PROLIFERATIVE KIDNEY DISEASE (PKD) IN PACIFIC SALMON AND STEELHEAD TROUT

Proliferative kidney disease (PKD) was detected for the first time in Pacific salmon at a California State Hatchery in June 1983. Both chinook ( Oncorhynchus tschawytscha ) and coho ( O. kisutch ) salmon and steelhead trout ( Salm gairdneri ) held at the hatchery contracted the disease. The cumulative mortality attributed to the disease was 95, 13 and 18% respectively. The mortality rate was greatest during the month of July when the water temperatures were 58–60°F (14–16°C). The protozoan causing PKD was detected in the kidney, spleen, liver, gills and pyloric‐ceca of affected fish. Although lesions typical of Renibacte‐rium salmoninarum infection were found in the kidney of one chinook salmon, all other fish examined were free of complicating bacterial or viral diseases. The ultrastructural features of the protozoan observed in the three species of affected fish were consistent with those described for the parasite in rainbow trout ( Salmo gairdneri ) in European outbreaks.