First report on Flavobacterium columnare causing disease outbreaks in cultured black carp, Mylopharyngodon piceus, in Northern Vietnam.

Columnaris disease has recently emerged as a concern for black carp (Mylopharyngodon piceus), an economically important aquaculture species in northern Vietnam. This study investigated the disease outbreaks from five affected farms. A total of 47 naturally diseased fish were sampled for bacterial isolation and identification. Five representative isolates resembling Flavobacterium were selected for phenotypic and genetic characterization. Through a combination of biochemical tests, PCR assays, and 16 S rRNA sequencing, these bacterial isolates were identified as Flavobacterium columnare. Experimental challenge tests on black carp revealed a median lethal dose (LD50) of F. columnare at 1.28 × 106 CFU/ml via immersion. Clinical signs and histopathological features observed in experimentally infected fish closely resembled those in naturally diseased fish. F. columnare isolates exhibited reduced susceptibility to several common antibiotic uses in Vietnam. This study represents the first documentation of F. columnare infection in black carp culture, enhancing our understanding of columnaris disease in fish and underscoring the need for preventive measures tailored to this cultured fish species.