Vaccine development

In teleosts, adaptive immune responses are well developed, and specifically acquired antibodies have been demonstrated towards a variety of organisms following artificial immunization (Leong, 1993). However, there are currently no vaccines available for saprolegniasis or other water-mould diseases of fish. Naturally occurring serum proteins have been demonstrated to react with mycelial extracts and culture filtrates in several fish species, and Seelinger (1960) showed that such reactions...

Introduction

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. 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...

Diagnostic Methods 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...

Chemical treatment methods

In recent years, the search for alternative methods and compounds for the control of oomycete outbreaks has increased and the efficacy of many potential fungicides have been tested, using in vitro screening methods. Bailey (1983a,b) outlined a method for screening aquatic fungicides using agar plugs containing fungal hyphae removed from the edge of actively growing colonies. Plugs were placed on to the depressions of spot plates containing the test compound. This approach enables many compounds...

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 SaproJegnia-infected mature brown trout has been correlated with...

Species of fish affected and geographical distribution

Infections of fish involving members of the Oomycetes are reported extensively 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 salmonids and, in the UK, Saprolegnia sp. has been isolated from Atlantic salmon Willoughby, 1986 , rainbow trout, Oncorhynchus mykiss, brown trout and Arctic char, Salvelinus salvelinus Pickering and...

Bacterial antagonists

Several studies have identified bacterial antagonists to S. parasitica and a variety of Pythium and Rhizophthora species Hatai and Willoughby, 1988 Petersen et al, 1994 . Hatai and Willoughby 1988 reported that the bacterium Pseudomonas fluorescens could strongly inhibit the radial growth of S. parasitica in vitro, and natural control might therefore be feasible. Fungal inhibition attributed to this bacterial antagonist has been suggested to result from the production of an antibiotic by the...

The Microorganism Taxonomic position

The identification and classification of Saprolegnia and other Oomycetes have historically been a problem, and the taxonomic position of this group remains uncertain. This is largely because oomycete taxonomy is essentially based upon morphology of reproductive structures and certain biochemical and physiological characteristics Dick, 1973 Dick et al, 1984 Beakes et al, 1994a . Delimiting genera on the basis of morphological features of reproductive structures is unsatisfactory Hughes, 1994 ,...

Culture methods

A wide range of media have been used for the isolation and culture of fungi from fish. In recent years, these have had antibiotics 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...

Secondary zoospores

Saprolegnia Life Cycle

Primary cysts germinate and release the secondary zoospore Fig. 17.4 , the main motile stage, which remains active for extended periods until encystment Fig. 17.2. Life cycle of Saprolegnia and other water moulds based on Neish and Hughes, 1980 Fig. 17.2. Life cycle of Saprolegnia and other water moulds based on Neish and Hughes, 1980 occurs on a suitable substrate or host. It is assumed that the secondary zoospore is an important dispersive phase of the life cycle and represents the main...

Histopathology

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...

General signs

Saprolegniasis

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 , which may spread over the entire body surface Richards and Pickering, 1979a . In severe cases, 80 of the body may be covered Fig. 17.1 . In early infections, skin lesions are grey or white in colour, with a characteristic circular or crescent shape Willoughby, 1989 , which can develop rapidly,...

Predisposing factors

It is generally assumed that fish are continually exposed to potentially pathogenic fungi. 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 sea water and therefore the...