The pathological characteristics at different stages and early diagnosis by duplex PCR of Nocardia seriolae disease in largemouth bass (Micropterus salmoides)

Abstract

Nocardia seriolae (N. seriolae) is the primary pathogen identified in Micropterus salmoides, causing nocardiosis, which results in substantial mortality. Although numerous studies have been carried out, there is currently no effective method for the early diagnosis of it to facilitate timely treatment. In this study, we used histological methods to comprehensively analyze the histopathological alterations in M. salmoides infected with N. seriolae at different infection stages to observe the dynamic pathological changes and explore the earliest affected organ. Additionally, we investigated the expression of genes related to inflammation and macrophage immunity in the infected tissues using qRT-PCR. Meanwhile, a duplex PCR method was developed. The results found that the histopathological process could be categorized into three stages: early (latent) stage, middle (reaction) stage, and late (terminal) stage depending on a histopathological scoring system. The genes related to inflammation and macrophage immunity expressed earlier before histopathology, which are precursor factors for granuloma formation. Among all tissues, the head kidney was one of the earliest tissue which was found white nodules with N. seriolae invading. Therefore, the head kidney tissue was selected as the mainly early diagnosis sample for duplex PCR test. Additionally, the sensitivity of the duplex PCR could reach 1.8764 × 10⁻¹ ng/μL. These findings provide a reference for the dynamic process of its pathogenesis, the mechanism of granuloma formation, and established a sensitive duplex PCR method by using the most earliest affected tissue as early diagnosis sample, offering new ideas for the control of N. seriolae.