R. Elston1; L. Harrell2, T. Scott2; M.
Wilkinson1
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.