Mode of infection and spread of Vibrio anguillarum in turbot Scophthalmus maximus larvae after oral challenge through live feed

The infection route of the marine fish pathogen Vibrio anguillarum was studied after oral challenge of juvenile turbot Scophthalmus maximus L, through a live feed. Artemia nauplii were incubated in a suspension of V. anguillarum cells, and subsequently fed twice to the fish. All challenged fish died within 4 d after the first challenge, while no mortality occurred in the non-challenged controls. The results of an immunohistochemical examination of the sectioned fish samples clearly demonstrated that V. anguillarum cells were ingested by the Artemia and that the latter were ingested by the fish. Bacteria were released from the Artemis mainly in the anterior part of the intestine. Most challenged fish started to show disease signs 24 h after the second challenge and died 2 d later. A histopathological analysis of moribund fish showed the development of septicaemia. Moreover, the sequential sampling, allowed the reconstruction of the infection route after oral challenge. Our results show that V. anguillarum was transported through the intestinal epithelium by endocytosis, after which the bacterium was released in the lamina propria. From there the bacterium was transported by the blood to the different organs, eventually leading to septicaemia and mortality.