Paramoeba pemaquidensis (Sarcomastigophora: Paramoebidae) infestation of the gills of coho salmon Oncorhynchus kisutch reared in sea water

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Abstract
Gill disease associated with Paramoeba pemaquidensis Page 1970 (Sarcomastigophora: Paramoebidae) infestations was observed in coho salmon Oncorhynchus kisutch reared in sea water. Fish reared in net pens in Washington and in land-based tanks in California were affected. Approximately 25% mortality was observed in the net pens in 1985, and the disease recurred in 1986 and 1987. Amoeba infesting the gill surfaces elicited prominent epithelial hyperplasia. Typical of Paramoeba spp., the parasite had a Feulgen positive parasome (Nebenkorper) adjacent to the nucleus and floating and transitional forms had digitiform pseudopodia. We have established cultures of the organism from coho gills; it grows rapidly on Malt-yeast extract sea water medium supplemented with Klebsiella bacteria. Ultrastructural characteristics and nuclear, parasome and overall size of the organism in study indicated it is most closely related to the free-living paramoeba P. pemaquidensis. The plasmalemma of the amoeba from coho gills has surface filaments. Measurements (in .mu.m) of the amoeba under various conditions are as follows: transitional forms directly from gills 28 (24 to 30), locomotive forms from liquid culture 21 .times. 17 (15 to 35 .times. 11 to 25), and locomotive forms from agar culture 25 .times. 20 (15 to 38 .times. 15 to 25). Nuclei and parasomes from liquid cultures were 5 to 7 and 3 to 5 .times. 2 to 3, respectively. The disease was not induced by laboratory exposure of coho salmon to amoeba cultures, but the amoeba was consistently isolated from exposed fish and aquarium detritus for 4 wk after exposure. This suggests that the organism is an opportunistic pathogen which only proliferates on fish gills and causes 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, while growth was greatly diminished below 10 ppt. This concurs with field observations where the amoeba was eradicated from fish held in sea water tanks following a reduction in salinity.

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