PISCIRICKETTSIA SALMONIS INFECTION IN ATLANTIC SALMON SALMO-SALAR IN NORWAY - EPIDEMIOLOGIC, PATHOLOGICAL AND MICROBIOLOGICAL FINDINGS

A rickettsia-like organism was isolated from diseased Atlantic salmon Salmo salar in Norway. Because of morphological and serological simila rities to the type strain the suggested name of the organism is Piscir ickettsia salmonis. The bacterium is considered the most probable caus e of a systemic disease diagnosed in 51 farms along the west coast of Norway. Most of the cases occurred in the autumn of 1988. The disease was only recorded in smelts after exposure to sea water and cumulative mortality has been low. In 63 % of fish with gross lesions examined d uring outbreak of disease in 14 of the affected farms, the typical mac roscopic finding was a normal coloured liver with white, circular, som etimes haemorrhagic foci. Of fish with gross lesions, 35 % showed pale gills, a yellow, mottled liver, and haemorrhages scattered throughout the skeletal muscles, perivisceral fat, the stomach wall and the swim bladder. Histomorphological changes were most often observed as necros is and granulomatous inflammation in the liver. Intracellular, intrava cuolar bacteria-like inclusions with an affinity for phagocytic host c ells were observed. Transmission electron microscopy revealed individu al or paired organisms enclosed in membrane-bound vacuoles.