Study on isolation and some pathological characteristics of Flavobacterium columnare causing infection in tilapia (Oreochromis spp.) cultured in Northern Vietnam

Abstract

The study aims to isolate, identify, and determine the pathological characteristics of Flavobacteria columnarecausing disease in tilapia cultured in Northern Vietnam. A total of 138 samples of tilapia suspected of being infected with F. columnare were collected from 34 disease outbreaks at 21 tilapia farms in Hoa Binh, Ha Nam, and Hai Duong provinces. The results showed that infected tilapia exhibited typical symptoms such as fibrous, silver gills, appearance of silvery white spots on the skin, fibrous fins. The outbreaks can occurr in tilapia farms anytime, but focusing and much damage in the winter-spring crop with accumulative mortality rates from 5-35%. A total of 87 isolates were retrieved from the fish samples; 34 representative isolates (1 isolate per outbreak) were successfully identified by phenotypic, biochemical tests, and PCR assays. Using TYES medium, the F. columnare isolates grew in yellow and rhizoid colonies after 36-48 h of culture. The bacteria were gram-negative, long filamentous, absorbed Congo red dye, produced flexirubin pigment, gliding motility, and were positive to catalase and oxidase tests. F. columnare grew well at a temperature of 25-28°C, pH 7-8 and NaCl concentration below 5‰. The present results provide critical information in developing strategies for the prevention and control of F. columnare infection in tilapia.