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ABSTRACT. Pure cultures of fungus were isolated from commercially raised channel catfish Ictalurus punctatus exhibiting overt signs of a winter syndrome locally termed 'winter kill' The fungal isolates were identified as members of the genus Saprolegnia. Histopathological examination of fungal associated skin lesions from diseased fish exhibited a complete lack of bacteria or of leukocytic infiltration around the site(s) of hyphal penetration. In order to determine if the fungus was the origin of disease or an opportunistic secondary pathogen, controlled laboratory studies were conducted which conclusively proved that if channel catfish were immunosuppressed by a rapid decrease in environmental water temperature from 22 to 1OeC, the Saprolegnia sp. isolates rapidly infected catfish to cause 92 O/O infection (skin lesions) and 67 % mortality within 21 d post-challenge. Examination of laboratory-infected fish revealed fungal associated skin lesions with histopathology identical to that obtained from fish in the field, i.e. there was a complete lack of bacterial or leukocytic infiltration around the lesion site. These results strongly suggest that 'winter lull' syndrome in catfish is an immunodeficiency disease of fungal rather than bacterial etiology and is probably better termed 'winter saprolegniosis'

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Bacteria capable of inhibiting the growth of Saprolegnia were isolated from commercial channel catfish pond water during January and November 1991. The isolates were typed as Pseudomonas fluorescens with 96% confidence and fluoresced under ultraviolet light. Only three out of the seven otherwise identical subcultures have retained the ability to inhibit Saprolegnia hyphal growth and were positive in gelatin liquefaction assays. Saprolegnia cyst germination inhibition assays indicated that each of the P. fluorescens subcultures inhibited cyst germination at ratios as low as 0.5:1 to 10:1, while Escherichia coli (used as a control) was only inhibitory at a ratio of 500:1. The inhibitory activity against Saprolegnia did not appear to reside in bacterial culture supernatants.

Previous studies have led to the development of a laboratory model for a disease termed "winter saprolegniosis'., locally referred to as "winter kill', occurring in channel catfish, huilurus pitncuiius Rafinesque, raised in commercial ponds in the southeastern United States. In the laboratory, the onset of disease was routinely brought about by a combination of two related faetors: (1) a rapid drop in water temperature whieh induces iminunosuppression in catfish: and (2) maintenance of low water temperatures ( -lOT). whieh favour the proliferation of an ubiquitous fungus of the genus Saprolegnia to produce high levels of fungal zoospores (^5 spores ml '). In this study, two eommereial catfish ponds were monitored for over one year to determine if the above factors occurred in the field and could be correlated with outbreaks of winter saprolegniosis. Il was noted that passages of severe cold weather fronts were able to drop pond water temperatures ^lCC within 24h and that sueh decreases in water temperature were associated with immunosuppression of the catfish in the ponds. Furthermore, when Saprolegnia sp. zoospore levels were ^5 spores mr', the immunocompromised catfish exhibited overt signs of winter saprolegniosis. If one of the faetors implicated in the induetion of disease was missing, fish in the ponds remained healthy, ln addition, the onset of disease in the ponds appeared independent of pH and oxygen, total ammonia nitrogen and un-ionized ammonia levels. Colleetively, these field results eontirm the laboratory based hypothesis coneerning the aetiology of winter saprolegniosis in channel catfish.

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