycosis, fish; mycotic dermatomycosis; Oomycetes infections in fish; progressive dermatomycosis; Saprolegnia infections in fish; saprolegniasis; Staff's disease; ulcerative dermal necrosis; ulcerative mycosis; water moulds; winter fungus disease; winterkill of catfish


Top of page Achlya
Achlya ambisexualis
Achlya americana
Achlya apiculata
Achlya bisexualis
Achlya caroliniana
Achlya diffusa
Achlya dubia
Achlya flagellata
Achlya hypnogyna
Achlya intricata
Achlya klebsiana
Achlya megasperma
Achlya oblongata
Achlya orion
Achlya proliferoides
Achlya racemosa
Achlya sparrowii
Aphanomyces invadans
Aphanomyces laevis
Aphanomyces piscida
Aphanomyces stellatus
Dictyuchus anomalus
Dictyuchus monosporus
Dictyuchus sterile
Isoachlya anisospora
Isoachlya monilifera
Isoachlya toruloides
Isoachlya unispora
Lagenidium rabenhorsti
Leptolegnia caudata
Leptomitus lacteus
Protoachlya paradoxa
Saprolegnia australis
Saprolegnia delica
Saprolegnia diclina
Saprolegnia ferax
Saprolegnia hypogyna
Saprolegnia invaderis
Saprolegnia litotalis
Saprolegnia luxurians
Saprolegnia megasperma
Saprolegnia mixta
Saprolegnia monica
Saprolegnia parasitica
Saprolegnia shikotsuensis
Saprolegnia subterranea
Thraustotheca clavata
Thraustotheca primoachlya


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Fungal infections of fish by oomycetes, commonly known as water moulds, are widespread in fresh water and represent the most important fungal group affecting wild and cultured fish. Most fungi are multicellular and assimilate nutrients by means of extracellular digestion. The Saprolegniaceae, in particular members of the genus Saprolegnia, are responsible for significant infections, involving both living and dead fish and eggs, particularly in aquaculture facilities. Oomycetes are classical saprophytic opportunists, multiplying on fish that are physically injured, stressed or infected (Pickering and Willoughby, 1982a). Members of this group are generally considered agents of secondary infection arising from conditions such as bacterial infections, immunosuppression, poor husbandry, and infestation by parasites and social interaction. However, there are several reports of oomycetes as primary infectious agents of fish (Willoughby, 1978; Pickering and Christie, 1980) and their eggs (Walser and Phelps, 1993; Bruno and Wood, (1999). These can readily become pathogens, resulting in epizootics among salmonids and other teleosts (Hatai and Hoshiai, 1992, 1993; Bly et al., 1994; Diéguez-Uribeondo et al., 1996). Saprolegnia may occur anywhere on the body of fish, but normally appears as a conspicuous, circular or crescent-shaped, white, cotton-like mycelium, particularly around the head and the caudal, adipose and anal fins (Neish and Hughes, 1980; Willoughby, 1989; Noga, 1993a; Hussein et al., 2001). It may spread over the body by radial extension until adjacent lesions merge.

The oomycetes are an economically important group of mycotic agents that affect salmonids and other teleosts. Four orders are recognised in this class and the most important are the Saprolegniales. Although eight genera (Achlya, Aphanomyces, Calyptratheca, Leptolegnia, Leptomitus, Pythiopsis, Saprolegnia and Thraustotheca) have been reported in naturally or artificially induced infections, only Achlya, Aphanomyces and Saprolegnia are significant in aquaculture (Aller et al., 1987; Hatai and Hoshiai, 1993; Bly et al., 1994). Some species are consistently isolated from fish and generally these are assigned to a single major cluster, which form a coherent, separate taxon, Saprolegnia parasitica Coker (synonym Saprolegnia diclina Humphrey type 1; Willoughby, 1978). The reader is referred to Taugboel et al. (1993), Diéguez-Uribeondo et al. (1994a), Mueller (1994) and Bruno and Wood, (1999) for reviews.

The ultrastructure, biochemistry and molecular sequences of the oomycetes show their phylogenetic roots with the Chromista, the chromophyte algae, and other Protista in the so-called Chromalveolate lineage rather than the true fungi (Beakes, 1989; Dick, 1990; Gunderson et al., 1987; Harper et al., 2005). Several characteristics, including cell-wall composition, mitochondrial morphology and ribosomal deoxyribonucleic acid (DNA) sequences, differentiate the water moulds from the true fungi. Polymerase chain reaction (PCR) techniques are improving our understanding of taxonomic relationships within the oomycetes, and developments such as random amplified polymorphic DNA (RAPD)-PCR (Diéguez-Uribeondo et al., 1996; Whisler, 1996; Bangyeekhun et al., 2003) will help to resolve taxonomic problems and enable additional comparative studies within the oomycetes.

The oomycetes have a complex life cycle, involving sexual and asexual stages (Kanouse, 1932; Noga, 1993a). Sexual reproduction involves the production of gametangia, the male antheridium and the female oogonium. Haploid antheridial male nuclei and egg nuclei arise following meiosis and fuse during the maturation of the resting oospores. Asexual reproduction may occur by the production of chlamydospores or gemmae, but generally zoospores are produced. In the Saprolegniales, motile primary zoospores, produced by the zoosporangium, encyst within minutes to produce primary cysts which normally germinate indirectly to produce a secondary motile zoospore. These more active secondary zoospores may swim for many hours before encysting to produce a secondary cyst. These in turn may undergo further cycles of zoospore formation and encystment (so called polyplanetism; Diéguez-Uribeondo et al., 1994b) or may germinate to produce a hypha, which may develop into a mycelium.

Fungal outbreaks among farmed fish stocks are frequently associated with poor water quality, injuries associated with handling and grading, temperature shock and spawning. For many years malachite green was used to control outbreaks (Bailey, 1983a; Willoughby and Roberts, 1992a). Unfortunately, the potential teratogenic or mutagenic properties of malachite green (Meyer and Jorgenson, 1983; Clemmensen et al., 1984; Fernandes et al., 1991) have resulted in banning its use in many countries. The search for alternative chemical treatments or other means of controlling fungal infections has resulted in many investigations being conducted. However, to date, only a limited number of chemical compounds show potential as fungicides and none is considered as effective as malachite green. Currently, the most successful strategy for the control of Saprolegnia in farmed fish is a combination of farm management, husbandry practices and chemical bath treatments. Generally, a combination of these practices is important in preventing or limiting fungal outbreaks.

Histological changes resulting from oomycete infection include loss of integrity of the integument, oedema and degenerative changes in the muscle mass. More severe lesions show deeper myofibrillar and focal cellular necrosis, spongiosis or intracellular oedema and sloughing of the epidermis (Copland and Willoughby, 1982), allowing hyphae to penetrate the basement membrane (Bootsma, 1973).

Most of the literature on oomycete infections relates to teleost fish in particular salmonids (Coker, 1923; Willoughby, 1985; Noga and Dykstra, 1986) and forms the predominant part of this review. The term fungal is used to represent fungal-like Chromalveolates or flagellated water moulds.

Topics for further study

Our understanding of the taxonomy of the Saprolegniaceae is beginning to be advanced by the application of molecular techniques. Molecular studies comparing the sequences of conserved genes show that the oomycetes have their phylogenetic origins with the chromophyte algae within a broader group that also encompasses Dinoflagellates, Apicomplexans and Ciliates, rather than with the true fungi (Harper et al., 2005). Molecular techniques are also enhancing our taxonomic knowledge within the oomycetes. Taxonomic analyses using the ITS and or large ribosomal subunit gene sequences in the Saprolegniaceae (Leclerc et al., 2000; Petersen and Rosendahl, 2000) and RFLP and RAPD analyses in the genus Aphanomyces (Lilley et al., 2003), are helping to resolve genus and species level concepts. Many traditional genera, particularly Achlya, are not monophyletic assemblages and the genus Saprolegnia is also likely to be problematic (Hulvey personal comm. 2004). However, these preliminary studies have indicated that the two important fish pathogens, Saprolegnia parasitica (Molina et al., 1995; Leclerc et al., 2000) and Aphanomyces invadans (Lilley et al., 2003), both appear to be well supported taxa. As shown already for A. invadans, it is hoped these approaches will soon lead to the development of diagnostic kits specific for S. parasitica and subgroups that can be applied in the absence of any reproductive structures. The importance of specific surface topography recognition at the host surface during the initial interaction with the disease agent has been demonstrated. Future studies of such interactions in oomycetes might be directed towards the molecular analysis of cell-surface components, using monoclonal antibodies, and the identification of the cell types involved in host defence. Monoclonal antibodies are likely to provide precise tools for probing and identifying receptor functions of antigens and therefore represent an area that would benefit from further development.

Many researchers have screened chemicals and advised on their suitability as fungicides. Despite this, no agent has been identified which matches the effectiveness of malachite green, although hydrogen peroxide, formalin and NaCl show some potential, and prophylactic bath treatments using a buffered iodophor are apparently successful for eggs.

The fact that oomycetes are more closely related to the chromophyte algae than true fungi should enable particular chemical groups to be selected, since many fungicides that are effective against higher fungi are ineffective against oomycetes and vice versa. Clearly, there is a need for safe and effective fungicides for use in aquaculture, and development in this area remains a priority. Supporting studies aimed at examining the permeability of the cell membrane to particular fungicides, the existence of suitable binding sites and the rate at which compounds accumulate and are detoxified are also required. Measurement of uptake is also essential for a thorough understanding of the mode of action of each fungicide.

Studies identifying mechanisms of immunostimulation in immunosuppressed fish may help to develop a vaccine or therapeutic approach for controlling saprolegniasis. This will require research into the absence of leucocytic infiltration in fungal-infected tissue seen in the winter months, its relationship to the inactivity of the complement system and an understanding of the connection between environmental temperature, fungal agents and the nature of the immune response. Studies aimed at producing antibodies to specific surface components, thereby reducing the ‘stickiness’ of the spores and their attachment to the fish surface, would be beneficial.

