DAO 21:177-186 (1995)  -  doi:10.3354/dao021177

The haematological and histopathological changes caused by Pasteurella piscicida or by its extracellular products (ECPs) are described for gilthead seabream Sparus aurata following experimental infection. Results indicate that the ECPs were haemolytic in vivo, causing a significant decrease in the number of circulating red blood cells. However, this decrease was not significant in fish injected with bacteria. The inflammatory 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 bacteria were frequently observed in the phagocytic vacuoles of these granulocytes. Macrophages containing phagocytosed bacteria were also noted. After 1 d, P. piscicida occurred in large numbers within the peritoneal macrophages. These bacteria were apparently intact. The histopathological study showed that the bacterium was mainly phagocytosed by macrophages and that the latter accumulated in several organs. Macrophages with engulfed bacteria appeared in the kidney and spleen at 6 h post-injection. After 2 d, high numbers of macrophages, singly or in aggregates, containing abundant phagocytosed bacteria, were observed in these organs. In later stages of the infection, the occurrence of degenerate macrophages full of intact-appearing bacteria and of bacterial colonies of different sizes suggested that macrophages played an important role in disseminating the pathogen throughout the fish. The lesions observed in the muscle adjacent to the site of injection and in the spleen 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 presence of toxin(s) which may be important in the pathogenesis of pasteurellosis.

Gilthead seabream . Pasteurella piscicida . Live cells . Extracellular products . Haematology . Histopathology