DAO prepress abstract  -  doi: 10.3354/dao01926

ABSTRACT: Farming of Atlantic cod Gadus morhua is one of the most rapidly growing sectors of Norwegian aquaculture. Classical vibriosis caused by Listonella anguillarum is a problem in cod aquaculture. In order to prevent disease outbreaks, a thorough understanding of the infection route and the impact of the bacteria on the host is important. The intestinal tract, skin and gills have all been proposed as routes of entry for bacterial infection such as vibriosis. The aim of this study is to further develop our understanding of L. anguillarum serotype O2α infections in cod larvae, including possible route of entry, pattern of infection and pathology. Cod eggs were transferred to a 24-well polystyrene multi-dish with 2 ml of sterile aerated 80% (28‰ salinity) seawater. Challenge doses were 10⁴ and 10⁶ CFU ml^-1, respectively. Unchallenged larvae were used as controls. Larvae for immunohistochemical examination were sampled from each group every day. In most of the larvae, either no or very few bacteria were observed. Typical findings were clusters of bacteria in the space between the primary gill lamellae. None of these bacteria seemed to have adhered to the gills. Three out of 161 larvae examined showed positively immunostained bacteria in the intestine. Some of the bacteria appeared to be attached to the microvilli, but none were observed inside epithelial cells. Only two larvae from the low-challenge dose group showed clear signs of pathology, which was found in the intestine. It is not possible to draw any conclusions regarding the portal of entry.