Pathogenicity of Saprolegnia spp. to Atlantic salmon, Salmo salar L., eggs

Live and dead Atlantic salmon eyed eggs were challenged with eight different Saprolegnia isolates, selected because of their varied origins, known morphological characteristics and growth/germination pattern. Some isolates were also tested for pathogenicity to Atlantic salmon parr. Challenge of eggs was performed by exposure to spores in suspension or by co-incubation of live eggs with infected dead eggs. The phenotypic characteristics of the isolates were evaluated in relation to their observed pathogenicity from the challenge experiment, to identify possible virulence factors leading to egg-infection by Saprolegnia. The results from the experiments confirm that live eggs are refractory to infection with Saprolegnia spores in suspension and that an infection of live eggs can only occur from an infection nucleus represented by dead eggs or debris. It was observed that strains pathogenic to salmon parr were not particularly infective towards eggs, and the isolates that gave the highest infection rates to eggs were species considered to be saprotrophs.