There is sufficient evidence to show that the infestation of fish eggs by zoospores results from a chemotactic response to compounds associated with the eggs themselves. Although further work is necessary to identify the role of specific compounds, it may become practical to manipulate these chemical messengers with amino acid baits, allowing the development of more targeted, specific control measures.

[Based upon material originally published in Woo PTK, Bruno DW, eds, 1999. Fish diseases and disorders, Vol. 3 Viral, bacterial and fungal infections. Wallingford, UK: CABI Publishing.]

Host Animals

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Animal nameContextLife stageSystem
Acipenser baerii baerii (siberian sturgeon)Domesticated hostAquatic: Egg
Acipenser fulvescens (lake sturgeon)
Acipenser oxyrinchus oxyrinchus (Atlantic sturgeon)Aquatic: Egg
Anabas testudineus (climbing perch)
Anguilla anguilla (European eel)Wild hostAquatic: Fry
Anguilla australis australis (shortfin eel)
Anguilla japonica (Japanese eel)Domesticated host, Wild hostAquatic: Adult|Aquatic/FryEnclosed systems/Ponds
Anguilla rostrata (american eel)
Aristichthys nobilis (bighead carp)Aquatic: All StagesEnclosed systems/Aquaria (marine / freshwater ornamentals)|Enclosed systems/Ponds|Enclosed systems/Tanks
Barbonymus gonionotus (java barb)Domesticated hostAquatic: FryEnclosed systems/Ponds
Bidyanus bidyanus (silver perch)Aquatic: AdultEnclosed systems/Ponds
Catla catla (catla)
Catostomus commersonii (white sucker)
Chondrostoma polylepis
Cirrhinus cirrhosus (mrigal)
Colossoma macropomum (tambaqui)
Coregonus lavaretus (common whitefish)
Ctenopharyngodon idella (grass carp)Aquatic: All StagesEnclosed systems/Aquaria (marine / freshwater ornamentals)|Enclosed systems/Ponds|Enclosed systems/Tanks
Cyprinus carpio (common carp)Domesticated host
Esox lucius (pike)Wild hostAquatic: Egg
Hypophthalmichthys molitrix (silver carp)
Ictalurus punctatus (channel catfish)Domesticated host, Wild hostAquatic: Adult
Labeo rohita (rohu)
Lampetra fluviatilis
Lates calcarifer (barramundi)
Leuciscus idus (ide)
Liza abu
Micropterus dolomieu (smallmouth bass)
Morone chrysops (white bass)Aquatic: Adult|Aquatic/Broodstock|Aquatic/Egg|Aquatic/Larval|Aquatic/FryEnclosed systems/Ponds|Enclosed systems/Tanks
Morone chrysops x Morone saxatilisAquatic: All StagesEnclosed systems/Ponds|Enclosed systems/Tanks
Mugil curema (white mullet)
Nematalosa erebi
Oncorhynchus tshawytscha (chinook salmon)Domesticated host, Wild hostAquatic: Adult|Aquatic/Egg
Oreochromis mossambicus (Mozambique tilapia)Domesticated host, Wild hostAquatic: All StagesEnclosed systems/Ponds|Enclosed systems/Raceways / running water ponds|Enclosed systems/Tanks
Oreochromis niloticus (Nile tilapia)Domesticated host, Wild hostAquatic: All StagesEnclosed systems/Ponds|Enclosed systems/Raceways / running water ponds|Enclosed systems/Tanks
Perca flavescens (yellow perch)Domesticated host
Perca fluviatilis (perch)Wild hostAquatic: Adult|Aquatic/Broodstock|Aquatic/Larval|Aquatic/Fry
Polyodon spathula (mississippi paddlefish)
Rutilus rutilus (roach)
Salmo salar (Atlantic salmon)
SalmonidaeDomesticated host, Wild hostAquatic: All StagesEnclosed systems/Ponds|Enclosed systems/Raceways / running water ponds|Enclosed systems/Tanks
Sander vitreus (walleye)
Sciaenops ocellatus (red drum)Aquatic: Broodstock|Aquatic/Egg|Aquatic/Larval|Aquatic/Fry
Tinca tinca (tench)Wild host
Xiphophorus maculatus (southern platyfish)

Hosts/Species Affected

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Descriptions of oomycetes infecting fish date from the mid-eighteenth century (Humphrey, 1893). In 1877, oomycetes were reported in association with an epizootic infection known as ‘salmon disease’ in rivers between England and Scotland. A report of the Royal Commission on the Salmon Disease in England and Scotland (1877-1879), published by Buckland et al. (1880) failed to reach any conclusions regarding the cause of the disease. Although some investigators believed this was due to a single species of fungus, namely Saprolegnia ferax (Stirling, 1878, 1880, 1881; Huxley, 1882a,b), others considered that the fungus was a secondary infection and that the primary cause of the disease was bacterial in nature (Rutherford, 1881; Hume Patterson, 1903). Retrospectively, the ‘salmon disease’ epizootic was clearly the first recognised case of ulcerative dermal necrosis (UDN) of salmonids (Roberts, 1972; Murphy, 1973).

Later, ulcerative dermal necrosis was noted in Atlantic salmon, Salmo salar, in the Republic of Ireland, with epizootics subsequently occurring in Great Britain (Munro, 1970) and Europe (de Kinkelin and Le Turdu, 1971). The condition is unlike other infections involving Saprolegnia, in that lesions occur on unscaled areas of the body, primarily the head. These lesions develop from small oval patches to become ulcerated and haemorrhagic, and only during late stage development become infected with Saprolegnia. Some authors reported that the primary aetiology was viral (Roberts, 1972; O’Brien, 1974), however there is some evidence for the role played by Saprolegnia from studies by Dunne (1970) and Roberts et al. (1971) who found that these early lesions would heal if treated with malachite green.

Species of fish affected and geographical distribution

Infection involving members of the oomycetes are extensive in both wild and farmed fish (Willoughby and Pickering, 1977; Neish and Hughes, 1980) and are considered ubiquitous in freshwater ecosystems (Neish and Hughes, 1980; Waterstrat, 1997). Many researchers reported infections on salmonid species and their eggs and, in the UK, Saprolegnia sp. has been isolated from Atlantic salmon (Willoughby, 1986), rainbow trout, Oncorhynchus mykiss, brown trout and Arctic charr, Salvelinus salvelinus (Pickering and Christie, 1980; Wood and Willoughby, 1986). Hatchery-reared and wild brown trout succumbed to infection by S. diclina at spawning time (Pickering and Christie, 1980), while increased losses of cultured rainbow trout fry have been associated with infection. Bruno and Stamps (1987) reported losses of farmed Atlantic salmon first-feeding fry due to S. diclina. Saprolegniasis is a serious problem in Atlantic salmon hatcheries in Ireland (Smith, 1994) and in Norway (Langvad, 1994), where infections of Saprolegnia sp. occur in wild Atlantic salmon parr. In Japan, epizootics due to S. parasitica have occurred in farmed coho salmon (Hatai and Hoshiai, 1992, 1993). Saprolegnia parasitica and S. diclina have also been implicated in mortality of cultured rainbow trout, coho salmon and ayu, Plecoglossus altivelis, in Japan (Yuasa and Hatai, 1995a). A Saprolegnia epizootic involving eggs and cultured salmon was attributed to S. parasitica, S. salmonis and S. australis in Japan by Hussein et al. (2001). Infection of salmonids with S. parasitica has occurred in Australia (Puckridge et al., 1989) and the USA (Mueller and Whisler, 1994). Furthermore, Chien (1980) discovered S. diclina and Aphanomyces laevis in spawning rainbow trout in Taiwan, and found Pythium debaryanum saprophytic on dead eggs.

Oomycete species reported as infections of live fish (updated from Neish and Hughes, 1980).

