Observation of a rickettsia-like organism in Atlantic salmon, Salmo salar L., in Ireland

Abstract. A rickettsia-like organism was observed in tissues of farmed Atlantic salmon, Salmo salar L., experiencing mortalities in the West of Ireland. Gross pathology of affected fish included haemorrhages in the skeletal muscle, splenomegaly, congestion of pyloric caeca and yellow mottling of livers, which appeared in ring formations in some cases. Histopathology was most obvious in the spleens, livers, kidneys and hearts, with widespread tissue necrosis and numerous basophilic granules present in the cytoplasm and cytoplasmic vacuoles of a variety of cells. Transmission electron microscopy revealed micro-organisms within membrane-bound cytoplasmic vacuoles similar in appearance to rickettsia.