A severe infectious systemic disease occurred in 3-year-old Chinook salmon (onchorynchus tschawytscha) brood stock held in saltwater net pens. Cumulative mortalities exceeded 95% (4,750 fish) over eight months. The causative agent replicates intracellularly in macrophages and endothelial cells. Accumulation and replication of the organisms occurs extensively in the filtering organs, i.e., spleen, kidney, and liver, and results in massive enlargement and compression necrosis of these organs but is accompanied by relatively little inflammatory response.

The organisms are spherical, 3.0 to 7.0 um in diameter, and have a cell wall with contains cellulose. They divide by daughter cell division and contain peripheral elongated mitochondria and a variety of cytoplasmic vacuoles.

The obligate intracellular parasite was isolated in vitro by organ explant cultivation and subsequent transfer to CHSE/214 cells. Mortalities and characteristic lesions were reproduced in juvenile salmon by inoculation with the isolate organisms were reisolated from moribund fish 25 days after inoculations. Antigenic identity was demonstrated between the isolate and the organism in the net-pen reared fish using a rabbit antiserum.