Order Saprolegniales, Family Saprolegniaceae

Fungal SpeciesReference
Achlya spp.Tiffney (1939a), Vishniac and Nigrelli (1957), Willoughby (1970), Bhargava et al. (1971), Norlard-Tintigner (1973, 1974), Srivastava (1976), Jha et al. (1977), Pickering and Willoughby (1977), Lartseva and Dudka (1990), Sati (1991)
Achlya ambisexualis RaperVishniac and Nigrelli (1957), Willoughby (1970), Norlard-Tintigner (1973, 1974)
Achlya americana HumphreyScott and O'Bier (1962), Scott and Warren (1964), Ogbonna (1989), Sati (1991)
Achlya apiculata de BaryOgbonna (1989)
Achlya bisexualis Coker and CrouchVishniac and Nigrelli (1957), O'Bier (1960), Scott and O'Bier (1962), Lartseva and Dudka (1990)
Achlya caroliniana CokerSrivastava (1976), Srivastava and Srivastava (1977a), Ogbonna (1989)
Achlya diffusa HarveySrivastava (1976), Ogbonna (1989)
Achlya dubia CokerBhargava et al. (1971), Srivastava (1976), Ogbonna (1989)
Achlya flagellata CokerTiffney and Wolf (1937), Tiffney (1939a), Domashova (1971), Srivastava (1976), Ogbonna (1989), Lartzeva and Dudka (1990), Sati (1991)
Achlya hypogyna Coker and PembertonOgbonna (1989), Lartzeva and Dudka (1990)
Achlya intricata BenekeHoward et al. (1970)
Achlya klebsiana PietersVishniac and Nigrelli (1957), Ogbonna (1989), Sati (1991)
Achlya megasperma HumphreyOgbonna (1989)
Achlya oblongata de BaryOgbonna (1989)
Achlya orion Coker and CouchSrivastava (1976), Ogbonna (1989), Sati (1991)
Achlya prolifera Nees von EsenbeckNorlard-Tintigner (1974), Srivastava (1976), Srivastava and Srivastava (1977c), Ogbonna (1989), Sati (1991)
Achlya proliferoides CokerSrivastava (1976), Ogbonna (1989)
Achlya racemosa HildebrandHoshina et al. (1960), Ogbonna (1989)
Achlya sparrowii ReischerVishniac and Nigrelli (1957)
Aphanomyces spp.Shanor and Saslow (1944), Willoughby (1970), Srivastava (1976), Pickering and Willoughby (1977), Dykstra et al. (1986), Sati (1991)
Aphanomyces laevis de BaryVishniac and Nigrelli (1957), Srivastava (1976), Chien (1980), Ogbonna (1989), Lartseva and Dudka (1990), Sati (1991)
Aphanomyces piscidaMiyazaki and Egusa (1972, 1973), Hatai et al. (1984)
Aphanomyces stellatus de BaryHoshina et al. (1960), Ogbonna (1989)
Calyptralegnia aclyoides (Coker and Couch) CokerVishniac and Nigrelli (1957)
Dictyuchus sp.Tiffney (1939a), Norland-Tintigner (1974)
Dictyuchus anomalus NagaiSrivastava (1976), Srivastava and Srivastava (1977b)
Dictyuchus monosporus LeitgebNorland -Tintigner (1974), Lartseva and Dudka (1990),
Dictyuchus sterile CokerSrivastava (1976), Ogbonna (1989), Sati (1991)
Isoachlya anisospora var.Srivastava (1976)
Isoachlya monilifera de Bary indica Saksena and BhargavaVishniac and Nigrelli (1957)
Isoachlya toruloides Kauffman and CokerOgbonna (1989)
Isoachlya unispora Coker and CouchDomashova (1971), Ogbonna (1989)
Leptolegnia caudata de BaryWilloughby (1970)
Protoachlya paradoxa CokerVishniac and Nigrelli (1957), Sati (1991)
Pythiopsis sp.Pickering and Willoughby (1977)
Saprolegnia spp.Huxley (1882a,b), Johnston (1917), Coker (1923), Tiffney (1939a,b), Chidambaram (1942), Chaudhuri et al. (1947), Aleem et al. (1953), Lennon (1954), Vishniac and Nigrelli (1957), Arasaki et al. (1958), O'Bier (1960), Scott and O'Bier (1962), Scott and Warren (1964), Stuart and Fuller (1968), Bhargava et al. (1971), Norland-Tintigner (1971, 1973), Bootsma (1973), Johnson (1974), Neish (1976, 1977), Srivastava (1976), Hatai et al. (1977), Pickering and Willoughby (1977), Sirikan (1981), Copland and Willoughby (1982), Gajdû¢sek and Rubcov (1985), Glazebrook and Campbell (1987), Oldewage and van As (1987), Okaeme et al. (1988), Pickering and Pottinger (1988), Lartseva and Dudka (1990), Pohl-Branscheid and Holtz (1990), Sati (1991), Bly et al. (1992), Mohanta and Patra (1992), Das and Das (1993)
Saprolegnia australis ElliotHatai et al. (1977), Pickering and Willoughby (1977), Papatheodorou (1981)
Saprolegnia delica CokerVishniac and Nigrelli (1957), O'Bier (1960), Scott and O'Bier (1962), Dudka and Florinskaya (1971), Norland-Tintigner (1973)
Saprolegnia diclina HumphreyMcKay (1967), Willoughby (1968, 1969, 1970, 1971, 1972, 1978), Norland-Tintigner (1970, 1973), Srivastava (1976), Hatai and Egusa (1977), Miyazaki et al. (1977), Chien (1980), Smith et al. (1985), Bruno and Stamps (1987), Ogbonna (1989), Lartseva and Dudka (1990), Sati (1991)
Saprolegnia ferax ThuretTiffney (1939a), Vishniac and Nigrelli (1957), O'Bier (1960), Hoshina et al. (1960), Scott and O'Bier (1962), Norland-Tintigner (1970, 1971, 1973), Bhargava et al. (1971), Srivastava (1976), Srivastava and Srivastava (1977b), Smith et al. (1985), Ogbonna (1989), Lartseva and Dudka (1990), Sati (1991)
Saprolegnia hypogyna PringsheimOgbonna (1989)
Saprolegnia litotalis CokerOgbonna (1989)
Saprolegnia invaderis Davis and LazarDavis and Lazar (1941)
Saprolegnia luxurians Seymour 
Saprolegnia megasperma CokerVishniac and Nigrelli (1957)
Saprolegnia mixta de BaryVishniac and Nigrelli (1957), Dudka and Florinskaya (1971), Lartseva and Dudka (1990), Sati (1991)
Saprolegnia monica PringsheimO'Bier (1960), Scott and O'Bier (1962), Domashova (1971), Sati (1991)
Saprolegnia parasitica Coker emend. KanouseRucker (1944), Hoshina et al. (1960), O'Bier (1960), Scott and O'Bier (1962), Norland-Tintigner (1973), Srivastava (1976), Toor et al. (1983), Ogbonna (1989), Krishna et al. (1990), Lartseva and Dudka (1990), Sati (1991), Hatai and Hoshiai (1992, 1993), Yuasa and Hatai (1995a)
Saprolegnia shikotsuensis Hatai, Egusa, AwakuraHatai et al. (1977)
Saprolegnia subterranea (Dissmann) SeymourPickering and Willoughby (1977)
Thraustotheca clavata (de Bary) HumphreyVishniac and Nigrelli (1957), Ogbonna (1989), Sati (1991)
Thraustotheca primoachlya Coker and CouchVishniac and Nigrelli (1957)

Order Leptomitales, Family Leptomitaceae

Leptomitus lacteus (Roth) AgardhLennon (1954), Scott and O'Bier (1962), Willoughby (1970), Pickering and Willoughby (1977), Ogbonna (1989)

Order Peronosporales, Family Pythiaceae

Pythium sp.Scott and O'Bier (1962), Scott and Warren (1964), Sati (1991)
Pythium ultimumScott and O'Bier (1962)

Order Lagenidiales, Family Lagenidiaceae

Lagenidium rabenhorsti ZopfKahls (1937)

Note. Species names given are those used by the original authors. Several have synonyms and many have subsequently been reclassified. Any references to non-fruiting Saprolegnia isolates have been included under Saprolegnia spp.

There have also been records of oomycete infections on non-salmonid fishes. In the UK, isolations of members of genera including Achlya, Aphanomyces, Leptolegnia, Leptomitus, Pythiopsis and Saprolegnia have been made from perch, Perca fluviatilis (Willoughby, 1970; Pickering and Willoughby, 1977; Bucke et al., 1979). Paxton and Willoughby (2000) reported that fungal spores from adjacent dead eggs did not infect perch eggs, suggesting that these eggs have some anti-fungal properties within the gelatinous mass. In France, mortality was caused by S. australis in cultured roach, Rutilus rutilus (Papatheodorou, 1981). Other coarse fish can be hosts for S. diclina and they include orfe, Leuciscus idus, carp, Cyprinus carpio, tench, Tinca tinca and pike, Esox lucius (Pickering and Willoughby, 1982b). These authors also isolated the fungus from eels and the river lamprey, Lampetra fluviatilis. Elvers of Anguilla anguilla intensively cultured in warm-water effluent suffered high mortality due to Saprolegnia sp. (Copland and Willoughby, 1982). Gajdusek and Rubcov (1985) reported damage to wild carp eggs in the former USSR caused by Saprolegnia sp. and, in the USA, Xu and Rogers (1991) isolated Saprolegnia from channel catfish, Ictalurus punctatus, which resulted in a massive mortality during the winter months (Bly et al., 1992; Bangyeekhun et al., 2001). Saprolegniasis also affects cultured Atlantic sturgeon, Acipenser oxyrinchus, at spawning (Smith et al., 1980) and cultured juvenile barramundi, Lates calcarifer, in Australia (Glazebrook and Campbell, 1987).

In Nigeria, Ogbonna (1989) showed that a wide range of oomycetes, including members of the genus Achlya, Aphanomyces and Saprolegnia, have been found in various freshwater hosts. Also in Nigeria, Okaeme et al. (1988) found the eyes of hatchery-reared tilapia, Oreochromis niloticus, infected with Myxosoma sarigi and with Saprolegnia sp. Fingerlings of another tilapia species, Oreochromis mossambicus, cultured in South Africa, were reported as common hosts for Saprolegnia sp. (Oldewage and van As, 1987) and El-Shrouny and Bodran (1995) recovered Achlya americana, A.dubia, A. flagellata, A. racemosa, S. ferax and S. diclina from this fish. In Brazil, silver mullet, Mugil curema, reacclimatized in fresh water were susceptible to Saprolegnia sp. (Conroy et al., 1986). Another species of mullet, Liza abu, and carp cultured in Iraq were infected with A. polyandra, S. ferax and S. terrestris (Butty et al., 1989).

Saprolegniasis has been noted in cultured teleosts in Indonesia (Sirikan, 1981) and India (Srivastava, 1980; Sati, 1991). Das and Das (1993) described an outbreak known as epizootic ulcerative syndrome (EUS), in which Saprolegnia was implicated with Aeromonas hydrophila as affecting a range of teleosts. High mortality, attributed to S. parasitica, was recorded by Krishna et al. (1990) in various species of carp cultured in ponds and in silver carp, Hypophthalmichthys molitrix, cultured in cages (Jha et al., 1984). Furthermore, infection with this oomycete species was the cause of losses in carp and Indian major carp, Labeo rohita, during the winter when the water had a high organic loading (Toor et al., 1983). Singhal et al. (1990) detected a significant suppression of growth in cultured carp, in which Saprolegnia sp. was associated with an Argulus indicus infection. Air-breathing teleosts, Anabas testudineus, from a river in India, were also found to carry S. parasitica infections (Mohanta and Patra, 1992). Freshwater tropical aquarium species commonly living in waters of 23°C have also shown clinical signs of saprolegniasis, and include Plecostomus spp. (Leibovitz and Pinello, 1980) and the Mexican platyfish, Xiphophorus maculatus (Vishniac and Nigrelli, 1957).

