Research Document 2019/016

Characterization of Aeromonas salmonicida and furunculosis to inform pathogen transfer risk assessments in British Columbia

By Boily, F., Malcolm, G. and Johnson, S.C.

Abstract

Aeromonas salmonicida is a gram negative bacterial pathogen known to cause disease in wild and cultured fish from freshwater, brackish and seawater. Due to its almost worldwide distribution and its economic importance to salmonid farming and conservation, A. salmonicida has been the focus of over 100 years of research. The subspecies A. salmonicida salmonicida, referred to as “typical A. salmonicida”, is associated with acute to chronic septicemic disease called “furunculosis” in salmonids. Other A. salmonicida subspecies are referred to as “atypical A. salmonicida” and may cause ulcerative and systemic diseases in a wide range of fish taxa, including salmonids. Typical and atypical strains of A. salmonicida are endemic in British Columbia (BC). Pathogen transmission is horizontal and all salmonid life stages are susceptible to infection. In endemic areas, furunculosis often develops following stress but covert and subclinical infections may occur in fish which become reservoirs (carriers) of the pathogen. Experimental studies demonstrated that infected, diseased and dead Atlantic Salmon (Salmo salar) shed large numbers of A. salmonicida cells. Although shedding by carriers is known to occur, patterns of shedding and the number of cells shed have not been determined. Survival of A. salmonicida in the marine environment could range between 2 and 26 days (median = 6 days) for temperature and/or salinity conditions occurring in the Discovery Islands, based on published studies using culture methods. Changes in virulence and infectivity of A. salmonicida cells in the environment are unknown, but based on published starvation studies, cells entering a “dormant” or non culturable but viable state (NCBV) have failed to induce infection and disease when injected into Atlantic Salmon. The relative susceptibility of Atlantic and Pacific salmon species is unknown but interspecific and intraspecific variations in resistance to furunculosis have been documented. No minimum infectious and lethal doses were found in the literature; these parameters would likely vary with factors related to the host, the pathogen and the environment. As A. salmonicida is endemic to BC, all three companies operating Atlantic Salmon farms in the Discovery Islands area vaccinate all their fish against furunculosis prior to seawater transfer, and implement disease surveillance and biosecurity measures at all stages of the production cycle to reduce the risks of disease.

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