Occurrence of nodular gill disease in farmed brown trout (Salmo trutta L.)

In March 2017, during an investigation on NGD in salmonid farms of the Northern Italy, the first cases of branchitis associated with amoebic infection in farmed brown trout (Salmo trutta L.) was observed. The episodes were detected in two commercial trout farms, where outbreaks of NGD in rainbow trout (Oncorhynchus mykiss) occurred periodically. Clinical examination of the affected brown trout evidenced respiratory distress with dyspnea and abnormal swimming behavior. In both tanks the cumulative mortality was less than 5%. The respiratory symptoms and mortality appeared less serious in brown trout compared to previous episodes of NGD in rainbow trout. From March to May 2017 twenty-five live and moribund brown trout from each tank were collected at monthly intervals and submitted for necropsy, parasitological analysis and histology. The gill tissue appeared pale and swollen at necropsy with whitish nodules in the distal parts of filaments. The histology showed multi-focal epithelial hyperplasia of the gills causing lamellar fusion with presence of limited number of amoebae along the surface of the affected filaments. Further investigations appear necessary to clarify pathogenesis, biological and environmental determinants of NGD in brown trout and to define the involvement of this species as potential reservoir of the disease.