Francisella spp. infections in farmed and wild fish

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Bacteria within the genus Francisella are non-motile, Gram-negative, strictly aerobic, facultatively intracellular cocco-bacilli. While the genus includes pathogens of warm-blooded animals including humans, and potential bioterror agents, there is also increasing evidence of a number of as yet unrecognised environmental species. Due to their nutritionally fastidious nature, bacteria of the genus Francisella are generally difficult to culture, and growth is also commonly inhibited by the presence of other bacteria within sample material. For these reasons, Francisella-related fish disease may be under-diagnosed. Following the discovery in 2004/2005 that a granulomatous disease in farmed and wild Atlantic cod (Gadus morhua) is caused by a previously undescribed member of this genus (Francisella philomiragia subsp. noatunensis), similar diseases have been identified in fish in at least seven countries around the world. These infections affect both freshwater and marine fish species and involve bacteria more or less closely related to F. philomiragia subsp. philomiragia, an opportunistic human pathogen. Recent work relating to characterisation of the disease/s, classification of fish pathogenic Francisella spp. and vaccine development will be presented. The potential impact of francisellosis in wild and farmed fish on a global perspective will also be discussed.