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.
KEY WORDS: Cryptobia salmositica · Oncorhynchus mykiss · Histopathology · Parasitaemias
Resale or republication not permitted without written consent of the publisherDis Aquat Org 46: [41][42][43][44][45] 2001
MATERIALS AND METHODSTwo groups (n = 50 in each group) of hatchery-raised juvenile rainbow trout (about 6 to 8 cm in length) were maintained in 2 fibre glass tanks at about 8°C for 2 wk before they were experimentally infected with Cryptobia salmositica. The inocula were prepared with blood from an infected rainbow trout and diluted with appropriate volumes of PBS (pH 7.4). The low dose (LD) group was inoculated intraperitoneally (ip) with 2000 parasites fish -1 and the high dose (HD) group with 20 000 parasites fish -1 . At weekly intervals, 4 fish from each group were anesthetised with MS-222 (100 ppm), bled and necropsied. In the first week after infection the parasitaemias in the blood were detected using the haematocrit centrifuge technique and wet mount method (24 microscopic fields examined, 10× objective). In subsequent weeks, the haemocytometer was used to estimate parasitaemias (Woo 1979). For histopathology, samples of visceral organs, skin, eye, brain and gills were fixed in 10% formalin, embedded in paraffin, sectioned into 4 µm thick pieces, and stained with haematoxylin and eosin. Smears of ascites fluid or blood were air dried, fixed in 100% Ethanol and stained with Giemsa's stain. A third group of uninfected fish (controls) was also sampled.
RESULTS
LD groupThe haematocrit centrifuge technique detected parasites in the blood of all fish at 1 wk post infection (pi).The parasitaemia increased rapidly, p...