Abstract
Gill disease associated with P. pemaquidensis infections was observed in O. kisutch reared in sea water in net pens in Washington and land-based tanks in California, USA. Approximately 25% mortality due to the parasite was observed in 1985 of fish kept in net opens. Infected gills exhibited predominent epithelial hyperplasia. The parasite had a Feulgen positive parasome (Nebenkörper) adjacent to the nucleus, and floating and transitional forms had digitiform pseudopodia. Cultures of the organism from O. kisutch gills were established on malt-yeast extract sea water medium supplemented with Klebsiella. Ultrastructural characteristics and nuclear, parasome and overall size of the organism indicated it is most closely related to the free-living paramoeba P. pemaquidensis. The plasmalemma of the amoeba has surface filaments. Measurements (in µm) of the amoeba under various conditions were: transitional forms directly from gills 28 (24 to 30), locomotive forms from liquid culture 21 × 17 (15 to 35 × 11 to 25), and locomotive forms from agar culture 25 × 20 (15 to 38 × 15 to 25). Nuclei and parasomes from liquid cultures were 5 to 7 and 3 to 5 × 2 to 3 µm respectively. The disease could not be induced by laboratory exposure of salmon to amoeba cultures, but the amoeba was consistently isolated from exposed fish and aquarium detritus for 4 weeks after exposure. This suggests that the organism is an opportunistic pathogen which only causes fish gill disease under certain situations yet to be clearly defined. In vitro studies indicated that the optimal salinity for growth of the amoeba was between 15 and 20 ppt, and that amoebae were eradicated from fish held in sea water tanks following a reduction in salinity.