Aphanomyces sp. and Saprolegnia sp. have been associated with an ulcerative mycosis in the USA from a range of estuarine species, primarily the Atlantic menhaden, Brevoortia tyrannus, in the northwestern Atlantic Ocean (Dykstra et al., 1986). Deep skin lesions were characteristic of the infection, which often involved the internal organs and induced an intense inflammatory reaction (Dykstra et al., 1986; Noga et al., 1988). This is in contrast to the superficial lesions and mild inflammatory response normally observed in oomycete infections of freshwater fish (Bly et al., 1992; Álvarez et al., 1995). In many regions of Asia, EUS is a serious disease of estuarine fishes and has been attributed to Aphanomyces invadans (Lilley et al., 1997a,b). This infection appears identical to isolates recovered from fish with red-spot disease in Australia and New Guinea and a mycotic granulomatosis in Japan (Callinan, 1985; Callinan et al., 1995; Lilley and Roberts, 1997). Recent studies have confirmed that the ulcerative mycosis of the Atlantic menhaden is also due to the same pathogen (Blazer et al., 2002), suggesting that many of the oomycete isolates previously identified with this syndrome were opportunists rather than the primary pathogen. Large but shallow ulcers resulted from this type of infection involving mullet, Mugil cephalus (Roberts et al., 1986); although in this latter case the fungus was not cultured, its morphology was characteristic of an oomycete.

Predisposing factors

Fish are continually exposed to potentially pathogenic fungi and it therefore follows that a change in some predisposing factor or factors is necessary for infection to occur. Salmonids are susceptible to saprolegniasis throughout the freshwater stage of their life cycle, particularly leading up to and during smoltification (Pickering, 1994). Although S. parasitica can survive low salinity (Langvad, 1994), it cannot withstand full salinity seawater and therefore the infection is absent from the marine phase in anadromous salmonid hosts. Saprolegniasis also shows a distinct seasonality, and this varies with the species of Saprolegnia. For example, S. diclina infections are more common in winter months (Hughes, 1962), whereas S. ferax occurs predominantly in the spring and autumn (Coker, 1923; Hughes, 1962; Klich and Tiffney, 1985).

Role of sexual maturation

The association of Saprolegnia infection with sexual maturation and a similar increase in susceptibility to some common skin parasites (e.g. Ichthyophthirius and Trichodina) are documented (Pickering and Christie, 1980). Sexual maturation in brown trout is accompanied by dramatic changes in epidermal structure and a decrease in mucus cells at the end of the spawning period (Pickering, 1977; Pickering and Richards, 1980), which is considered to exacerbate their susceptibility (Noga, 1993a). Although precocious mature male Atlantic salmon parr are susceptible to saprolegniasis, they also have an increased number of mucus cells (Murphy, 1981). However, no decrease in mucus cells during the prespawning period occurs, even with marked differences in susceptibility to fungal infections (Pickering and Christie, 1980). The retention of zoospores of S. diclina on the epithelium of rainbow trout was also enhanced in experimentally challenged fish which had previously been implanted with the androgen 11-ketotestosterone (Cross and Willoughby, 1989). Interestingly, the gross crescentic patterns of fungal growth reflected those previously seen only on wild salmonids. Cross and Willoughby (1989) postulated that viable hyphae persisted only at the circumference of the advancing colony and this form of growth created the characteristic pattern.

Richards and Pickering (1978) noted that fungal lesions were common on the dorsum in mature males and on the caudal fin in mature females. The activation of the pituitary-interrenal axis in teleosts is recognised as an almost ubiquitous component of the response to many different factors, most of which are considered stress related (Pickering, 1981). An increase in circulating corticosteroids has been used to assess the importance of the stress response in fish with Saprolegnia infection (Pickering and Duston, 1983). Prolonged oral administration of cortisol or natural increases in this hormone resulted in a marked increase in the susceptibility of the fish to fungal infection. However, their plasma cortisol levels were within the levels capable of being produced by fish under natural stress (Pickering and Pottinger, 1985). Several authors have reported the immunosuppressive role of raised cortisol in salmonids (Pickering and Pottinger, 1985; Bennett and Wolke, 1987). A chronic increase in corticosteroid levels in brown trout from 1-4 ng ml-1 to 9-10 ng ml-1 increased the susceptibility of the fish to Saprolegnia infection (Pickering and Pottinger, 1985; Pottinger and Day, 1999). Observed changes may be due to the fungal infection in salmonids being associated with an increase in cortisol and certain sex hormones and therefore an increase in susceptibility to saprolegniasis (Pickering, 1977). When brown trout parr were treated with the chemosterilant methallibure during the later stages of spermatogenesis, hyperplastic changes in the epidermis were prevented and the prevalence of saprolegniasis was reduced (Pottinger and Pickering, 1985). A similar finding was noted by Murphy (1981) with precocious male 1+ Atlantic salmon parr.

Integument integrity

Increased susceptibility to Saprolegniaceae from damage to the epidermis has been shown in fish under experimental conditions (Tiffney and Wolf, 1937; Tiffney, 1939b; Hoshina and Ookubo, 1956; Vishniac and Nigrelli, 1957; O’Bier, 1960; Scott and Warren, 1964; Srivastava and Srivastava, 1977a,b; Hatai and Hoshiai, 1994). Mechanical damage from high stocking densities of farmed brown trout was considered responsible for an increased incidence of Saprolegnia infection (Richards and Pickering, 1978). The association of sexual maturity with the elevated occurrence of infection may, in part, be attributable to tegument damage sustained during spawning (Richards and Pickering, 1978).

Concurrent infection

Many species of Saprolegnia act as secondary invaders, and prior infection with a primary pathogen renders the host more susceptible to the opportunistic fungus. The condition ulcerative dermal necrosis (UDN) is a classical example where the disease was characterized by secondary Saprolegnia infection following an initial viral infection (Stuart and Fuller, 1968; Willoughby, 1968; O’Brien, 1974).

Primary bacterial infection associated with Saprolegnia sp. has been recorded in the Japanese eel, Anguilla japonica (Hoshina and Ookubo, 1956; Egusa, 1965; Egusa and Nishikawa, 1965). Concurrent infestations with Saprolegnia sp. were observed in wild Atlantic salmon infected with Gyrodactylus salaris (Johnsen, 1978) and Gyrodactylus sp. (Heggberget and Johnsen, 1982), which damaged the skin of the host.

Environmental stress

Environmental stress factors, including poor water quality, adverse water temperatures and, in aquaculture, handling or overcrowding, can all result in increased occurrences of fungal infections (Bailey, 1984). Annual outbreaks of saprolegniasis in wild brown trout were partially the result of an increase in organic debris in the water and a decreased flow rate (White, 1975). High organic loadings were identified as a cause of increased infection by S. parasitica (Toor et al., 1983). Furthermore, rainbow trout exposed to sublethal levels of ammonia and nitrite increased their susceptibility to experimental infection with S. parasitica (Carballo and Muñoz, 1991). Social aggression in rainbow trout can increase susceptibility to this fungus (Cross and Willoughby, 1989).

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Last updated: 10 Jan 2020
Continent/Country/Region Distribution Last Reported Origin First Reported Invasive Reference Notes


NigeriaPresentCABI (Undated);
South AfricaPresentCABI (Undated)Original citation: Oldewage and As (1987)


IndiaPresentSinghal et al. (1990); Khulbe (1992); CABI (Undated);
IndonesiaPresentCABI (Undated)Original citation: Sirikan (1981)
IraqPresentCABI (Undated)Original citation: Butty and et al. (1989)
JapanPresentHussein and Hatai (2002); CABI (Undated);
TaiwanPresentCABI (Undated)Original citation: Chien (1980)


FrancePresentCABI (Undated)Original citation: Papatheodorou (1981)
IrelandPresentCABI (Undated)Original citation: Smith (1994)
NorwayPresentCABI (Undated)Original citation: Langvad (1994)
RussiaPresentCABI (Undated)Original citation: Gajdüsek and Rubcov (1985)
United KingdomPresentCABI (Undated);

North America

United StatesPresentCABI (Undated);


AustraliaPresentCABI (Undated);
Papua New GuineaPresentCABI (Undated)Original citation: Callinan (1985)

South America

BrazilPresentCABI (Undated)Original citation: Conroy and et al. (1986)
ChilePresentZaror et al. (2004)


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Early circular or crescent-shaped skin lesions associated with Saprolegnia infection in salmonids are often characterized by growths of thin, white or grey threads (Willoughby, 1989). Microscopic examination of hyphal growth reveals the characteristic, branched, coenocytic mycelium, with many zoosporangia. The histopathology associated with early, superficial infection in salmonids shows rapid degenerative changes in the epidermis and dermis. More aggressive lesions, with deeper myofibrillar and focal cellular necrosis, spongiosis or intracellular oedema and ultimate sloughing of the epidermis, may follow (Neish, 1977; Pickering and Richards, 1980). Associated inflammatory reactions are absent (Hatai and Hoshiai, 1992). As the fungus radiates from the focus of infection, more of the epidermis is destroyed, and consequently hyphae can penetrate the basement membrane, with growth sometimes continuing into the hypodermis and musculature (Neish, 1977). Thrombi are frequently observed in the blood-vessels as a result of the penetrating hyphae. Primary infectious lesions with many hyphae have been reported in the pyloric region of the stomach of amago salmon, and these hyphae may also invade other abdominal tissues (Miyazaki et al., 1977). Furthermore, the gill lamellae and pharynx have been the sites for primary infection of farmed Atlantic salmon fry (Bruno and Stamps, 1987).

