Abstract
In February 2025, a mortality event of unknown aetiology occurred at a hybrid grouper ( Epinephelus fuscoguttatus × Epinephelus lanceolatus ) aquaculture farm in southern Taiwan, with sporadic daily deaths persisting for a month. Four moribund groupers were collected over three sampling intervals for diagnostic investigation. Gross pathology revealed opaque white discoloration with multifocal petechial haemorrhages in the lateral musculature. Gram‐stained muscular viscous fluid smears showed abundant Gram‐positive, filamentous bacilli (2–100 μm in length). Histopathology of the skeletal muscle confirmed extensive Zenker's necrosis, with dense bacterial aggregates infiltrating necrotic myofibres. On blood agar at 28°C, smooth, greyish, α‐haemolytic colonies developed within 16–24 h; growth was restricted at 32°C and absent at 35°C. Full‐length 16S rRNA gene sequencing identified the isolates as Erysipelothrix . High‐resolution gyrB ‐based phylogenetic analysis clustered the isolates nearly identically with the dolphin‐derived E. rhusiopathiae strain KC‐Sb‐R1 (CP033601), distinct from swine‐derived E. rhusiopathiae (97.0%), E. piscisicarius (91.2%) and E. tonsillarum (83.9%). The isolates belonged to spa type B. To our knowledge, this is the first documented case of an E. rhusiopathiae ‐like infection causing myositis and intramuscular abscesses in grouper aquaculture, highlighting a potentially marine‐associated lineage with significant implications for aquatic veterinary medicine and public health.