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Diseases of Aquatic Organisms

    DAO prepress abstract   -  DOI: https://doi.org/10.3354/dao03641

    Vannella mustalahtiana sp. nov. (Amoebozoa, Vannellida) and rainbow trout nodular gill disease (NGD) in Russia

    Alexander Kudryavtsev, Ekaterina Kondakova, Ekaterina Volkova, Aleksey Parshukov*

    *Corresponding author:

    ABSTRACT: An outbreak of nodular gill disease (NGD) in farmed rainbow trout Oncorhynchus mykiss (Walbaum, 1792) was recorded in Ladoga Lake (Karelia, north-western Russia) in Spring 2020. The disease was characterised by typical clinical signs including asphyxiation, distended opercula, loss of activity, and swimming upside down under the water surface. Maximum monthly mortality was 15.2%. The histological examination of the gills showed deformation and clubbing of lamellae, epithelial hypertrophy and hyperplasia, lamellar fusion and fusion of filaments. Granulomas were located within the epithelial layer and/or rose above its surface. Light microscopic in vivo observations of the mucus smears from the affected gills revealed numerous amoeboid protists demonstrating flattened body when adhering to the substratum and blunt, radiating pseudopodia when afloat. Based on these morphological characters these amoebae could be assigned to Discosea (Amoebozoa), and analyses of their small-subunit (SSU) rRNA gene sequences showed that they belonged to the genus Vannella Bovee, 1965. The results herein support the designation of a new species, V. mustalahtiana sp. nov. In spite of being isolated from the gills of the freshwater fish, the species belongs to a clade of Vannella comprising mostly species isolated from marine and brackish water habitats. These findings may be essential for the aetiology and treatment of the disease.