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Observations on the pathology of saprolegniasis of Pacific salmon and on the identity of the fungi associated with this disease Neish, Gordon Arthur

Abstract

The objectives of this study were to isolate and characterize the fungi associated with saprolegniasis of Pacific salmon and to determine the conditions which allow the infections to become initiated. The fungi isolated from the salmon were either sterile Saprolegnia species, or Saprolegnia species with affinities to the S. diclina-S. parasitica complex. Two morphologically and physiologically distinct strains were recognized in this latter group. Difficulties associated with the identification of these and other Saprolegnia isolates focussed attention on the inadequacy of existing species concepts and also showed that more attention should be directed toward the effect of nutritional and environmental factors on the morphology and growth of these fungi. A study of the DNA base compositions of eleven isolates belonging to the genus Saprolegnia sensu stricto showed that, contrary to expectations based on the literature, this character could not be used to distinguish groups at the infrageneric level, but the results did suggest that isolates included in the genus may have relatively homogeneous genomes. The existence of a satellite DNA was confirmed and was found in all isolates examined. This satellite DNA separates Saprolegnia species from all other Oomycetes which have been similarly examined. An argument is present favouring the rejection of the name Saprolegnia parasitica Coker as a nomen ambiguum and it is shown that all existing oogonium producing isolates included in this species can be legitimately considered to be Saprolegnia diclina Humphrey. Infection experiments, and observations on the histo- and gross pathology of saprolegniasis, when considered in the light of modern concepts concerning the nature of infectious diseases and existing knowledge of the physiology of Pacific salmon, suggest that there is a direct link between increased plasma corticosteroid levels in the fish and their susceptibility to saprolegniasis and other infections caused by normally non-pathogenic organisms. It is hypothesized that natural increases in the plasma corticosteroid levels of the salmon, either alone, or in conjunction with further stress-induced increases, create a situation where natural immunity and the ability to repair tissue damage are greatly impaired. This combination of factors allows an infection to be initiated and, once established, it becomes progressively worse, and ultimately terminal, at a rate which can be directly correlated with increasing temperature.

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