A Comparison of Pathological Changes Caused by Vibrio anguillarum and Its Extracellular Products in Rainbow Trout(Oncorhynchus mykiss).

Abstract

Some haematological parameters and histopathological changes in the liver, spleen, kidney, gills, gut, skin, muscle and heart of rainbow trout injected intraperitoneally with virulent Vibrio [Listonella] anguillarum or with V. anguillarum extracellular products (ECPs) were compared. The number of circulating red blood cells, the haematocrit and haemoglobin values decreased in the fish injected with either bacteria or ECPs compared with controls. The erythrocyte sedimentation rate increased in both groups of injected fish; however, the serum levels were similar to those observed in control fish. Light and electron microscopic observations of samples obtained from fish injected with V. anguillarum cells or its ECPs revealed similar histopathological changes in most tissues. Both groups of fish showed degeneration and necrosis of hepatocytes, vasodilation of the glomerular capillaries, cloudy swelling and hyaline droplet degeneration of the epithelial tubular cells, oedematous gills, desquamation of the intestinal mucosa and necrosis of the muscle fibres at the site of injection. However, the bacterium multiplied extensively in the kidney and spleen where it caused more acute alterations than ECPs. Based on these results, it was concluded that the extracellular products play an important role in the virulence mechanisms of V. anguillarum.