Pathological and immunohistochemical investigations in rainbow trout (Oncorhynchus mykiss Walbaum, 1792) experimentally infected with Vibrio anguillarum

The objectives of the study were to investigate and to compare the clinical, pathological and immunohistochemical data observed in rainbow trouts experimentally infected with Vibrio anguillarum according to the inoculation route (intraperitoneal injection vs. immersion). For that, 20 fishes were intraperitoneally inoculated with 5.5x10(7)cfu V anguillarum serotype 01 and 20 other animals were exposed to 50 mL of bacterial inoculum (5.5x10(5) cfu/mL) diluted in water (30 L) for 30 minutes whereas 2 groups of control fishes (n = 8 in each croup) were similarly manipulated and only exposed to the vehicle. Mortality and clinical signs were daily recorded and necropsy. histopathology using conventional staining and immunohistochemistry using polyclonal chicken antibodies targeted against the V anguillarum antigens were performed on fishes spontaneously died or killed. Mortality rates were 55% when trouts were inoculated intraperitoneally and 40% when they were exposed to bacteria by immersion. The majority of deaths have occurred between the 3rd and the 9th days and between the 5th and the 11th days, respectively. Hyperaemia and haemorrhages associated to oedema were predominantly observed in skin, muscles and gills, and at a lesser extend in visceral organs (liver, heart, spleen, stomach) and in the abdominal fat tissues in intraperitoneally inoculated trouts. Histopathological investigations have also confirmed the nature of the lesions and have also evidenced cellular degeneration and necrosis associated to bacteria clumps as well as the increase in eosinophilic granular cells (EGCs) and degranulation in gills and in stomach. In trouts inoculated by immersion, similar but milder lesions were observed on a smaller number of infected fishes except that the gill lesions were systematically present and well marked. Immunopositivity to bacterial antigens in injuried tissues was observed only during the acute phase of the disease (e.g. until the 15th and the 11th days according to the inoculation route). These results clearly demonstrate the pathogenecity of V anguillarum in rainbow trouts mainly after intraperitoneal inoculation and suggest that gills during immersion may be the primary site of the bacterium entry.