Francisella orientalis (Fo) is considered to be one of the major pathogens of tilapia because of the high mortalities observed during outbreaks. Other cichlids belonging to the same family (Cichlidae) as tilapia are also quite susceptible to this pathogen. On various occasions, Fo has also been isolated from other warm water fish, including three-line grunt, hybrid striped bass, French grunt, Caesar grunt, and Indo-Pacific reef fish. However, only a few studies have reported the pathogenicity of Francisella orientalis in ornamental cichlid fish. This study fulfills Koch's postulates by showing that a strain of Fo obtained from green Texas cichlid (Herichthys cyanoguttatus) was able to produce the same pathogenicity in healthy fish. A mortality of 100% was observed after healthy green Texas cichlid were experimentally injected with Fo at a dose of 8.95 × 105 CFU/fish. DNA extracted from the organs of predilection (spleen, head kidney) gave positive results by PCR for all fish that died during the experimental period. Spleen and head kidney presented with multifocal white nodules in the affected fish, corresponding to typical vacuolated granulomas on histopathological examination of the tissues. Based on the results of this study, it is evident that Fo can indeed infect green Texas cichlid and produce a disease typical of francisellosis.
Keywords: Cichlidae; Francisella; green Texas cichlid; pathogenicity.