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

Abstract

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.

Saprolegnia species in Norwegian salmon hatcheries: field survey identifies S. diclina sub-clade IIIB as the dominating taxon

“…The observation that S. diclina sub-clade IIIB isolates also dominated in fected eggs (73%) seems natural considering the high overall prevalence in the hatchery water systems. It is also in concordance with Thoen et al (2011), who demonstrated through a challenge experiment that a presumed saprotrophic isolate was the single most pathogenic isolate to salmon eggs. Even though S. diclina is often believed to be a saprophytic species, our results may suggest that this species is responsible for a considerable portion of the eggs lost in saprolegniosis in Norway.…”

Identification of causative agent for fungal infection and effect of disinfectants on hatching and survival rate of common carp (C. carpio) larvae

“…Padmakumar et al, (1985) found that fungal infected common carp eggs were enveloped with a cloudy cover and did not hatch and succumbed in 12-36 hrs. Thoen et al, (2011) noticed that the live egg contact with dead infected salmon egg had developed foci of white discoloration then death with total whitening. Saprolegnia species were grown on a surface of egg shell which affected it by direct adhesion mechanism.…”

Species composition of the genus Saprolegnia in fin fish aquaculture environments, as determined by nucleotide sequence analysis of the nuclear rDNA ITS regions

“…In addition, it is difficult to determine whether these species are opportunistic pathogens, or saprophytes that successfully colonized dead fish eggs that had died due to other causes. In the study of Thoen et al (2011), the ability of S. parasitica, S. diclina and Saprolegnia spp. isolates to successfully colonize and kill a live egg of Atlantic salmon was dependent on the quantity of inoculum and on the isolate tested.…”