Immunohistochemistry of Atlantic cod larvae Gadus morhua experimentally challenged with Vibrio anguillarum.

Farming of Atlantic cod Gadus morhua is one of the most rapidly growing sectors of Norwegian aquaculture. Classical vibriosis caused by Vibrio anguillarum is a problem in cod aquaculture. 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 infections such as vibriosis. We aimed to further develop understanding of V anguillarum serotype O2alpha infections in cod larvae by elucidation of a possible route of entry, the pattern of infection and its histopathology. Cod eggs were transferred to a 24-well polystyrene multi-dish with 2 ml of sterile aerated 80% (28 per thousand salinity) seawater. Challenge doses were 10(4) and 10(6) CFU ml(-1). Unchallenged larvae were used as controls. Larvae for immunohistochemical examination were sampled daily from each group. In most of the larvae, either no or very few bacteria were observed. Typical findings were clusters of bacteria in the spaces between the primary gill lamellae. None of these bacteria seemed to have adhered to the gills. Intestines of 3 out of 161 larvae examined contained positively immunostained bacteria. Some bacteria appeared attached to the microvilli, but none was observed inside epithelial cells. Only 2 larvae from the low-challenge dose group showed clear signs of histopathology, which occurred in the intestine. It is not possible to draw any conclusions regarding the portal of entry.