Francisella infections in farmed and wild aquatic organisms

Abstract

Over the last 10 years or so, infections caused by bacteria belonging to a particular branch of the genus Francisella have become increasingly recognised in farmed fish and molluscs worldwide. While the increasing incidence of diagnoses may in part be due to the development and widespread availability of molecular detection techniques, the domestication of new organisms has undoubtedly instigated emergence of clinical disease in some species. Francisellosis in fish develops in a similar fashion independent of host species and is commonly characterised by the presence of multi-organ granuloma and high morbidity, with varying associated mortality levels. A number of fish species are affected including Atlantic cod, Gadus morhua; tilapia, Oreochromis sp.; Atlantic salmon, Salmo salar; hybrid striped bass, Morone chrysops × M. saxatilis and three-lined grunt, Parapristipoma trilinineatum. The disease is highly infectious and often prevalent in affected stocks. Most, if not all strains isolated from teleost fish belong to either F. noatunensis subsp. orientalis in warm water fish species or Francisella noatunensis subsp. noatunensis in coldwater fish species. The disease is quite readily diagnosed following histological examination and identification of the aetiological bacterium by culture on cysteine rich media or PCR. The available evidence may indicate a degree of host specificity for the various Francisella strains, although this area requires further study. No effective vaccine is currently available. Investigation of the virulence mechanisms and host response shows similarity to those known from Francisella tularensis infection in mammals. However, no evidence exists for zoonotic potential amongst the fish pathogenic Francisella.

Figure 1

Phylogenetic tree showing the two major lineages of Francisella inferred from the 16S rDNA sequences of reference strains and environmental samples. Taken from Barns et al. [11].

Figure 2

Phylogenetic relationship of the Francisella sp. bacterium isolated from a diseased giant abalone Haliotis gigantea, inferred from the 16S rDNA sequences. Taken from Kamaishi et al. [3].

Figure 3

South Carolina tilapia with the gross signs of the severe, chronic stage of the PLO disease. Granulomas in the gills A, and in the spleen B. Taken from Mauel et al. [4].

Figure 4

Inhibition of Francisella noatunensis subsp. noatunensis by contaminating bacteria in spleen homogenate; a) 10^-1 dilution; b) 10^-2 dilution; c) 10^-3 dilution thin arrow = contaminants thick arrow = F. noatunensis bacteria.