First Report of Nocardiosis Caused by Nocardia seriolae in Farmed Spotted Scat (Scatophagus argus).

Nocardiosis is a chronic bacterial disease causing significant losses in marine aquaculture worldwide. This study reports the first confirmed outbreak of nocardiosis caused by Nocardia seriolae in farmed spotted scat (Scatophagus argus) in Vietnam, with cumulative mortalities reaching up to 70%. Diseased fish exhibited skin ulceration and disseminated white nodules in internal organs and musculature. Histopathological examination revealed typical granulomatous lesions containing abundant Gram-positive, branching filamentous bacteria. A bacterium with consistent morphological characteristics was isolated from affected organs, and molecular identification based on 16S rRNA gene sequencing and species-specific PCR confirmed the pathogen as N. seriolae. The experimental challenge reproduced the clinical signs and pathological features of natural infection, with an estimated LD50 of 5.3 × 103 CFU/fish, confirming high pathogenicity in this host. Antimicrobial susceptibility testing showed reduced susceptibility to amoxicillin and universal reduced susceptibility to trimethoprim-sulfamethoxazole, while all isolates remained wild type or susceptible to florfenicol, oxytetracycline, erythromycin, and doxycycline based on previously reported cut-off values. This study extends the known host range of N. seriolae and highlights nocardiosis as an emerging disease threat to spotted scat aquaculture, emphasising the need for early diagnosis and targeted health management strategies.