Scanning Electron Microscopy and Pathogenicity ofChilodonella piscicola(Ciliophora) on Juvenile Salmonids

Abstract

Abstract An ectoparasitic ciliate, Chilodonella piscicola (=C. cyprini), became epizootic and caused chronic mortalities in juvenile masu (cherry) salmon Oncorhynchus masou and pink salmon O. gorbuscha being reared at the Nemuro Hatchery, eastern Hokkaido, Japan, in the spring of 1988. Cumulative mortalities amounted to 20% in a masu salmon population over 5 months and 10% in a pink salmon population over 2 months. Transmission experiments in masu salmon fry confirmed that C. piscicola can cause severe proliferation of the gill epithelium in the absence of other stressors. The infected fish exhibited reduced growth and chronic mortalities after the mean intensity had reached a peak (619 parasites per fish) at week 4 postinfection. The marked epithelial hyperplasia was followed by intense fusion of adjacent gill lamellae and filaments. Respiratory failure due to gill epithelial hyperplasia may have been a primary cause of the deaths.