RESUMO
One group (n = 50) of juvenile rainbow trout Oncorhynchus mykiss was inoculated intraperitoneally with 2000 (low dose [LD]) and another group (n = 50) with 20,000 (high dose [HD]) Cryptobia salmositica fish(-1). The histopathology was a generalised inflammatory reaction, and lesions were in connective tissues and in the reticulo-endothelial system. In the LD group, the first lesions were observed in the liver, gills and spleen at 2 wk post infection (pi) while in the HD group they were in the liver and gills at 1 wk pi. Endovasculitis and mononuclear cell infiltration were observed at 3 wk pi in the HD group. These were followed by tissue necrosis and extravascular infiltration of parasites at 4 wk pi. The severity of lesions was directly related to parasitaemias in the blood and extravascular location of parasites. In the HD group, the most extensive tissue necrosis was at 4 wk pi and in the LD group it was 6 to 7 wk pi. Necrosis in the vital organs (liver, kidney and depletion of the haematopoeitic tissues) and anaemia were probably in part responsible for mortality of fish during acute disease. Regeneration and replacement of necrotic tissues were seen at 7 to 9 wk pi in the HD group, and it was most noticeable in haematopoietic and reticular tissues. These occurred during the recovery phase of the disease and were associated with significant reduction in blood parasitaemia.