First Report of Streptococcus iniae Infection in Cultured Spotted Scat (Scatophagus argus) in Thailand.

Streptococcus iniae is a known pathogen in aquaculture, but its role in disease outbreaks in spotted scat (Scatophagus argus) has not been previously reported. This study documents the first outbreaks of S. iniae in spotted scat from two farms in Southern Thailand. Affected fish showed typical streptococcosis symptoms. Bacteria recovered from internal organs were identified as S. iniae by VITEK 2 system, MALDI-TOF MS and 16S rRNA sequencing. Phylogenetic analysis showed high similarity to S. iniae isolates from Nile tilapia and Asian seabass in Thailand and other countries. Multilocus sequence typing classified isolates as ST4 within clonal complex 1, indicating low genetic diversity and broad host range. Experimental infections confirmed pathogenicity, with mortalities peaking 2-7 days post-injection. High-dose groups experienced 100% mortality. The median lethal dose (LD) was 1.83 × 10 CFU/fish for S21 (Phatthalung) and 4.22 × 10 CFU/fish for S36 (Songkhla). Histopathology revealed encephalitis, myocarditis and renal tubular necrosis. All isolates were highly susceptible to tested antimicrobials. These findings highlight S. iniae as a new pathogen in spotted scat aquaculture in Thailand and emphasise the need for integrated management focused on water quality, parasite control and husbandry practices to mitigate streptococcosis risk.