PISCIRICKETTSIA-SALMONIS, A MAJOR PATHOGEN OF SALMONID FISH IN CHILE

Piscirickettsiosis has resulted in extensive losses among salmonid fishes cultured in southern Chile. It occurs at many seawater sites near Puerto Montt, where recurring epizootics have frequently resulted in mortality approaching 90% among the coho salmon (Oncorhynchus kisutch) cultured there. Three other salmonid species reared in netpens in these waters are also subject to the disease. A unique rickettsial agent, Piscirickettsia salmonis (type strain LF-89), has been isolated in cell culture and demonstrated to be the causative agent of piscirickettsiosis. The obligately intracellular nature of P. salmonis and its occurrence only in salt water suggest that a natural reservoir of infection is maintained by one or more poikilotherms in the marine environment. Identification of this source of infection and elucidation of the natural mode(s) of transmission of the agent are high research priorities. Annual importation of eggs into Chile from salmonid-growing regions in the Northern Hemisphere provides a supply of fish for culture, but one which may have limited natural resistance to fish pathogens unique to Chilean waters. The epizootics of piscirickettsiosis amply demonstrate one unfortunate consequence of the interaction of native species with non-indigenous fish species introduced for aquaculture.