Saprolegniasis has been correlated with extensive haematopoietic pathology in brown trout (Álvarez et al., 1988). In addition to a marked lymphocytopenia (Pickering and Pottinger, 1988), significant impairment of the haematopoietic organs has been reported. Lymphoid cell degeneration, cell depletion, vascular alterations within blood vessels and enlargement and hypertrophy of sinusoidal endothelial cells also occur (Álvarez et al., 1988). Álvarez et al. (1995) recorded histological changes in the thymus and, although the hyphae had not directly invaded this organ, the tissue was oedematous, with epithelial hypertrophy, increased pyknosis and phagocytic activity, involving macrophages and epithelial cells. Considerable changes occurred in the structure of the parenchyma, with large areas showing a marked decrease in cellular density. Most thymocytes were pyknotic, and both epithelial cells and macrophages contained engulfed dead cells. Darkly staining epithelial cells showed cytoplasmic vesicles and clear signs of degeneration, including a vacuolar cytoplasm, with no inflammatory response to the fungal invasion observed.

Copland and Willoughby (1982) reported a loss of integrity of the integument, an oedema of the hypodermis, with degenerative changes in the muscle mass, accompanied by marked myofibrillar degenerative changes in farmed elvers infected with Saprolegnia. Severe infection causes swelling in the intermyotomal connective tissue, which has a fenestrated appearance, due to loss of nuclei. In farmed pike, muscle lesions attributed to fungal infection are uncommon, but hyphae occasionally invade deeper areas, including nervous tissue (Bootsma, 1973). The location of the hyphae suggests that pathogenic Saprolegnia are not generally tissue-specific.

Saprolegnia lesions in channel catfish initially occur at the site of injury, containing a central zone of either necrotic skin, with fungal mycelia throughout the lesion, or, in more severe lesions, a necrotic core of sloughed tissue, which leaves a crater-shaped cavity (Xu and Rogers, 1991). In some lesions, the epidermis was completely sloughed, leaving the dermis exposed (Xu and Rogers, 1991; Bly et al., 1992). Adjacent tissue becomes infected following the spread of hyphae on the skin surface, and mucus cells present in normal skin are absent in the infected skin.

Observations in India, on pond-reared Indian glass barb, Chela laubuca, showed typical colonization and destruction of the epidermis and hypodermis by S. diclina and a profuse hyphal growth, associated with inflammation of the cornea and concavity of the retina (Srivastava et al., 1994). Infection of the cornea in other oomycete outbreaks is uncommon.

Oomycetes other than Saprolegnia have been reported as pathogens of wild and farmed fish; species include Aphanomyces and Achlya (Miyazaki and Egusa, 1973; Fraser et al., 1992; Kitancharoen et al., 1995). Hatai et al. (1994) and Wada et al. (1994) observed multiple granulomas within the internal organs and musculature of the dwarf gourami, Colisa lalia, infected with Aphanomyces. These consisted of mononuclear cells, neutrophils, macrophages and fibrillar structures and were considered to resemble the mycotic granulomatosis recorded in farmed ayu infected with Saprolegnia (Hatai, 1980b). Multinuclear giant cells were absent in the Aphanomyces and Achlya infections.

In the eastern USA, Noga et al. (1988) assigned an ulcerative mycosis in Atlantic menhaden to an oomycete infection. Aphanomyces are more commonly cultured from these lesions, although Saprolegnia is also isolated (Dykstra et al., 1986). The lesions are classified based upon gross and histological features. Advanced lesions consist of open ulcers containing many hyphae, interspersed with necrotic muscle. Hyphae are surrounded by intense inflammation, with many basophilic granulomas possessing necrotic centres. Some lesions are haemorrhaged and infiltrated by lymphocytes and granular cells. Peduzzi and Bizzozero (1977) speculated that the secretion of lytic enzymes ahead of the fungal hyphae may account for tissue damage. Deeply basophilic granulomas and other haemorrhagic lesions, infiltrated with lymphocytes and occasional multinucleated giant cells, are also observed. However, it is considered unusual for granuloma formation to occur in association with oomycete infection. Hatchery mortality among juvenile ayu has been attributed to S. diclina type 1 infection, and diagnosed as a mycotic granulomatosis (Hatai, 1980b; Wada et al., 1993). Histologically, numerous hyphae observed in the stomach penetrate into the pyloric caeca and other visceral organs, resulting in severe necrosis. Multinuclear giant cells are abundant in these lesions. Wada et al. (1993) recognised that primary infectious lesions in farmed ayu were initially established in the pyloric region, with hyphae invading from the mucous membrane into these regions.

Ultrastructural studies

The first electron-microscopy study on the ornamentation of zoospore cysts was carried out by Manton et al. (1951), who recorded curious double-headed hooks on slender stalks, likened to boat-hooks. Meier and Webster (1954) extended these early observations, and demonstrated that the secondary rather than the primary cysts from several S. parasitica isolates bore stalked, double-headed hooks. Shadow-cast preparations, displaying the presence of hooked hairs on secondary zoospore cysts, have been used to differentiate species of Saprolegnia (Pickering et al., 1979; Hallett and Dick, 1986; Hatai et al., 1990). Ornamentation of the cyst surface in Saprolegnia species isolated from salmonid lesions was also established on whole-mount preparations by electron microscopy. Bundles of four and 16 elongate (2.5-14.0 µm), bifurcate, hooked spines or hairs, often called boat-hooks (Meier and Webster, 1957), have been observed (Pickering et al., 1979; Beakes, 1983; Willoughby et al., 1984; Puckridge et al., 1989; Hatai et al., 1990, Beaks et al., 1994b; Mueller and Whisler, 1994). Similar structures are also present in cysts from saprophytic species, including S. diclina (sensu stricto) and S. ferax. However, they are shorter (0.5-1.0 µm) and occur singly or in small bundles of around four spines. The hooked spines in S. hypogyna also occur singly, but are thicker and longer than in other saprophytic species (Pickering and Willoughby, 1982b; Hatai et al., 1990). Variation in the number and arrangement of these hooks and their microarchitecture is reported (Ford and Beakes, 1983; Beakes et al., 1994a), with some isolates demonstrating a high degree of variation in cyst ornamentation (Hatai et al., 1990). However, Beakes et al. (1994a) and Mueller and Whisler (1994) were unable to relate any of these differences in Saprolegnia to variation in morphological type within a geographical area. Interestingly, several workers have reported a direct relationship between the number of hooks in a bundle and their length (Pickering et al., 1979; Puckridge et al., 1989; Hatai et al., 1990; Fregeneda Grandes et al., 2000), and the significance of this is uncertain. An analysis of secondary zoospore cyst formation of Saprolegnia from infected wild brown trout by Fregeneda Grandes et al. (2000) indicated the presence of two distinct morphotypes among long-spined isolates. The isolates with a higher number of bundles per cyst and bundles with a greater number and length of hairs, were included in cyst morphological Group I and those with a smaller number and shorter length of these bundles and hairs were included in cyst morphological Group II. What was interesting was that Group II isolates were all from salmonids with saprolegniosis, whereas most isolates from mucus and water belonged to Group I. So, whilst the presence of bundles of spines over 2 µm in length on the secondary cysts are reliable markers for S. parasitica as a taxon their size and number does not appear to be an indicator of aggressiveness and their role in infection is therefore still unclear. Until genetically modified pathogenic isolates without spines can be generated and tested, then a question mark has to be placed over their role as determinants of pathogenicity.

Willoughby (1985) examined the mode of cyst germination in water alone, along with secondary zoospore cyst ornamentation, using electron microscopy or phase-contrast light microscopy to identify cultures of Saprolegnia from fish. Using these methods, Willoughby (1985) could distinguish between parasitic and saprophytic species of Saprolegnia, based upon the presence of long hooked hairs on the zoospore cyst in the former group.

Electron-microscopy studies of experimentally infected channel catfish revealed the appearance of lesions typically between seven and nine days post-infection (Xu and Rogers, 1991). Hyphae penetrated the epidermis and dermis, with necrosis and sloughing in areas next to the hyphae. Damage to fibroblasts and change in the collagen orientation were noted in both naturally and experimentally infected fish. Some epithelial cells were more electron-dense than normal cells and the nuclear membrane in other cells had ruptured, showing an increase in cytoplasmic vacuoles. Earlier, Álvarez et al. (1988) had hypothesized that cytoplasmic vesicles observed in the endothelial cells were derived from the fungi. Xu and Rogers (1991) also noted the destruction of collagen, with significant debris containing melanosomes between the lamellae.

The invasion of brook charr eggs by S. diclina thalli was shown, using scanning electron microscopy, to occur from a combination of mechanical pressure and extracellular enzyme digestion (Rand and Munden, 1992). Brook trout eggs exposed to zoosporulating hyphae were colonized by encysted spores, spore germlings and young thalli within 15 min postexposure (Rand and Munden, 1993a). Some thalli penetrated the outer layer of the chorion and appeared to spread in a lateral direction just beneath the membrane surface. Between 1 and 24 h post-exposure, infected eggs were covered by a few branching thalli, either spreading over or penetrating the chorionic membrane. By 24 h post-infection, however, a light to moderately heavy mycelial mat covered the egg surface.

Osmoregulatory failure

The primary sequel of uncomplicated saprolegniasis is an osmotic imbalance, due to loss of epithelial integrity and tissue destruction, caused by penetration of the hyphae (Copland and Willoughby, 1982; Noga, 1993b). It is generally accepted that death is due to severe haemodilution, caused by haemorrhage, and the progressive destruction of the epidermis by hyphae (Hatai and Hoshiai, 1994). A significant decrease in serum ions in Saprolegnia-infected mature brown trout has been correlated with infection (expressed as the percentage of body-surface area covered by the fungus), serum osmotic pressure and sodium ion concentration (Richards and Pickering, 1979b; Duran et al., 1987). The pathology observed can be directly attributed to the tissue destroyed in the immediate area of the hyphae (Neish, 1977), and the oedema in infected fish can be assigned to osmoregulatory changes (Richards and Pickering, 1979b; Noga, 1993a). However, Noga et al. (1988) considered that these observations did not fully explain the lesions in deep muscle fibres in menhaden.

