Abstract
Francisella noatunensis subsp. orientalis (Fno) (syn. F. asiatica) is an emergent warm water fish pathogen and the causative agent of francisellosis in tilapia (Oreochromis sp.).1 To study the pathogenesis of this fish bacterium, tilapia fingerlings were experimentally infected by immersion challenge with wild type Fno, and the bacterial distribution in different organs was investigated at different time points by real-time PCR, histopathological and immunohistochemical examination of infected tissues. Moreover, a side-to-side comparison was made with fish challenged with an Fno strain with a mutation in the intracellular growth locus C (iglC) gene.2 The time course of the appearance of bacterial antigens and tissue lesions in various tissues was coincident with the levels of the bacterial DNA loads at the infection sites. Initially (time point 0, or 3 h post-inoculation of bacteria), samples obtained from mucous swabs contained the highest amount of Fno genome equivalents (GE). After 96 h, significant increases of Fno GE were detected in several tissues obtained from fish challenged with the WT strain. This coincided with the appearance of granulomatous inflammatory infiltrate and granulomas in several tissues, especially the head kidney, spleen, and posterior kidney. When compared to the wild type, the mutant iglC was present in similar numbers in the tissues at early time points (< 48 h post infection) and persisted greater than 8 days in several tissues. In contrast to the wild type, it did not replicate in any tissue, and no apparent lesions were confirmed by microscopic analysis.
Acknowledgments
The authors wish to thank Dr. Barrington Brown of the St. Kitts & Nevis Aquaculture Pilot Project & Environmental Research. The authors will also like to thank Ross University School of Veterinary Medicine Research Grants for their support in the project.
References
1. Birkbeck TH, Feist SW, Verner DW. Francisella infections in fish and shellfish. J Fish Dis. 2011;34:173–187.
2. Soto E, Fernandez D, Hawke JP. Attenuation of the fish pathogen Francisella sp. by mutation of the iglC gene. J Aquat Anim Health. 2009;21:140–149.