SEQUENTIAL PATHOLOGY OF EXPERIMENTAL PASTEURELLOSIS IN GILTHEAD SEABREAM SPARUS-AURATA - A LIGHT-MICROSCOPIC AND ELECTRON-MICROSCOPIC STUDY

The haematological and histopathological changes caused by Pasteurella piscicida or by its extracellular products (ECPs) are described for g ilthead seabream Sparus aurata following experimental infection. Resul ts indicate that the ECPs were haemolytic in vivo, causing a significa nt decrease in the number of circulating red blood cells. However, thi s decrease was not significant in fish injected with bacteria. The inf lammatory response induced by bacteria and ECPs was similar, including lymphopenia, granulocytosis, an increase in the number of peritoneal exudate cells, and mobilization and degranulation of the eosinophilic granular cells. The study of peritoneal exudate cells showed that at 1 and 6 h post-injection numerous peritoneal granulocytes had engulfed 1 or 2 bacteria per cell. Granule discharge occurred, and altered bact eria were frequently observed in the phagocytic vacuoles of these gran ulocytes. Macrophages containing phagocytosed bacteria were also noted . After 1 d, P. piscicida occurred in large numbers within the periton eal macrophages. These bacteria were apparently intact. The histopatho logical study showed that the bacterium was mainly phagocytosed by mac rophages and that the latter accumulated in several organs. Macrophage s with engulfed bacteria appeared in the kidney and spleen at 6 h post -injection. After 2 d, high numbers of macrophages, singly or in aggre gates, containing abundant phagocytosed bacteria, were observed in the se organs. In later stages of the infection, the occurrence of degener ate macrophages full of intact-appearing bacteria and of bacterial col onies of different sizes suggested that macrophages played an importan t role in disseminating the pathogen throughout the fish. The lesions observed in the muscle adjacent to the site of injection and in the sp leen ellipsoids of fish injected with ECPs were rare, possibly due to the low proteolytic activity of ECPs. In contrast, fish injected with ECPs developed severe lesions in the Liver and gills, suggesting the p resence of toxin(s) which may be important in the pathogenesis of past eurellosis.