Nodular gill disease in farmed rainbow trout (Oncorhynchus mykiss) in Italy

Nodular gill disease (NGD) is a serious parasitic condition associated with amoebic infection affecting farmed rainbow trout (Oncorhynchus mykiss) in several countries worldwide. Outbreaks of NGD have been recently observed in several rainbow trout farms in Italy, with clinical signs strongly indicative of respiratory distress and high mortality registered mainly during winter. From December 2013 to April 2015 a total of 494 rainbow trout were collected from 13 Italian farms and subjected to anatomo-pathological and histological exams. Gross gill lesions consisted in excessive mucus production, swelling, whitish nodules in the distal parts of filaments and presence of alternate discolored and congested areas. In 261 (52.8%) fish histology allowed to identify the presence of amoebae as showed also by the parasitological exam of fresh gill samples. Histological lesions, in presence of amoebae, consisted in strong hyperplasia of the gill epithelial cells producing extensive lamellar fusion with obliteration of interlamellar spaces. In more severe cases mucous cell hypertrophy, lamellar edema, necrosis and exfoliation of epithelial cells were observed. Among the epithelial cells, inflammatory infiltrate with numerous lymphocytes, macrophages and eosinophilic granule cells/mast cells was detected. Further analyses are in progress in order to identify the species of amoebae involved as causative agent of NGD in Italy.