Abstract:
Since 2013, several 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. The etiological agent of this disease in Thailand, 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. Ten diseased red tilapia were collected from two affected farms for bacterial isolation, histological studies, and polymerase chain reaction (PCR) diagnosis for the suspected pathogen, Francisella sp.. All fish samples were positive for the 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 nucleotide sequencing of the 16S rRNA gene. BLAST analysis revealed 100 % identity to Francisella noatunensis subsp. orientalis strains available in the GenBank database. Subsequently, two groups of healthy red tilapia fingerlings were challenged with 1.08 x 103 CFU per fish and 1.08 x 105 CFU per fish by intraperitoneal injection. The accumulative mortality was 86.7 ± 23 % at 5 days post-injection (dpi). On the contrary, fish injected with 1.08 x 103 CFU per fish had died much slower, but the same accumulative mortality was observed at 16 dpi. Clinical signs and histopathological manifestations of typical granulomas were found in multiple organs of both the naturally and experimentally infected fish. Moreover, lethal dose 50 % (LD50) of VMCU-FNO131 in red tilapia was 2.88 x 105 CFU per fish at days 12 via intraperitoneal injection. In contrast, LD50 value could not calculate in immersion trial, and the dose 7 x 106 CFU mL-1 has just caused 25 % cumulative mortality within 21 days.
