Severe gill disease, resulting in significant mortality, has been associated with a Paramoeba sp. (Sarcomastigophora: Paramoebidae) in seawater reared salmonid fishes in Washington and California, USA, and in Tasmania, Australia. The parasite has been associated with massive mortalities in rainbow trout Salmo gairdneri and Atlantic salmon S. salar in Tasmania, and the disease has been observed in coho Salmon Oncorhynchuskisutch reared in seawater tanks in California and net pens in Puget Sound, Washington.

In Washington the disease is most prevalent in the fall, and approximately 25% mortality was observed in coho salmon at one site in the fall of 1985. The organism infests gill surfaces and elicits prominent epithelial hyperplasia. Typical of Paramoeba sp., the parasite has a Feulgen positive Nebenkorper (parasome) adjacent to the nucleus and floating forms have digitiform pseudopodia. We have established cultures of the organism from coho gills, and it grows rapidly on Malt-Yeast Extract Seawater medium supplemented with Klebsiella bacteria.

Two Paramoeba sp. have been described as parasites of aquatic invertebrates; P. perniciosa in blue crabs and P. invadens in sea urchins. However, ultrastructural characteristics and nuclear, parasome and overall size of the organism in study indicate it is most closely related to the free-living paramoeba, P. pemaquidensis. The plasmalemma of the amoeba from coho gills has surface hairs, and the overall diameter, nuclear and parasome diameters are 11-30 Am, 4.8-5.8 Am, and 3.2-5.6 X 2.2-3.2 Am respectively.

Studies in Australia indicate that baths with formalin, copper sulfate or malachite green are not efficacious, but a decrease in salinity appears to quickly eradicate the parasite. Transmission studies with the cultured organism are underway in our laboratory and these results will be presented.