Isolation and Identification of Erysipelothrix sp. in Hybrid Grouper (Epinephelus fuscoguttatus × E. lanceolatus) in Taiwan.

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.