Pathological Progress of Two Types of Nodules in Micropterus salmoides Infected with Nocardia seriolae

The typical clinical signs of Nocardia seriolae infection include white nodules, ranging from 0.1 to 10 mm in diameter, distributed across various internal organs. However, the structural composition of nodules of different sizes remains unexplored. In this study, natural cases of largemouth bass (Micropterus salmoides) were collected, and pathogenic bacteria were isolated and confirmed through a re-infection experiment. The isolated bacteria were identified as N. seriolae through 16S rRNA and gyrB gene sequencing. Healthy largemouth bass were infected with the isolate using an immersion infection and observed continuously over 56 days. Samples were successfully obtained from the incubation, prodromal, symptomatic, and convalescent phases, allowing for gross, histological, and ultrastructural observations of nodular lesion progression. Results demonstrated two types of nodular lesions: necrotic foci and granulomas. Macroscopically visible nodules larger than 1 mm, observed primarily in the liver, spleen, kidney, and muscle tissues of moribund fish, exhibited coagulative necrosis and were identified as the principal cause of mortality. Conversely, granulomas, with diameters less than 1 mm, were consistently present in the spleen, kidney, and liver during the symptomatic and convalescent phases.