Concurrent infections of Streptococcus iniae and Aeromonas veronii in farmed Giant snakehead (Channa micropeltes)

Abstract The giant snakehead, Channa micropeltes , is an increasingly important economic freshwater fish in Thailand and other regions of Asia. Presently, giant snakehead are cultured under intensive aquaculture conditions, leading to high stress and conditions favouring disease. In this study, we reported a disease outbreak in farmed giant snakehead with a cumulative mortality of 52.5%, continuing for 2 months. The affected fish exhibited signs of lethargy, anorexia and haemorrhage of the skin and eyes. Further bacterial isolations revealed two different types of colonies on tryptic soy agar: small white, punctate colonies of gram‐positive cocci and cream‐coloured, round and convex colonies of rod‐shaped gram‐negative bacteria. Additional biochemical and species‐specific PCR analysis based on 16S rRNA confirmed the isolates as Streptococcus iniae and Aeromonas veronii . Multilocus sequence analysis (MLSA) placed the S . iniae isolate into a large clade of strains from clinically infected fish worldwide. Gross necropsy findings showed liver congestion, pericarditis and white nodules in the kidney and liver. Histologically, the affected fish showed focal to multifocal granulomas with inflammatory cell infiltration in kidney and liver, enlarged blood vessels with mild congestion within the meninges of the brain and severe necrotizing and suppurative pericarditis with myocardial infarction. Antibiotic susceptibility tests revealed that S . iniae was sensitive to amoxicillin, erythromycin, enrofloxacin, oxytetracycline, doxycycline and resistant to sulfamethoxazole‐trimethoprim, while the A . veronii was susceptible to erythromycin, enrofloxacin, oxytetracycline, doxycycline, sulfamethoxazole‐trimethoprim and resistant to amoxicillin. Conclusively, our findings highlighted the natural concurrent bacterial infections in cultured giant snakehead, which support the implementation of appropriate treatment and control strategies.