Enzyme activity

Peduzzi and Bizzozero (1977) showed that the thalli of certain fish-pathogenic Saprolegnia exhibit chymotrypsin-like activity and postulated that this enzymatic activity contributed to pathogenesis. The work was extended by Rand and Munden (1992), who reported high lipase and alkaline phosphatase activity surrounding the thalli of S. diclina infesting the egg membrane of brook charr. These enzymes may alter the integrity of the chorionic membrane by solubilizing structural polymers, thereby easing penetration of the thalli. Enzyme changes noted in Saprolegnia-infected brown trout include lactate dehydrogenase, glutamate-oxalate transaminase (GOT), glutamate-pyruvate transferase (GPT), creatine phosphokinase, alkaline phosphatase and acid phosphatase, with the presumptive liver enzymes GOT and GPT being elevated, which suggests hepatocellular damage (Duran et al., 1987). Fish dying from Saprolegnia infection suffer a severe haemodilution, associated with elevated serum enzymes (Noga, 1993a; Hatai and Hoshiai, 1994) and a hypoproteinaemia in several fish species, including brown trout (Richards and Pickering, 1979b), Atlantic salmon (Mulcathy, 1969) and coho salmon (Hatai and Hoshiai, 1994). Severe hypoproteinaemia and a significant reduction in the albumin:globulin ratio is reflected in the electrophoretogram of the serum proteins from infected fish (Richards and Pickering, 1979b).


Resistance to infectious diseases may be innate or specifically acquired. In fish, there is increasing evidence that fungal infections are associated with immunosuppression (Álvarez et al., 1988; Hayman et al., 1992), with many outbreaks occurring after a sharp decrease in water temperature, to levels near the physiological minimum for a particular fish species. Fungal growth occurs rapidly, with the formation of characteristic skin lesions, accompanied by significant fish mortalities (Bly et al., 1994; Quiniou et al., 1998) and, at temperatures less than or equal to 12°C, the culture of channel catfish is under threat from outbreaks of Saprolegnia spp. Many farmed catfish in the southern USA die following such infections, known locally as 'winter kill' or 'winter saprolegniasis' (Bly et al., 1992, 1993, 1994). Affected fish show fungal-associated skin lesions and mucus-depleted skin. Experimental studies demonstrated that this condition involved complex reactions between a rapid decrease in water temperature, in vivo immunosuppression, lack of an inflammatory response and the Saprolegnia (Xu and Rogers, 1991; Bly et al., 1992; Álvarez et al., 1995). Work by Quiniou et al. (1998) noted there was a marked decline in mucous cells in catfish subjected to a water temperature drop of 12°C and there was no recovery of these cells when they were also challenged with Saprolegnia. Low temperature-induced mucus loss may explain how fungal cysts attach to and infect catfish skin and result in disease. Hayman et al. (1992) found that channel catfish were also complement-deficient in the winter months. Serum CH50 values were severely depressed, suggesting that immune and non-immune functions generally associated with complement did not function correctly. These authors proposed that the lack of leucocytic infiltration in fungal-infected tissue was related, in part, to the inactivity of the complement system, but it is possible that low water temperature also affected the synthesis of complement proteins. Bly and Clem (1992) considered that low water temperature, elevated corticosteroid and an increase in cytotoxic factors secreted by the fungus may account for immunosuppression. Bly et al. (1994) suggested that catfish and possibly other fish could clear fungal infection via a cell-mediated response. When catfish were held at 22°C and injected intramuscularly with viable Saprolegnia, the hyphae were rapidly destroyed in a classical foreign-body response (Bly et al., 1994). Álvarez et al. (1995) further postulated that the disappearance of the thymic parenchyma in infected brown trout and a combination of some or all of the above factors were related to Saprolegnia infection.

There are a few reports on the relationship between genetic variation and disease resistance of farmed salmonids (reviewed by Chevassus and Dorson, 1990). In a study, Nilsson (1992) reported a positive relationship between mortality of replicated families of Arctic charr with Saprolegnia infection. Heritability estimates for mortality showed that tolerance to fungal infection by the charr could be improved by selective breeding of fish.


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Saprolegniasis is frequently observed as a superficial and chronic infection, with the appearance of cotton-wool-like tufts on the integument and gills of host fish or eggs (Neish and Hughes, 1980; Hussein et al., 2001), which may spread over the entire body surface (Richards and Pickering, 1979a). In severe cases, 80% of the body may be covered. In early infections, skin lesions are grey or white in colour, with a characteristic circular or crescent shape (Willoughby, 1989), which can develop rapidly, causing destruction of the epidermis. As infection proceeds, lethargy and loss of equilibrium follow, making the fish more susceptible to predation.

Pathogenic members of the Saprolegniaceae may penetrate major organs (Bootsma, 1973; Norlard-Tintigner, 1973, 1974; Dukes, 1975; Wolke, 1975; Hatai and Egusa, 1977), and the terms progressive dermatomycosis or mycotic dermatomycosis have been proposed (Hatai, 1980b; Wada et al., 1993). The actual cause of death is likely to be associated with impaired osmoregulation (Gardner, 1974; Hargens and Perez, 1975). Respiratory difficulties may also feature when infection is associated with the gills (Bruno and Stamps, 1987).

Lesions do not normally appear at random, but are initially localized in specific areas associated with physical insult, concurrent infection with another pathogen (Neish and Hughes, 1980) or sexual differences of the host (White, 1975; Richards and Pickering, 1978). The latter seem attributable to differences in the number of goblet cells in the skin of male and female fish (McKay, 1967; Neish, 1977). However, the initial site of infection may not be confined to the skin or gills. Staff's disease in carp involves a saprolegniasis of the olfactory pits (Bauer et al., 1973), infecting parts of the body such as the lateral line and cornea (Leibovitz and Pinello, 1980). Outbreaks of S. ferax involving the gut epithelium have been reported in brook trout, Salvelinus fontinalis (Agersborg, 1933), and rainbow trout fingerlings (Davis and Lazar, 1941), and the alimentary tracts of rainbow trout and amago salmon, Oncorhynchus rhodurus, fry were sites of infection for S. diclina (Hatai and Egusa, 1977; Miyazaki et al., 1977).

Members of the genus Saprolegnia can be considered opportunistic facultative parasites, which are both saprotrophic and necrotrophic (Cooke, 1977). Peduzzi and Bizzozero (1977) suggested isolates of the S. parasitica-S. diclina complex and S. ferax were capable of progressing from saprotrophs to necrotrophs, due to their possessing a proteolytic enzyme, which resembles chymotrypsin. However, there is evidence that some Saprolegniaceae act as primary pathogens (Neish, 1977; Willoughby and Pickering, 1977; Willoughby, 1978; Noga et al., 1988). Many workers have successfully infected fish with Saprolegnia under experimental conditions (Tiffney, 1939a; Vishniac and Nigrelli, 1957; Hoshina et al., 1960; Scott and Warren, 1964; Norlard-Tintigner, 1970, 1971, 1973, 1974; Srivastava and Srivastava, 1977a,b,c), but it is questionable whether these parasites can cause primary infection.


Isolation techniques

One primary difficulty encountered when investigating fungal diseases of fish is isolating the fungus. Many Saprolegnia infections occur in the dermis and therefore more than one species of fungus may easily occur in lesions at the same time (Pickering and Willoughby, 1982a,b). Many saprophytic species may be present and their growth in culture may be rapid, thereby masking the primary species. This is particularly true of older infections, where the number of secondary saprophytic organisms is likely to be higher (Alderman, 1982). Consequently, sampling of dead animals should be avoided wherever possible (Willoughby, 1971). A squash preparation of a skin scrape from the fungal lesion can be a useful preliminary screening method for fungi and other ectoparasites (Pickering and Christie, 1980).

Initial isolation is usually made from small pieces of the affected tissue (approximately 5 mm3). Hatai (1989) advised that deep-seated tissues should be used wherever possible, to limit contamination with bacteria or other fungi. All tissues should receive some form of pre-treatment to remove any contaminant species. Several regimes have been employed to achieve this, including surface sterilization by immersion in ethanol, washing with sterile water, and irradiation (Tiffney, 1939b). Glass rings were included by Raper (1937) to reduce bacterial contamination. Willoughby (1978) described a procedure whereby infected salmonid tissues were cut into small pieces and incubated in sterile lake water for up to 5 days at 7°C, and observed regularly for different forms and a decision made whether to make isolations from zoospores or hyphae. This method, although extremely thorough, is impractical as a routine diagnostic procedure. Instead, viable fungi can be extracted from the tissues, following washing, and cultured using an appropriate artificial medium.

To ensure unifungal growth, isolations should be carried out from single spores or hyphal tips of the initial culture wherever possible (Neish and Hughes, 1980; Alderman, 1982). Furthermore, these isolates should be subjected to infection experiments using the same host species to determine pathogenicity. In practice, however, this is rarely carried out, and, for salmonids, it would appear that small numbers of isolations from affected fish are sufficient to determine the pathogenic role of the fungal isolate (Neish, 1977; Willoughby, 1978). This was supported by Neish and Hughes (1980), who concluded that Saprolegnia lesions in salmonids are frequently unifungal and that this species is an aggressive saprotroph.

Impression smears (Krishna et al., 1990) or squashes (Pickering and Christie, 1980) are used to provide a presumptive diagnosis. These techniques are useful in providing an indication of the organism prior to cultivation and are used for identification to genera. Imprints stained with a lactophenol blue (Krishna et al., 1990) or methylene blue (Bruno and Stamps, 1987) are viewed by light microscopy. Squashes are usually observed using dark-field or phase-contrast microscopy.

Culture methods

Isolation media employed for infectious Oomycetes.

