Experimental studies on uptake and interaction of spores of the Saprolegnia diclina-parasitica complex with external mucus of brown trout (Salmo trutta).
The uptake and survival of spores of the pathogen S. parasitica and of the saprophyte S. diclina in the surface mucus of brown trout was investigated. Spores of both the pathogen and saprophyte accumulated to a similar extent in the mucus, although the pathogen attached more rapidly. Spores of both species were rapidly shed from inoculated fish placed in water containing very few background spores, but after 24 h viable propagules of the pathogen were retained on the fish in greater numbers than those of the saprophyte. It is suggested that the natural defence of the fish against Saprolegnia has several components, one of which is the purely physical removal of spores in shed mucus. There was also evidence for the presence of a morphogen in the mucus, which resulted in growth inhibition and abnormal differentiation of colonies after their transfer to assay media. In addition, colonies with granular lysed contents, which had developed over a 24h period in situ in the mucus, often had fish cells attached to them, and it is suggested that a cell-mediated defence mechanism also operates. In water that previously contained fish, secondary zoospores of the pathogen germinated, whilst saprophyte spores simply encysted and did not germinate. The possible significance of this, in the challenge experiments on fish, is discussed.