Received: 23-02-2022 / Accepted: 05-04-2022
This study was conducted to identify the pathogen F. columnarefrom diseased grass carp cultured in northern Vietnam and to examine the susceptibility of this bacterium to various common antibiotics used in aquaculture. A total of 103 grass carp samples suspected to be infected with F. columnare were collected and screened from 25 grass carp farms in Ha Noi, Hai Duong and Bac Ninh provinces. The grass carps infected with F. columnareexhibited the symptoms of decayed, white-grey gill filaments, rotten fins, and skin discoloration. From 67 yellow and rhizoid isolates recovered from the collected samples, 25 representative isolates (one per farm) were identified by the phenotypic tests and PCR assays. Grass carps (20-25g) challenged with F. columnareat the dose of 2 ×106 CFU/ml by immersion in 2h resulted in 100% mortality within 48-72 hpi. Theantibiotic test showed that high proportion of F. columnareisolates were susceptible to doxycycline, amoxicillin, florfenicol, and oxytetracycline (92-96%), but resistant to sulfamethoxazole/trimethoprim (44%) and trythromycin (16%). The present study provided necessary information for diagnosing and controlling F. columnarein grass carp.