Abstract
Neoparamoeba sp., including the putative aetiological agent of amoebic gill disease in cultured fish ( N. pemaquidensis ), were incubated in vitro with an Atlantic salmon gill epithelium (RGE‐2) cell line. Proliferation by the amoeba population was dependent upon culture osmolarity; no growth occurred at 330 m m kg −1 but a sixfold increase was observed at 1000 m m kg −1 . At 780 m m kg −1 there was a fourfold increase in the amoeba population but a concurrent decrease in RGE‐2 cell density that was significantly greater than that caused by the high culture osmolarity alone. This apparent cytopathic effect (CPE) developed rapidly and resulted in complete cytolysis of the monolayer in 5 days. CPE occurred in multiple foci and presented as cell vacuolation, rounding and clumping, and the rapid clearance of large areas of the cell monolayer. The possibility that CPE is because of the presence of Neoparamoeba sp. derived cytolytic products is discussed in the context of the pathology of the disease in vivo and the occurrence of secreted cytopathogenic compounds in other amoeba species.