Host Range, Geographic Distribution
SRS disease in Chile typically occurs in marine waters during the on-growing process from smolt to harvest. It has also been isolated from freshwater cages of Coho salmon and trout. The disease was originally predominant in Coho salmon (Oncorhynchus kisutch) but is now recognized to cause serious losses in all farmed salmonid fish species including Atlantic salmon (Salmo salar), rainbow (steelhead) trout (O. mykiss), Chinook salmon (O. tshawytscha), pink salmon (O. gorbuscha) and masu salmon (O. masou). Piscirickettsia sp. are commonly found in fish worldwide (e.g., Chile, Canada, Ireland, Scotland and Norway) but are of major economic importance only in Chile to date. The distribution of P. salmonis and RLO is therefore wide spread. Several reports describing RLO infections in non-salmonid finfish exist. For example, about 10 years ago, a RLO was identified as the causative agent of an outbreak with mass mortality among pond-reared tilapia in Taiwan. Also, RLO-related mortalities in juvenile European sea bass at 12 – 15 °C in sea cages have been reported along the French Mediterranean coast.

Transmission & Epidemiology
At present there are few reports of P. salmonis coming from wild salmonids, although it is likely that the bacterium is present in naturally occurring populations of marine fish. Horizontal transmission has been reported in marine-farmed salmon 2 weeks after the introduction of pathogen-free fish into infected sites. The extended extracellular survival time of this organism in salt water (several weeks at 5-20 °C) may be of sufficient duration to permit horizontal transmission without a vector. Experimentally it is documented that the bacterium can enter through the intact skin and gills although the mode of entry is still not clear.

The possibility of vertical transmission of P. salmonis now looks more and more likely due to recent research in Chile. Apparently there is an adhesion complex that allows the pathogen to enter the salmon egg. There is even a suggestion that this complex may be involved in fish to fish transmission.

Currently, no alternative host has been identified and the source, reservoir and means of transmission of P. salmonis remain important areas of research.

The course of the clinical disease is typically chronic to subacute in nature with mortalities typically developing 10 - 12 weeks after the transfer of fish to seawater and lasting approximately 10 weeks before they diminish. Virtually all stocks become infected and usually experience more then one clinical episode, typically in the spring and autumn seasons.

Disease reprinted courtesy of OIE Diagnostic Manual for Aquatic Animal Diseases, OIE (World Organisation for Animal Health), Paris, France.