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Francisella noatunensis subsp. orientalis IS AN EMERGING THREAT OF RED TILAPIA (Oreochromis sp.) FARMS IN THAILAND

Vuong V. Nguyen, Ha T. Dong, Saengchan Senapin, Nopadon Pirarat, Channarong Rodkhum*
Department of Veterinary Microbiology
Faculty of Veterinary Science
Chulalongkorn University
Bangkok 10330, Thailand
channarong_r@yahoo.com  or Channarong.R@chula.ac.th

Outbreaks of an emerging disease characterized by multiple white granulomas in internal organs have occurred in cultured red tilapia (Oreochromis sp.) in some provinces in Thailand since 2012. The etiological agent of the disease, however, has not yet been determined. The objective of the present study was to identify the causative agent of this infectious disease by fulfilling Koch's postulates. Diseased fish were collected from two affected farms for bacterial isolation, histological studies, and Francisella-specific PCR diagnosis. All fish samples were positive with Francisella genus-specific PCR. One bacterial strain, designated VMCU-FNO131, was successfully recovered from the kidney of diseased fish. Species identification of this strain was established by amplification and nucleotides sequencing of 16S rRNA gene. BLAST analysis revealed 100% identity to Francisella noatunensis subsp. orientalis strains available in the Genbank database. Subsequently, two groups of apparently healthy red tilapia fingerlings were challenged with 1.08 x 104 CFU mL-1 and 1.08 x 106 CFU mL-1 by intraperitoneal injection. The same accumulative mortality of 86.7 ± 23% was observed within 16 days and 5 days post-injection respectively. Typical clinical signs and histopathological manifestation of granulomas were found in multiple organs of the both naturally and experimentally infected fish. This is the first report of Francisellosis in cultured tilapia in Thailand.

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