Isolation, identification and histopathological observation of Francisella noatunensis subsp. orientalis from Nile tilapia (Oreochromis niloticus)

• Published in: Aquaculture 2025-01, Vol.595, p.741532, Article 741532
• Main Authors: Xu, Ming, Li, Fulong, Chen, Baipeng, Deng, Yongqiang, Chen, Defang, Geng, Yi, Ouyang, Ping, Huang, Xiaoli
• Format: Article
• Language: English
• Subjects: aquaculture; ceftazidime; China; cytoplasm; disease outbreaks; F. noatunensis subsp. orientalis; farms; Francisella; genes; gentamicin; hemorrhage; hepatitis; Histopathology; imipenem; kidneys; macrophages; minocycline; mortality; nephritis; Nile tilapia; nucleotide sequences; Oreochromis niloticus; pathogens; PCR detection; polymerase chain reaction; sequence analysis; spleen; TEM; transmission electron microscopy; vancomycin; vasculitis
• ISSN: 0044-8486
• DOI: 10.1016/j.aquaculture.2024.741532

In April 2023, a substantial mortality event struck the primary Nile tilapia (Oreochromis niloticus) breeding areas in Maoming City, Guangdong Province, resulting in considerable economic losses. To unravel the etiology of this disease outbreak, thorough farm field necropsies were conducted, encompassing detailed descriptions of gross clinical signs and diagnostic sample collection. Clinically, affected tilapia primarily manifested multifocal superficial skin ulcers, mild focal congestion, and hemorrhage in the jaws, eyes' irises, and other integumentary regions. Tissue specimens from diseased fish were screened for Francisella spp., using a polymerase chain reaction (PCR) assay. The PCR product targeting polymerase gene showed 99.64 % nucleotide sequence identity to the published sequence of Francisella noatunensis subsp. orientalis (F. noatunensis subsp. orientalis). Histopathological examination of selected tissues revealed acute to subacute, severe necrotizing bacterial splenitis accompanied by diffuse vasculitis, marked necrotizing nephritis, mild necrotizing hepatitis, and prominent proliferative vasculitis. Transmission electron microscopy (TEM) further demonstrated that F. noatunensis subsp. orientalis was predominantly found in macrophage cytoplasm, appearing as round, spheroidal bacteria. Koch's postulates were successfully confirmed through challenge testing. Moreover, F. noatunensis subsp. orientalis exhibited high sensitivity to Gentamicin, Vancomycin, Minocycline, Imipenem, and Ceftazidime. In conclusion, F. noatunensis subsp. orientalis has emerged as a significant pathogen affecting Nile tilapia in China, characterized by systemic multi-organ necrotizing vasculitis with the spleen and head kidney being the primary targets. •The isolation and characterization of a new pathogen of Nile tilapia in Guangdong, China.•F. noatunensis subsp. orientalis is the pathogen causing disease and mortality in Nile tilapia.•The spleen and head kidney are the main target organs of F. noatunensis subsp. orientalis infection.•It may be a serious infection disease in this region of China.