FIRST DESCRIPTIONS OF NODULAR GILL DISEASE IN ITALIAN FARMED RAINBOW TROUT (ONCORHYNCHUS MYKISS)

Amoebic gill disease is a severe parasitic condition affecting salmonid farming in marine and freshwater environment. Several outbreaks of nodular gill disease associated with amoebic infections in freshwater farms of rainbow trout (Oncorhynchus mykiss) have been reported in Denmark, Poland, Germany, USA and Canada. Some cases of nodular gill disease in juvenile and adult rainbow trout have been observed during the last three years in some trout farms in northern and central Italy. High mortality rates occurred throughout the year, with a higher severity and incidence at low temperatures. Clinical signs were indicative of respiratory distress. Gross pathology observations of gills showed excessive mucus production, whitish nodules in the distal regions of filaments and alternated anemic and congested areas. The nodules commonly were composed of several adjacent filaments clubbed and fused together, suggesting diffuse and severe epithelial hyperplasia. The lesions observed were evaluated utilizing a categorical field evaluation of gross gill score. In some samples examined in spring 2014 the microscopic exam of fresh gill showed the presence of roundish elements of about 20 µm in diameter referable to amoebae lined up along the gill surface or piled between the secondary lamellae. The identity of amoebae has not been determined so far and is still under study. Histological examination of gills revealed extensive proliferation of the epithelial and mucous cells, often with complete fusion of the lamellae. Lamellar oedema and leucocyte infiltration in the hyperplastic tissue were found. The filaments in severe cases were totally coalescing, sometimes affecting the entire gill arch. The apical portion of the filament showed marked spongiosis accompanied by cellular sloughing. Amoebae, better evidenced by Giemsa staining, were observed adhering to the gill epithelium surface and within interlamellar spaces. Bacteriological exams of the gills did not support a correlation between presence of bacteria and amoebic infections. Nodular gill disease of trout is an emerging problem which requires particular attention by fish pathologists. Studies are in progress to clarify etiopathogenesis and main environmental and biological determinants of the disease in order to define appropriate measures for its prevention and control.