Development of improved vaccines effective against emerging strains of the fish pathogen Yersinia ruckeri

Despite advances in vaccine development and chemotherapy, bacterial pathogens remain a significant constraint to aquaculture production, as well as a potential threat to wild stocks. In addition to so-called 'new' bacterial pathogens, the threat often comes from established pathogens evolving to circumvent developed control strategies. An example of such an evolving pathogen is the Gram-negative bacterium Yersinia ruckeri which causes economically-important infections of Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss). Y. ruckeri has been identified as the aetiological agent of enteric redmouth disease (ERM) of farmed rainbow trout in England and Wales for more than thirty years. The organism also causes significant outbreaks of disease in farmed Atlantic salmon but very little is known about the epidemiology of Y. ruckeri in this species. Until recently, Y. ruckeri posed a reduced risk to farmed trout and salmon due largely to the extensive use of vaccination. However, increasing worldwide reports of vaccine breakdown in the field are believed to be associated with the emergence of new pathogenic strains of Y. ruckeri. Traditionally, the majority of ERM infections in rainbow trout in the USA and Europe have been caused by a subgroup (biotype) of closely-related serotype O1 Y. ruckeri strains represented by the so-called Hagerman strain. However, a new and distinct serotype O1 biotype has emerged in recent years that was first identified in the UK but which is now increasingly affecting rainbow trout production across Europe and the USA. Isolates recovered from infected Atlantic salmon appear to be more diverse and represent a wider range of O-serotypes (O1, O2 and O5). Preliminary evidence suggests that salmon-pathogenic serotypes are significantly less pathogenic to rainbow trout than they are to Atlantic salmon. The outer membrane (OM) of Gram-negative bacteria is at the interface between pathogen and host and outer membrane proteins (OMPs) play key roles in host-pathogen interactions such as adherence and colonization, nutrient uptake (specifically iron acquisition), evasion of the host immune response and secretion of virulence factors. Because many of these OMPs are under strong selection pressures (e.g. from the host immune system) they can exhibit varying degrees of diversity within a single bacterial species. This diversity is likely to confer upon strains different virulence attributes and host tropisms. In addition, many OMPs are surface-exposed and immunogenic and are therefore likely to serve as protective antigens. Despite the importance of Y. ruckeri as a pathogen of farmed salmonids, very little is known about the diversity of its OMPs in different strains or their roles in the disease process. Furthermore, very little is known about the host immune response to Y. ruckeri in salmon and trout or about the bacterial factors responsible for providing protection against infection. The aims of this project are as follows: 1. To carry out an epidemiological survey of the prevalence of Y. ruckeri in Scottish salmon and assess the effectiveness of transmission of the organism in seawater. 2. To characterize the OM proteomes of key selected isolates of Y. ruckeri recovered from salmon and trout under different growth conditions and identify cross-protective antigens (and potential vaccine candidates) by immunoproteomics. 3. To compare the virulence of selected isolates for Atlantic salmon and trout. 4. To compare the kinetics of the immune response to selected isolates of Y. ruckeri in Atlantic salmon and trout. 5. To study serotype specificity of immunity and the protection afforded by vaccines prepared from different isolates of Y. ruckeri in Atlantic salmon and trout.