Prevalence, Virulence Gene Distribution and Alarming the Multidrug Resistance of Aeromonas hydrophila Associated with Disease Outbreaks in Freshwater Aquaculture

Abstract

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.