Date of Award


Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)


Marine Biology


Ian R. Bricknell

Second Committee Member

John T. Singer

Third Committee Member

Paul Rawson


The production of Atlantic cod (Gadus morhua L.) in aquaculture continues to be hindered by large losses during juvenile production. These losses are often the result of the bacterial disease vibriosis, caused by the marine pathogen Vibrio anguillarum. The preventative vaccination of Atlantic cod against this pathogen has become standard practice in order to decrease the frequency of disease outbreaks. While vaccination by intra-peritoneal injection and bath immersion are consistently effective at preventing pathogen invasion through the skin and gills, neither of these methods fully protect the mucosal surfaces of the intestinal tract, which is an important site of immune response in many teleost species. The oral delivery of vaccines in feed pellets could provide a cheaper, easier method of vaccine delivery; however, this method is currently not effective enough for large-scale use, due to the degradation of vaccine antigens in the acidic environment of the fish’s stomach.

Juvenile Atlantic cod were exposed to 104-106 CFU of V. anguillarum 02α by intra-peritoneal injection, bath immersion, oral intubation and anal intubation and monitored for three weeks post-infection. There were significant differences in the postinfection mortality of the fish in these different treatments, as well as the bacterial colonization and the severity of physical damage to the host tissues during the first three days post-infection. Semi-quantitative RT-PCR also illustrated that the invasion of V. anguillarum 02α through these different entry points affected the magnitude and composition of the up-regulation of immune factors such as Interleukin-1β. Significant correlations were also observed between the bacterial load of the host tissues and the up- regulation of the resulting immune response during the first three days post-infection.

The fish that were orally intubated with V. anguillarum 02α in this experiment exhibited a less intense infection, delayed mortality, and only minor changes in gene expression when compared with the infected fish of the other three treatments. The survival of cultured V. anguillarum in the laboratory was also reduced in phosphate- buffered saline below pH 3.3, which is representative of the pH of Atlantic cod stomach. While these results suggest that entering orally may decrease the ability of V. anguillarum to cause disease in juvenile Atlantic cod, the condition of this pathogen and its ability to invade the intestinal tract after acidic exposure it is still unclear.

This research has illustrated that the particular entry point of V. anguillarum into juvenile Atlantic cod influences the localized progression of pathogen invasion and the intensity of the resulting infection. Having a better understanding of these host-pathogen interactions at the point of exposure will assist other researchers in improving vaccine design as well as administration methods. This will be especially important for oral vaccines, in order to protect the intestinal tract of Atlantic cod and other species from the invasion of pathogens like V. anguillarum.