Since 1981 the coho salmon reared in saltwater have had high mortality of unknown etiology mainly in Autumn and Spring.
The sick fish locate in the corners of the cages and are lethargic and inappetent. Externally they show darkened body coloration and severe pallor of the gills, without other signs. Internally, the kidneys are swollen, particularly in the posterior regions; the liver occasionally has a gray mottling and the spleen is enlarged. The gut is without food and filled with yellow mucus.
The hemathocrit values are approximately 27% in moribund fish and 40-45% in apparently healthy fish. In blood smears it is possible to observe structures the size of bacteria. These are also in smears of kidney, spleen and liver. These structures were found within the cells (macrophages?), free in the smear and also in clusters.
It has not been possible to isolate any pathological agent; however, histological studies have shown several lesions with various grades of severity and the presence of unidentified cells in the kidney as well as in the liver and spleen.
These cells are PAS(+) and contain eosinophilic cytoplasmic granules; some are multinucleated or with amoeboi form. Electron microscopic observation has confirmed the presence of these cells.
In this season (Autumn 89) the coho salmon syndrome has been very severe, causing high mortalities in a large number of fish farms in the south of Chile. The mortality level registered is 30 to 90% in coho salmon only, although some coho salmon are reared with other salmonid species (Rainbow trout and Atlantic salmon) that look healthy and without mortality.