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Research Document 2019/020

Characterization of Piscirickettsia salmonis and salmonid rickettsial septicaemia to inform pathogen transfer risk assessments in British Columbia

By Jones, S. R. M.

Abstract

Infection with the facultative, intracellular Gram-negative bacterium Piscirickettsia salmonis causes piscirickettsiosis (SRS), a potentially severe septicaemia of farmed marine fish in many parts of the world. The bacterium has a broad host range and several species of non-salmonid and salmonid fishes are susceptible to the infection and disease. There are no data on susceptibility in Sockeye Salmon (Oncorhynchus nerka), but Chinook (O. tshawytscha), Coho (O. kisutch) and Pink (O. gorbuscha) Salmon and Rainbow Trout (O. mykiss) are susceptible, as are Atlantic Salmon (Salmo salar). Waterborne movement of the bacterium is the most likely route of transmission, although there are no data on shedding rate or minimum infectious dose in any species. Environmental and husbandry-associated stressors contribute to the severity of the disease. The development of control strategies has been hampered by a poor understanding of the transmission, epidemiology and virulence factors of P. salmonis, and by an absence of efficacious vaccines and reliable antibiotic therapies. In British Columbia (BC), SRS has been reported in farmed Atlantic and Chinook salmon since the 1980s. There are very few data on the occurrence of P. salmonis in wild salmonids in BC. Nucleotide sequences from BC isolates of P. salmonis are remarkably similar to those from Chile and Europe. In conclusion, the wide host range of P. salmonis indicates a high likelihood that Sockeye Salmon will be susceptible. SRS outbreaks appear to be triggered by environmental stressors, whether of environmental or farm origin. Studies on P. salmonis are required to confirm susceptibility and pathogenesis in Sockeye Salmon, to determine salmon species-specific bacterial shedding rates and to characterize the attenuation of bacterial viability under natural conditions.

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