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.

Francisellosis in fish: an emerging challenge

“…in approximately 40% losses, presenting a major impediment to the expansion of cod aquaculture 7 . Initially thought to be a Rickettsia-like organism 5,6,8 or Piscirickettsia-like organism 8,9 , the pathogen was later confirmed as a g-Proteobacteria in the family Francisellaceae, order Thiotrichales 7 . Francisella noatunensis subsp.…”

“…orientalis (Fno) causes francisellosis in tilapia (a fresh and warm water fish species) and F. noatunensis subsp. noatunensis (Fnn) in Atlantic cod 7,8 (a marine and cold water fish species).…”

“…In Atlantic cod, emaciation, haemorrhagic skin and heart nodules also occur while in tilapia, gills can have the nodules in addition to exopthalmia and skin haemorrhages and scale loss 7,8 . Histopathology in affected organs feature granulomas consisting of vacuolated macrophages with the Francisella organisms, associated central necrosis and fibrous encapsulation 5,[7][8][9]12,13 , in the sub-acute (7 days post challenge 5 ) to chronic disease. Acute disease has been experimentally replicated causing 100% mortality by 72 h post intraperitoneal inoculation of approximately 10 7 colony-forming unit (cfu) per fish where bloody ascites, increased melanomacrophage centres but no granulomas were observed 6 .…”

“…Acute disease has been experimentally replicated causing 100% mortality by 72 h post intraperitoneal inoculation of approximately 10 7 colony-forming unit (cfu) per fish where bloody ascites, increased melanomacrophage centres but no granulomas were observed 6 . For tilapia, epizootics typically occur in cooler, winter water temperatures with higher mortalities at 158C than 308C 7 or no mortalities at 26.5-29.28C 8 . Francisellosis causes more mortalities as water temperatures increase towards 208C in summer for Atlantic cod 7 .…”

“…Francisellosis causes more mortalities as water temperatures increase towards 208C in summer for Atlantic cod 7 . Epidemiologically, piscine Francisellae cause disease in both fresh and marine waters and morbidity can be extremely elevated for Atlantic cod and tilapia 8 . Fish pathogenic Francisella can enter a viable but non-culturable state in cold water after 30 days at 88C and 16 days at 128C 8 , meaning that they are nonvirulent.…”