MediaFungal speciesReference
Glucose yeast extract agar (GYA)Various Saprolegnia speciesWilloughby and Pickering (1977)
 S. dicilinia and S. feraxHatai and Egusa (1979)
 S. parasiticaSmith et al. (1985), Hatai and Hoshiai (1992)
 S. diclina, S. parasitica and S. hypogynaHatai et al. (1990)
Glucose peptone agar (GPA)Various Saprolegnia speciesWilloughby and Pickering (1977), Willoughby et al. (1984), Wood and Willoughby (1986)
Sabouraud's dextrose agar (SDA)S. diclinaChien (1981), Bruno and Stamps (1987)
 S. parasiticaKrishna et al. (1990)
Corn-meal agar (CMA)Various Saprolegnia speciesWilloughby and Pickering (1977), Bullis et al. (1990), Bly et al. (1992), Dykstra et al. (1986)
 Aphanomyces sp. 
Peptone glucose yeast-extract agar (PYGEA)Lagenidium callinectesLightner (1988)
 S. diclinaRand and Munden (1993a)
Peptone yeast glucose sea-water agar (PYGS)Lagenidium sp.Fuller et al. (1964), Bian et al. (1979)

A wide range of media have been used for the isolation and culture of fungi from fish and antibiotics have been incorporated to help reduce contamination. However, in some circumstances, the fungi may be too sensitive to permit the use of antibiotics, as with Aphanomyces astaci, the causative agent of crayfish plague (Unestam, 1965; Lilley and Inglis, 1997). Penicillin and streptomycin are the most commonly used antibiotics and are incorporated into appropriate media at 150-1000 iu ml-1 and 250-1000 mg ml-1, respectively. Other antimicrobial agents, such as carbenicillin, chloramphenicol, gentamycin, neomycin, oxolinic acid and potassium tellurite, are used to a lesser extent. Agar plates are inoculated with pieces of infected tissue, using aseptic technique. The plates are incubated and observed at frequent intervals for emerging hyphal tips (Noga and Dykstra, 1986) and then transferred to fresh media to obtain a unifungal culture. Isolations by inoculating mycelium directly from fish or eggs on to an agar medium have been made (Neish, 1975; Willoughby and Pickering, 1977). Alderman (1982) suggested that, whenever bacterial contamination can be kept to a minimum by using antibiotics, flooding agar plates with a thin layer of fresh or sea water, as appropriate, is advantageous.

A broad range of media have been used for the culture of fungi from fish. Often, the media are supplemented with antibiotics and a low nutrient medium is preferred, to reduce growth of saprophytic species and bacteria (Alderman, 1982; Seymour and Fuller, 1987). For marine or estuarine species, media can be supplemented with salt, usually at a concentration up to 3% (Hatai, 1989), or prepared using sterile sea water (Fuller et al., 1964; Bian et al., 1979; Lightner, 1988). Stevens (1974) described a range of media devised specifically for fungal isolation from the marine environment. Incubation temperatures range between 5 and 37°C; however, temperatures of 10, 15 and 20°C are most common.

An alternative method to using agar plates is the more traditional system of 'baiting'. Using techniques described by Johnson (1956), Seymour (1970) and Stevens (1974), infected fish tissues, soil or water samples are incubated with hemp seeds. The resulting fungal colonies are transferred either to fresh hempseed media or to sterile water to obtain unifungal cultures. Currently, most workers appear to use an agar medium. However, Abdul-Karim et al. (1989) used hemp seeds for the initial isolation of S. ferax, S. terrestris and A. polyandra from carp and mullet. Chien (1981) isolated S. diclina and A. laevis from rainbow trout, using the same technique as Ogbonna (1989) in his investigations of saprolegniaceous fungi in freshwater fish in Nigeria. Furthermore, Willoughby (1985) advocated the use of culturing on hemp seeds in water as a satisfactory and rapid method for obtaining germination of Saprolegnia zoospores, essential if the species of an isolate is to be determined. Following culture for 1-3 days, the water was filtered to produce a filtrate containing high numbers of viable zoospores. This application of hemp-seed culture has subsequently been employed by many investigators, including Rand and Munden (1993b), who studied zoospore attachment of S. diclina in brook trout, and by Wood and Willoughby (1986) in their survey of the Saprolegniaceae present in Lake Windermere, England. The presence of zoospores and oogonia and the mode of cyst germination of S. parasitica from coho salmon were also determined following hemp-seed culture (Hatai and Hoshiai, 1992).

Wallpaper paste ('Polycell') has been used as an alternative to traditional agar (Willoughby et al., 1984), particularly for the culture of Saprolegnia from water samples (Willoughby, 1986; Wood and Willoughby, 1986) and fish (Singhal et al., 1987). The paste seems to act as a good physical support for the fungus, allowing separation of the growing colonies. Samples are mixed with nutrients and the paste in a Petri dish and incubated at 20°C for 24-72 h, after which time single colonies may be transferred to sterile water for identification of sporangia, zoospores and oogonia. Improved recovery of oomycetes using hydroxyethyl cellulose (Natrosol 250) as a replacement for wallpaper paste, was reported by Celio and Padgett (1989).

Preservation of cultures requires frequent subculturing (Bullock, 1989) or conservation in mineral oil (Buell and Weston, 1947).


Identification of oomycete fungi has relied largely upon micromorphology and sporulation characteristics, the limitations of which have already been discussed above (Coker, 1923; Sparrow, 1960; Seymour, 1970; Willoughby, 1978). Sexual reproductive stages are usually required to enable accurate identification of a species (Wood and Willoughby, 1986). However, one problem facing the diagnostic mycologist is the lack of sexual structures produced under culture conditions, making accurate identification to species very difficult (Pickering and Willoughby, 1982b). The origin of the antheridial branch is another important diagnostic feature. Antheridial cells may be hypogynous, i.e. borne immediately beneath the oogonium on the oogonial stalk, or androgynous, whereby the antheridial branch originates from the oogonial stalk. A monoclinous antheridium does not come from the oogonial stalk, but originates from the same hypha as the oogonium, whereas a diclinous antheridium arises from a different hypha. In spite of such difficulties, secondary cyst ornamentation, including features such as size, shape and nature of the oogonia surface and wall, has been successfully described in S. parasitica (Pickering et al., 1979; Hallett and Dick, 1986; Puckridge et al., 1989; Hatai et al., 1990; Söderhäll et al., 1991).

Yuasa and Hatai (1994) attempted to identify Achlya, Aphanomyces and Saprolegnia from their biological characteristics. They investigated optimal growth temperature and sensitivity to a range of chemicals and found differences between genera and their sensitivity to malachite green, sorbose and Polyphenon-100. They were also able to differentiate between a saprophytic and a pathogenic strain of Achlya piscicidia.

Initial indications suggest that developing an immunological assay to identify oomycete infections in clinical outbreaks may be feasible (Bullis et al., 1990). Murine polyclonal antibodies to S. parasitica were observed to cross-react with other Saprolegnia species, but with further investigation these authors were confident a more specific assay could be developed. Unfortunately, cross-reaction with all Saprolegnia was observed with the monoclonal antibodies raised against the same species (Burr and Beakes, 1994). These authors also found that the monoclonal antibody cross reacted with other genera within the Saprolegniales, including A. astaci and Achlya sp.

The DNA base composition analysis has been examined as a potential taxonomic tool for infrageneric separation of oomycetes (Green and Dick, 1972) but was quickly superseded by much more sensitive molecular tools (e.g. Molina et al., 1995; Bangyeekhun et al 2001; 2003). Neish and Green (1976) subsequently suggested that Saprolegnia may be a homogeneous taxon and showed that DNA base analyses cannot be employed in the separation of species of this genus. None the less, these authors reported that the guanine-cytosine (GC) ratios of all members of the genus fell within a very narrow range and they were therefore able to exclude species on this basis. In contrast, Chaplitski et al. (1986), using a different technique, obtained lower GC ratios in S. terrestris and S. mixta.

Rapid methods

Preliminary investigations using polyclonal (Bullis et al., 1990) and monoclonal antibodies (Beakes et al., 1994a; Burr and Beakes, 1994, Bullis et al., 1996) raised against various epitopes of Saprolegnia indicate that developing a rapid antibody-based assay for the detection of oomycete infections of fish may be possible. However, no such techniques have been developed to date. The successful development of a PCR technique for the identification of the S. diclina-parasitica complex has been reported (Molina et al., 1995; Diéguez-Uribeondo et al., 1996; Bangyeekhun et al., 2001; 2003). This latter method provides a sensitive and rapid assay for the assessment of genetic distance between different isolates. Bangyeekhun et al. (2003) used the RAPD-PCR technique and the presence of repeated zoospore emergence to characterize Saprolegnia sp. isolates. They reported the majority of the isolates belonged to a single genetically defined group.

A phenitic and cladistic analysis of ITS and LSU rDNA from forty species of the genera Achlya, Aphanomyces, Brevilegnia, Dictyucleus, Keptolegnia, Plectospira, Phythiopsis, Saprolegnia and Thraustotheca was carried out (Leclerc et al., 2000). This group reported that Brevilegniabispora did not belong to the family Saprolegniaceae, and that Plectospira myrianda clustered with Aphamonyces spp. and constitute an ancestral group. The genus Achlya appeared polyphyletic, corroborating more or less the three known subgroups, defined by their sexual spore type (eccentric, centric and subcentric).

A PCR to examine ribosomal DNA from Saprolegnia isolates obtained from many geographical locations was developed by Molina et al. (1995). They discovered that the use of endonuclease BstUI produced identical fingerprints from all strains of S. parasitica examined, and suggested that this could form the basis of a diagnostic test to be applied in the absence of antheridia and oogonia. Diéguez-Uribeondo et al. (1996) have also developed a method employing RAPD, using PCR with DNA from S. diclina-parasitica complex isolates. They showed that Spanish isolates of this fungus had a genetic similarity of 85-100%, compared with only a 20-45% similarity with other strains of the S. diclina- parasitica complex. Whisler (1996) noted that S. parasitica isolates from the Columbia River basin, USA, recognised by RAPD analysis, were representative of the vast majority of the isolates cultured from fungal lesions. The use of single and paired primers with PCR amplification permitted identification of pathogenic groups and their distinction from other species of the genus considered more saprophytic in character (Whisler, 1996).

