Comparative pathological description of nocardiosis in largemouth bass (Micropterus salmoides) and other Perciformes

As a major cultured species of Perciformes, Micropterus salmoides has become an important commercial fish in China. However, Nocardia sp. has struck the M. salmoides production and caused the morbidity of more than 70%. This study described the pathological characteristics of M. salmoides infected with Nocardia sp. and analyzed the pathological characteristics differences since the first infection in M. salmoides and other Perciformes. Some differences of the pathological features in recent years emerged comparing with the previous cases, which manifested as the nodules in ulcerated tail muscle, granulomas in the dermis layer, fin, the muscle of eyes and head kidney. Moreover, this study revealed more detailed histopathological features including the decrease of lymphocytes and renal interstitial hematopoietic cells in the spleen, congested of the glomeruli, and dilated of the renal capsule. Besides, hepatocytes were loosely arranged, and mucosal epithelial cells of intestines were necrotic and disintegrated. Our results showed that there are no visible differences between M. salmoides nocardiosis and other species of Perciformes, but in recent years this disease in M. salmoides showed more serious pathological alterations. These results indicated that the pathogenicity of Nocardia sp. infecting M. salmoides may have improved, and it is urgent and important to build a program to prevent and control the relevant diseases.