NODULAR GILL DISEASE IN RAINBOW TROUT (ONCORHYNCHUS MYKISS): EMERGENCE IN ITALY

Nodular gill disease (NGD) affecting rainbow trout (O. mykiss), firstly described by Daoust and Ferguson (1985) in U.S.A., has been subsequently reported in Denmark and Poland. The present outbreaks have been surveyed in rainbow trout farmed in 6 fish farms in north Italy, during the spring season (T=10°C). Symptomatic fish showed hyperventilation, anoxia, gills hyperplasia and loss of reactivity. Mortality reached 60-70%. Macroscopically the gills showed multi-focal pearly white nodular proliferation of lamellae and increased mucus secretion. Lesions were often bilateral. Fish sizes ranged from 100 to 150 g. Forty-six symptomatic fish were sampled and euthanized for the investigation. Samples of skin/muscle, gills, liver, heart, spleen, kidney, were fixed in Formalin and Bouin; gills samples were fixed also in 2.5% glutaraldehyde for T.E.M.. The histological evaluation highlighted a massive epithelial proliferation that often lead to a complete obliteration of the affected arches. Very frequently, we detected lamellar fusion affecting the entire filament or its distal portion. In the gills epithelium numerous cells, possibly ascribable to amebae, were detectable, showing a large central nucleus containing a thin peripheral layer of chromatin which was separated by a clear halo from the nucleoplasm. They were adherent or included in the epithelium. Mitotic figures were common and cellular debris were present in the external part of the hyperplastic lamellae. T.E.M. analysis revealed few trophozoites. They were irregularly round (8-12 µm), showing a slight electron-dense filamentous coat and an abundant cytoplasm containing numerous mitochondria, evident endoplasmic reticulum, scattered free ribosomes and few clear small vacuoles. Most of the mitochondria had lamellar cristae, only few of them had discoid shape. These findings may suggest to ascribe them to Naegleria spp., as described by Dykova and Kostka (2013). This is the first report concerning NGD outbreaks in rainbow trout farmed in Italy.