Nocardia seriolae Infection Causes Granulomatous Pathology, High Mortality and Growth Impairment in Cultured Rice Field Eel ( Monopterus albus )

ABSTRACT During a recent epidemiological survey of rice field eel ( Monopterus albus ) aquaculture farms in Jiangxi Province, China, we observed a characteristic presentation of multifocal granulomatous nodules suggestive of nocardial infection. Bacterial isolation yielded light‐yellow, pigmented colonies with a granular texture and dry, wrinkled surface. Microscopy revealed branching filamentous rods characteristic of Nocardia , while histopathology demonstrated granulomatous inflammation with severe necrosis. Molecular identification, supported by whole‐genome sequencing, confirmed the isolate as Nocardia seriolae , designated strain HZAU2505. Targeted sampling of clinically symptomatic fish across fifteen batches revealed a Nocardia seriolae PCR detection rate of 22.86% (8/35) among diseased animals. Notably, specific PCR surveillance targeting the Nocardia seriolae ITS gene detected Nocardia seriolae DNA in 90.9% (10/11) of frozen feed samples from affected facilities. Disk diffusion screening for aquaculture‐approved drugs indicated that strain HZAU2505 was susceptible to doxycycline, neomycin, florfenicol, intermediate susceptible to ciprofloxacin, and resistant to enrofloxacin and penicillin. Complementary broth microdilution assays (CLSI M24) revealed consistent resistance patterns: isolates were susceptible to aminoglycosides (amikacin), tetracyclines (doxycycline), macrolides (clarithromycin), and oxazolidinones (linezolid), showed intermediate susceptibility to quinolones (ciprofloxacin), but exhibited strong resistance to β ‐lactams (amoxicillin‐clavulanate), sulfonamides (trimethoprim‐sulfamethoxazole), and glycopeptides (vancomycin). Experimental infection with 10 6 CFU/fish of strain HZAU2505 led to 80% cumulative mortality. Infected Monopterus albus displayed perianal lesions, internal white nodules, and clinical signs consistent with natural infections. Bacterial loads reached up to 10 5 copies/mg sample. Infected fish also exhibited significantly reduced growth, with a 17.7% lower weight gain rate after 21 days compared to controls, alongside markedly elevated total triglyceride (TG) concentrations. This study provides evidence of Nocardia seriolae infection in Monopterus albus , which manifests as severe necrotizing lesions, significant growth impairment, and high mortality. These findings highlight a critical risk to eel aquaculture productivity and disease management.