First isolation and characterisation of Aeromonas dhakensis from hybrid African catfish ( Clarias gariepinus × Heterobranchus longifi

We investigated a case of increased mortality in juvenile hybrid African catfish ( Clarias gariepinus × Heterobranchus longifilis ) reared at an intensive aquaculture facility in Hungary. Affected fingerlings exhibited severe intestinal distension, multifocal petechial haemorrhages, and extensive enteritis. Bacteriological analyses consistently yielded a dominant haemolytic bacterial colony morphotype comprising Gram-negative, rod-shaped bacteria, which were identified as Aeromonas dhakensis by 16S rRNA and rpoD gene sequencing, whereas Aeromonas veronii and Shewanella xiamenensis were isolated only sporadically. Phylogenetic analysis clearly separated the A. dhakensis strains from those of A. hydrophila , supporting their distinction as separate species. Antimicrobial susceptibility testing showed that A. dhakensis was susceptible to several antimicrobials used in aquaculture, and treatment with medicated feed containing neomycin and oxytetracycline was followed by a rapid decline and subsequent cessation of mortality. The predominance of A. dhakensis in the affected intestinal tissues, together with the clinical and pathological findings and the favourable response to antimicrobial treatment, supports its potential involvement in the observed enteritis. However, a direct causal relationship could not be conclusively established. To the best of our knowledge, this is the first report of A. dhakensis associated with hybrid African catfish and the first documented occurrence of this bacterium in Hungary.