Detection of Francisella noatunensis subsp. orientalis in farmed tilapia (Oreochromis sp.) in Northern Vietnam

Abstract

The study was conducted in March 2025, when water temperatures ranged from 22 to 24°C. Tilapia and red tilapia cultured in Bac Ninh and Hai Duong (prior to the merger) exhibited clinical signs including popeyes, body hemorrhages, and white spots in the kidney and spleen. These clinical signs, along with the environmental conditions (22-24°C), were consistent with previously reported descriptions of infection caused by Francisella noatunensis subsp. orientalis (Fno). The bacterium was isolated using culture methods on modified Thayer-Martin agar and cysteine heart agar supplemented with bovine haemoglobin. Suspected colonies were further analysed using duplex PCR with two specific primer pairs (F11/R5 and FnoF1/FnoR1), and 16S rRNA gene sequencing was performed and compared to sequences in the NCBI database using the BLAST tool. Results showed that characteristic colonies of Fno emerged after 5-7 days of incubation at 25-26°C. All 08 suspected samples were tested positive for Fno by duplex PCR, displaying two distinct amplicons of 1140 and 203 bp. The 16S rRNA sequence analysis confirmed 100% identity with reference strains of Fno and the challenge experiment confirmed the presence of Fno in kidney tissue and demonstrated its rapid lethality in Nile tilapia at a dose of 10⁷-10⁸ CFU/ml.