Infectivity of a rickettsia isolated from coho salmon Oncorhynchus kisutch

Two species of salmonids were tested for their susceptibility to infection by a rickettsia isolated in cell culture from diseased coho salmon Oncorhynchus kisutch in Chile. Mortality approached 100% in coho and Atlantic salmon Salmo salar injected with 10-fold dilutions of cell culture medium containing the rickettsia. Typical disease signs were present in the coho salmon but were not observed in inoculated Atlantic salmon. However, the rickettsia was recovered in pure culture from moribund fish in each of the injected groups of either species. The rickettsia was thereby demonstrated to be pathogenic and the cause of the ongoing epizootic affecting salmonids cultured in Chile. Horizontal transmission was not demonstrated in a group of uninoculated coho salmon held in the same tank with experimentally infected fish.