First descriptions of nodular gill disease in italian farmed rainbow trout (Oncorhynchus mykiss)

FIRST DESCRIPTIONS OF NODULAR GILL DISEASE IN ITALIAN FARMED 
\nRAINBOW TROUT (ONCORHYNCHUS MYKISS) 
\nF. QUAGLIO 1 *, A. PEROLO 1 , P. BRONZATTI 2 , A. GUSTINELLI 3 , V. MENCONI 3 , G. 
\nCAVAZZA 3 , M. CAFFARA 3 , A. MANFRIN 4 , E. GALLO 1 , M.L. FIORAVANTI 3
\n1 University of Padova, Italy 
\n2 Veterinarian fish pathologist 
\n3 Alma Mater Studiorum University of Bologna, Italy 
\n4 Istituto Zooprofilattico Sperimentale delle Venezie, Padova, Italy 
\nAmoebic gill disease is a severe parasitic condition affecting salmonid farming in marine and 
\nfreshwater environment. Several outbreaks of nodular gill disease associated with amoebic 
\ninfections in freshwater farms of rainbow trout (Oncorhynchus mykiss) have been reported in 
\nDenmark, Poland, Germany, USA and Canada. Some cases of nodular gill disease in juvenile 
\nand adult rainbow trout have been observed during the last three years in some trout farms in 
\nnorthern and central Italy. High mortality rates occurred throughout the year, with a higher 
\nseverity and incidence at low temperatures. Clinical signs were indicative of respiratory distress. 
\nGross pathology observations of gills showed excessive mucus production, whitish nodules in 
\nthe distal regions of filaments and alternated anemic and congested areas. The nodules 
\ncommonly were composed of several adjacent filaments clubbed and fused together, suggesting 
\ndiffuse and severe epithelial hyperplasia. The lesions observed were evaluated utilizing a 
\ncategorical field evaluation of gross gill score. In some samples examined in spring 2014 the 
\nmicroscopic exam of fresh gill showed the presence of roundish elements of about 20 μm in 
\ndiameter referable to amoebae lined up along the gill surface or piled between the secondary 
\nlamellae. The identity of amoebae has not been determined so far and is still under study. 
\nHistological examination of gills revealed extensive proliferation of the epithelial and mucous 
\ncells, often with complete fusion of the lamellae. Lamellar oedema and leucocyte infiltration in 
\nthe hyperplastic tissue were found. The filaments in severe cases were totally coalescing, 
\nsometimes affecting the entire gill arch. The apical portion of the filament showed marked 
\nspongiosis accompanied by cellular sloughing. Amoebae, better evidenced by Giemsa staining, 
\nwere observed adhering to the gill epithelium surface and within interlamellar spaces. 
\nBacteriological exams of the gills did not support a correlation between presence of bacteria and 
\namoebic infections. Nodular gill disease of trout is an emerging problem which requires 
\nparticular attention by fish pathologists. Studies are in progress to clarify etiopathogenesis and 
\nmain environmental and biological determinants of the disease in order to define appropriate 
\nmeasures for its prevention and control.

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