Francisellosis in fish: an emerging challenge

Abstract

Francisellosis is a bacterial disease with increasing economic impacts in the culture of tilapia and Atlantic cod since emerging in 1992. Two main strains – Francisella noatunensis subsp. orientalis (Fno) and F. noatunensis subsp. noatunensis (Fnn), have been identified, causing both acute and chronic granulomatous systemic disease. The piscine host range is increasing and Francisella culture should be included in routine diagnosis. Differentiation from the major zoonotic F. tularensis and opportunistic zoonotic F. philomiragia when dealing with environmental soil and water samples from fish farms is important. Diagnosis can be challenging but presentation of granulomatous pathology in fish should require use of cysteine supplemented selective media, culture at 15–28°C or culture in fish cell lines and specific PCR to exclude piscine Fno or Fnn. Control of infections in fish rely on appropriate antibiotic selection although in the long term an effective commercial vaccine that includes the pathogenic species of Francisella is required.

Granulomatosis in fish aquaculture: a mini review

“…Francisellosis is, therefore, a bacterial disease that mostly occurs in juvenile fish during winter months; according to Chong (2016), for tilapia, epizootics typically occur in cooler winter water temperatures with higher mortalities at 15°C than 30°C; while for the Atlantic cod, Francisellosis causes more mortalities as water temperatures increase towards 20°C. This disease provoked economic impacts in the culture of tilapia and Atlantic cod since registering the first cases three decades ago (Chong 2016); herein, the mortality rate of infected fish depends upon several factors; however, the Atlantic salmon undergoing francisellosis registered mortality of 20%, while mortality of 95% was reported for tilapia (Chern & Chao 1994;Ostland et al 2006). Several Francisella species can be transmitted by direct contact between infected animals, through contaminated water or food, or by vectors (Colquhoun & Duodu 2011).…”