Winter saprolegniosis in channel catfish

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'