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

Abstract

The study was conducted in March 2025, when aquaculture water temperatures commonly ranged from 22 to 24°C. Tilapia and red tilapia cultured in Bac Ninh and Hai Duong provinces (prior to the administrative merger) exhibited clinical signs including popeye, 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 disease caused by Francisella noatunensis subsp. orientalis (Fno). The bacterium was isolated by culture on modified Thayer-Martin agar and cysteine heart agar supplemented with bovine blood. Suspected colonies were further analysed using duplex PCR with two specific primer pairs (F11/R5 and FnoF1/FnoR1), and 16S rRNA gene sequences were compared with sequences in the National Centre for Biotechnology Information (NCBI) database using the BLAST tool. The results showed that characteristic colonies of Fno appeared after 5-7 days of incubation at 25-26°C. All eight suspected samples tested positive for Fno by duplex PCR, yielding two amplification products of 1140 and 203 bp. The 16S rRNA sequence analysis confirmed 100% identity with reference strains of Fno, while the challenge experiment confirmed the presence of Fno in kidney tissue and demonstrated its rapid lethality in tilapia at doses of 10⁷-10⁸ CFU/ml.