The expression of spore-specific marker transcripts at different stages of the asexual life cycle of S. parasitica was examined by Andersson and Cerenius (2002). They reported that one of the markers, designated puf1, was found to be expressed transiently upon each of several cycles of zoospore encystment and re-emergence. The transcript is induced immediately upon zoospore encystment and is rapidly lost when a cyst is triggered to germinate. In non-germinating cysts, puf1 is maintained until a time point when the cysts can no longer be triggered to germinate and thus have become determined for zoospore re-emergence. The results show that the cyst stage has two phases of about equal duration (which are physiologically and transcriptionally distinct) and that the transcriptional machinery of oomycetes is also active in non-germinating spores.

List of Symptoms/Signs

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SignLife StagesType
Finfish / Generalised lethargy - Behavioural Signs Aquatic:Adult Sign
Finfish / Loss of balance - Behavioural Signs Aquatic:Adult Sign
Finfish / Mortalities -Miscellaneous Aquatic:Adult Sign
Finfish / Skin erosion - Skin and Fins Aquatic:Adult Sign

Disease Course

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Pathogenesis and immunity

Fungal disease attributed to the genus Saprolegnia results from opportunistic and primary infection (Willoughby and Pickering, 1977; Noga et al., 1988) and is generally assigned to a single major cluster and separate taxon, S. parasitica Coker (syn. S. diclina Humphrey type 1), in salmonids. The secondary zoospore cysts of S. parasitica (S. diclina type 1) and Saprolegnia sp. were shown by Pickering et al. (1979) to bear bundles of long (5-10 mm) hooked spines, whereas S. diclina types 2 and 3, S. ferax and S. australis produced secondary cysts with short <1.0 mm), single, hooked hairs. This, and subsequent studies suggest that the long hairs associated with the former species could facilitate infection by enabling the spores to attach to fish more efficiently by remaining at the water-air interface (Hallett and Dick, 1986). Willoughby and Roberts (1992a) considered that the elongate hairs present on the zoospore cysts of S. parasitica might allow them to remain suspended in the water column for longer periods, thereby increasing the probability of them encountering a fish host. These observations have promoted the suggestion that the length of hooked hairs is related to pathogenicity (Beakes, 1983; Hatai and Hoshiai, 1993).

Manton et al. (1951) and Meier and Webster (1954) considered that the long bifurcate hooks of the secondary cyst of Saprolegnia play a role in attachment to the fish host, and Roberts (1989) proposed that these ornamentations are important in the pathogenicity of saprolegniasis. In support of this, a series of challenge experiments, using brown trout and Arctic charr, showed that S. parasitica remained localized on the fish surface for longer periods than the saprophytic S. diclina. However, the number and size of hooks are not necessarily good indicators of aggressiveness, with the most highly pathogenic isolates typically possessing relatively small bundles of relatively short stiff hairs (Hatai et al., 1990; Fregeneda Grandes et al., 2000). Furthermore, Rand and Munden (1993a) found no evidence that attachment of S. diclina zoospores to brook trout eggs was mediated by the cyst-wall appendages. Instead, they proposed that attachment was facilitated by the release of an adhesive substance, supporting earlier findings regarding the adhesion of this species to non-living surfaces (Willoughby, 1977; Beakes, 1983).


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Fish pathogens, including members of the class oomycetes, are transmitted via several sources, including wild and farmed fish, their eggs, the water supply, transport vehicles, movement of staff between aquaculture facilities, and farm equipment, such as nets. Transmission within the oomycetes occurs directly between fish and/or eggs, with no intermediate host being involved (Singhal et al., 1987). Susceptibility to infection changes with prevailing circumstances, and several key factors are known to affect both the sensitivity of the fish and the growth of the fungus. These include pollution, low water levels, overcrowding, mechanical trauma, including handling, the failure to remove moribund and dead fish or ova, changes in hormonal status and the result of infection by other organisms (Piper et al., 1982; Plumb, 1984). In addition, there may also be some seasonal variation in inoculum potential (Hunter, 1975). Among wild fish, redd digging and spawning contribute to physical damage and therefore the possibility of increased fungal infection (Richards and Pickering, 1978). There are at least three lines of defence against Saprolegnia infection following the challenge of fish with zoospores (Wood et al., 1988). The skin is the first point of contact for infection, and increased secretion of fish mucus following contact with secondary zoospores may serve to reduce the number of parasites on the fish surface. Pickering (1994) concluded that there was sufficient circumstantial evidence linking a decrease in mucification from fish with increased susceptibility to fungal infection. Willoughby (1989) also suggested this process represented an important defence against infection. Secondly, a morphogen from the external mucus could inhibit mycelia growing from spores (Wood et al., 1988), and finally a cellular response was detected in the external mucus. Thus, the mucosal layer acts primarily as a physical barrier to colonization by fungi or other infectious agents. Components in the cyst-coat matrix aid physical entanglement with the fish surface (Beakes et al., 1994a), and, in the presence of a sufficiently high inoculum, surface mucus readily accumulates propagules at 20-60 times the concentration of spores in the surrounding water (Wood et al., 1988). However, Willoughby and Pickering (1977) noted that the number of spores on the skin of brown trout was greatly reduced during the first 24 h following exposure of the fish to Saprolegnia. Mucus removed from the surface of fish triggered encystment of zoospores from pathogenic strains of Saprolegnia, with mycelial growth following rapidly (Pickering and Willoughby, 1982b). As a result of damage to the epithelium and loss of goblet cells, mucus production ceases and it is at this stage that fungal spores are more likely to attach.

Adhesion of Saprolegnia cysts to substrates is believed to involve the production of a non-fibrous conconavalin A-binding material by the fungus (Beakes et al., 1994a). The formation of discrete points of attachment to the egg appear raised, and suggests that the associated thalli are firmly attached to the surface (Rand and Munden, 1993a). Shortly after exposure, encysted spores and young thalli can be recorded on the egg. The development of thalli and their spread and penetration into the egg are considered important factors for the establishment of infection in hatcheries. Within 1-24 h post-exposure, the egg surface becomes covered by branching thalli, spreading over or sometimes invading the chorionic membrane. Dead eggs are particularly susceptible and infection may spread rapidly to viable eggs (Rand and Munden, 1993a). In this context, it is not surprising that chorionic membrane extracts have been identified as factors stimulating positive chemotaxis of zoospores towards live eggs (Rand and Munden, 1993b). A positive chemotactic response towards arginine and alanine was also detected, but no such response was observed to other amino acids or sugars tested. These results indicate that chemotaxis may have an important role in attracting zoospores of S. diclina towards live salmonid eggs. Rand and Munden (1993b) concluded that chemoattractants might be useful in the development of strategies to control or eliminate Saprolegnia among salmonid eggs raised in hatcheries.

Infectious disease outbreaks always pose a risk to farmed stock, despite the quality of husbandry practices employed. The main factor determining whether Saprolegnia infection occurs is considered to be the physiological state of the fish (Pickering and Christie, 1980; Cross and Willoughby, 1989). Sexually mature, stressed or damaged fish are the most susceptible to infection. Fungal outbreaks in hatcheries can result in substantial egg mortality (Smith et al., 1985), as hyphal growth on the chorionic membrane spreads rapidly, especially if dead eggs are not removed.

Trade in aquatic animals, especially the movement of fish to countries where they are not indigenous, represents a risk to fisheries and aquaculture (Lilley et al., 1997b). The use of RAPD-PCR enabled Lilley et al. (1997b) to show that isolates of A. invadens, obtained from fish reared in several different countries, had identical nucleotide sequences and could not be differentiated by restriction fragment length polymorphism. Lilley et al. (1997b) concluded that the lack of genetic diversity might explain many cross-border outbreaks of EUS, ulcerative mycosis or red-spot disease recorded over the last 25 years.

Impact Summary

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Fisheries / aquaculture Negative

Impact: Economic

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Fish mycoses are considered difficult to prevent and treat, particularly in intensive freshwater systems, and are reported to be second only to bacterial disease in economic importance to aquaculture (Meyer, 1991). Water mould epizootics are generally rare in wild fish populations, but during the 1960s, thousands of wild Atlantic salmon died during their migration into rivers in the British Isles and southern Ireland from ulcerative dermal necrosis (UDN) (reviewed by Roberts, 1993). Such outbreaks increased public awareness of the disease in wild salmon stocks and consequently promoted an increase in research. Saprolegnia parasitica has been identified from many fishes and is considered by several investigators as a significant pathogen (Willoughby, 1969). Despite a great deal of work, the cause of UDN was controversial (Stuart and Fuller, 1968) and was not established (Carberry, 1968).

In the eastern USA, up to 80% of the commercial catch of Atlantic menhaden were affected with the disease ulcerative mycosis (synonyms: epizootic ulcerative syndrome (EUS), red-spot disease) (Callinan, 1985; Callinan et al., 1995; Lilley and Roberts, 1997). The ulcerative mycosis was attributed to an Aphanomyces infection, which occurred during the unusually wet spring and summer of 1984, which created conditions of low salinity. These losses, with sporadic outbreaks in earlier years, represented a severe threat to this industry, which, at the time, was worth $27 million per annum (NCDMF, 1983). In economic terms, Aphanomyces, particularly within the Australo-Pacific and southern Asia, is considered the most significant infection affecting cultured fish (Frerichs et al., 1986; Hatai et al., 1994).

Outbreaks of Saprolegnia spp. in farmed fish are usually restricted to chronic but steady losses. However, egg mortality can increase rapidly, causing significant numbers to die, which may have a major economic impact. During severe winters, some farmers in the USA have reported losses of up to 50% in farmed catfish, with an annual economic cost of about $40 million (Bly et al., 1994). Saprolegnia infection is believed to contribute significantly to overall losses in elver, Anguilla anguilla, culture (Copland and Willoughby, 1982). In Japan, significant losses, exceeding 50% per annum, attributed almost exclusively to S. parasitica, have been reported in farmed coho salmon, Oncorhynchus kisutch (Hatai and Hoshiai, 1992, 1993). Significant mortality from fungal outbreaks has also occurred in wild and farmed brown trout, Salmo trutta, in Spain (Aller et al., 1987; Diéguez-Uribeondo et al., 1996).


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