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 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 well documented (Pickering and Christie, 1980). Sexual maturation in the 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 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 if experimentally challenged fish were previously 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 recognized 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). 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 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 thought to be 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 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 also 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).

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