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.
Highlights
Tularemia i.e. F. tularensis is known to have a very small least infectious dose of 10 bacterial cells or less [61]. This trait appears to be shared with fish and mollusc pathogenic Francisella, as few as 1 - 23 cfu F. noatunensis subsp. orientalis injected intraperitoneally were capable of causing disease in tilapia while 32 cfu of the abalone pathogenic Francisella sp. described by Kamaishi et al [8] killed 100% of intramuscularly injected abolone within 16 days of infection
Further the fish pathogenic Francisella may be readily distinguished from F. philomiragia by their requirement for cysteine in culture media and their inability to grow at temperatures of 35°C or above and lack of production of cytochrome oxidase [13]
Concluding remarks Despite previous recognition of the disease, the aetiological agents of francisellosis were not identified until recently. As these bacteria are not always readily observed histologically and cannot be cultured in the laboratory media used in routine fish disease investigations, it is likely that diseases caused by this group of bacteria remain under-diagnosed
Summary
The diversity of as yet undescribed Francisella.
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