Francisellosis in Fish: A Disease Guide

Francisellosis, caused by Francisella noatunensis, is a chronic granulomatous disease of fish. Its intracellular nature limits treatment, so control relies on biosecurity.

Overview

Francisellosis is a chronic, systemic, granulomatous disease of farmed and wild fish caused by Francisella noatunensis, a non-motile, gram-negative, facultatively intracellular cocco-bacillus. The disease typically causes high morbidity with multiple white granulomas in internal organs and variable mortality. Because the bacterium lives inside host cells, it is difficult to treat and to culture, which complicates both control and diagnosis.

Subspecies and affected hosts

Two main subspecies infect fish according to water temperature. Francisella noatunensis subsp. noatunensis affects cold-water species such as Atlantic cod (Gadus morhua) and Atlantic salmon (Salmo salar). Francisella noatunensis subsp. orientalis (also referred to as Francisella orientalis) affects warm-water species, especially tilapia (Oreochromis sp.), as well as ornamental cichlids and other species. Disease progression tracks water temperature, with warm-water infections accelerating as temperature rises and cold-water strains progressing slowly at very low temperatures.

Clinical signs

• Multifocal white nodules (granulomas) in the spleen, kidney, and liver
• Enlarged spleen (splenomegaly)
• Pale gills and anaemia
• Lethargy and erratic swimming
• Hemorrhagic ascites and an atrophic liver in some cases
• High morbidity within affected stocks

Transmission

The bacterium spreads through direct contact and contaminated water. The infectious dose can be very low, with as few as a handful of bacterial cells reported to cause disease for the warm-water subspecies, which means the pathogen can establish readily once introduced into a population.

Diagnosis

Diagnosis is based on histopathology showing extensive chronic granulomatous inflammation with multiple granulomas, combined with PCR and culture. The organism is notoriously difficult to grow and requires cysteine-enriched media, so PCR is often used when culture fails. The granulomatous lesions can resemble those of mycobacteriosis, so laboratory differentiation is important.

Treatment, control and prevention

Antibiotic therapy is of limited effectiveness because the bacterium hides inside host cells, and no broadly effective commercial vaccine has been established. Management therefore relies on biosecurity and reducing the conditions that favor outbreaks.

• Stock from tested, disease-free sources and quarantine new fish
• Manage water temperature and reduce stress that drive clinical disease
• Maintain strict biosecurity, equipment disinfection, and separation of stocks
• Remove and properly dispose of dead and moribund fish to reduce environmental load
• Use any antibiotics only under veterinary direction, recognizing limited efficacy against an intracellular pathogen