Abstract:The visceral white-spots disease is a serious bacterial disease in cage cultured large yellow croaker (Pseudosciaena crocea) in recent years in Zhejiang Province. The principal clinical signs of visceral white-spots disease among infected fish include multi-focal granulomas in the internal organs, especially in spleen and posterior kidney. In present study, the tissues including liver, spleen, kidney, heart, intestine and muscular tissues collected from infected large yellow croaker, P. crocea were examined histologically and ultrastructurally to elucidate the histological features after being infected by the bacterial disease. The collected diseased fish showed clinical signs of numerous gray, tan or white foci present in the spleen, liver or kidney internally without any external lesions. Histopathological analysis revealed severe degeneration and necrotic changes in multiple internal organs, including liver, spleen and kidney. Moreover, fibrosis in the liver tissue, proliferation of lymphocytes, infiltration of neutrophils and sedimentation of hemoferrin in spleen, and atrophy of glomeruli, deformation of renal tubules and amyloidosis in kidney were observed in this study. Furthermore, serious vacuolization, inflammatory cell infiltration and pathological nodules were observed. The results of ultrastructural analysis showed swelling and distortion of visceral cells in liver, spleen, and kidney. Rarefaction and vacuoles of the mitochondrial structure, disruption and disappearance of the mitochondrial cristae and condensation and margination of the chromatin were also observed in the liver, spleen and liver cells of diseased fish. In addition, large numbers of rod-shaped bacterial cells were packed in kidney and spleen. Our results clearly demonstrated the pathological changes in the cell or tissues among the diseased fish. The changes were characterized by serious visceral granulomas, and the release of toxic metabolites and loss of appetite leading to metabolic disorder and serious mortality of the fish. The study provided valuable data for understanding the pathogenesis of this bacterial pathogen in large yellow croaker.
