| Summary: | The study aims to evaluate the infection prevalence, virulence gene distribution and antimicrobial resistance of <i>Aeromonas hydrophila</i> associated in diseased outbreaks of cultured freshwater fish in Northern Vietnam. The confirmed <i>A. hydrophila</i> were screened for the presence of the five pitutative-virulence genes including aerolysin (<i>aerA</i>), hemolysin (<i>hlyA</i>), cytotonic enterotoxin (<i>act</i>), heat-labile cytotonic enterotoxin (<i>alt</i>), and heat-stable enterotoxin (<i>ast</i>), and examined the susceptibility to 16 antibiotics. A total of 236 <i>A. hydrophila</i> isolates were recovered and confirmed from 506 diseased fish by phenotypic tests, PCR assays, and <i>gyrB</i>, <i>rpoB</i> sequenced analyses, corresponding to the infection prevalence at 46.4%. A total of 88.9% of <i>A. hydrophila</i> isolates harbored at least one of the tested virulence genes. The genes <i>aerA</i> and <i>act</i> were most frequently found (80.5% and 80.1%, respectively) while the <i>ast</i> gene was absent in all isolates. The resistance to oxacillin, amoxicillin and vancomycin exhibited the highest frequencies (>70%), followed by erythromycin, oxytetracycline, florfenicol, and sulfamethoxazole/trimethoprim (9.3–47.2%). The multiple antibiotic resistance (MAR) index ranged between 0.13–0.88 with 74.7% of the isolates having MAR values higher than 0.2. The results present a warning for aquaculture farmers and managers in preventing the spread of <i>A. hydrophila</i> and minimizing antibiotic resistance of this pathogen in fish farming systems.
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