Abstract:
Francisellosis caused by Franciscella orientalis (formerly named: Franciscella noatunensis subspecies orientalis) has been reported as one of the most important emergent diseases of warm water fish. However, little is known about its transmission, fastidious nature, survival, and persistence in multiple environments and global presence. This study aims to investigate vertical transmission, horizontal transmission by vectors and effect of other organisms on the outbreak of francisellosis in hybrid red tilapia. To prove the transmission of F. orientalis from subclinically infected tilapia mouthbrooders to their offspring through the current practice of fry production in tilapia hatcheries, experimentally infected hybrid red tilapia broodstock were mated and their offspring were examined for the presence of F. orientalis. The results showed that the ovary and testis of all 3 pairs of the broodstock, as well as their fertilized eggs, yolk-sac larvae, 5, and 30-day old fry were F. orientalis positive by F. orientalis-specific PCR and in situ DNA hybridization. Upon the vector transmission experiment, mosquito larvae Aedes aegypti was able to acquire F. orientalis in immersion challenge test at a dose of 0.895 x 107 CFU mL-1 and cohabitated with infected fish. Hybrid red tilapia fed by infected pupae at 25oC showed more severe histopathological lesions of typical granulomas resembling for francisellosis infection and positive F. orientalis detection by specific PCR comparing with 30oC. Additionally, co-infection challenge of F. orientalis and Ichthyophthirius multifiliis in hybrid red tilapia successfully mimicked typical signs and histopathological manifestations of both diseases. Synergistic effect of the two pathogens infection in fish leading to the exacerbated mortality. In conclusion, the study provided evidence of vertical transmission of F. orientalis in hybrid red tilapia with the current practice in tilapia hatcheries. Our finding suggested mosquito larvae are available to acquire and transmit F. orientalis to healthy red tilapia and might be an important environment reservoir for the bacterium in nature. The ectoparasite Ich infection can enhance the severity of franciscellosis caused by F. orientalis infection in hybrid red tilapia.
