Prophylactic measures in rainbow trout aquaculture: Further development of a DNA vaccine for proliferative kidney disease.

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Our proposed work will study an important disease, affecting the rainbow trout aquaculture industry in the UK and central Europe, called proliferative kidney disease (PKD). The disease is caused by a microscopic parasite distantly related to jellyfish, which causes a severe immune response in fish characterized by chronic kidney pathology. Recent studies discovered colonial freshwater invertebrates, called bryozoans, can harbour the parasite stage infective to fish with a clear link between increasing water temperatures and increased parasite proliferation in bryozoans. This causes an increase in the release of infective parasite spores suggesting that the disease is likely to become more problematic in farmed and wild salmonid stocks as water temperature trends increase due to climate change. This has already been clearly seen in wild brown trout populations in Switzerland and wild Atlantic salmon stocks in Norway and in the increasing prevalence of PKD in recent years in trout farms in Southern England. There are no current treatments or means of controlling the disease. Importantly, however, there is a great deal of potential to develop a treatment since it is known that fish surviving parasite infection are resistant to the disease upon parasite re-exposure. Thus, the vaccination of fish is expected to be one way to control the disease. Vaccination will be based on finding molecules secreted from or found on the surface of the parasite in direct contact with the fish immune system. Such molecules can prime the immune system to react quickly when exposed to the parasite preventing or reducing chronic infection and associated fish mortalities. In recent studies we used parasite material isolated from bryozoans in molecular biology techniques to create a "gene library". Gene pools created from this library enabled us to examine the ability of thousands of parasite molecules to prime the fish immune system and protect against disease pathology or to slow the onset of advanced clinical disease. Different gene pools were administered to fish by DNA vaccination in line with recent advances in vaccine technology, representing a more cost effective approach to discover suitable vaccine candidates than using conventional protein-based methods. Fish were vaccinated at a commercial rainbow trout farm that suffers from this disease every year. Results were assessed in terms of the extent of disease pathology and parasite abundance in kidney tissue. After two sequential rounds of subdivision and retesting of the most protective gene pool, we were able to identify a single batch of 182 parasite molecules slowing the onset of kidney pathology. The commercial exploitability of such vaccine studies is highly dependent on the identification and full characterization of single parasite protective molecules. So in this proposal we aim to take the next steps towards PKD vaccine development by subdividing and retesting this gene batch in order to identify and characterize the parasite molecules conveying disease protection. The majority of parasite molecules will be found in both infected hosts, with a minority being host-specific. Parasite molecules capable of protecting fish against the disease must either be found in both hosts or are fish-specific. From our previous studies, we have created a parasite gene library from infected fish. We, therefore, further propose to use a high throughput molecular biology technique to determine which parasite molecules are predominantly or solely found in the fish host relative to the bryozoan host and to test those "fish-specific" molecules predicted to be secreted from or found on the surface of parasites within the infected fish kidney. Overall, we will take a systematic approach to further vaccine development for PKD, exploiting our past success in uncovering a single protective parasite gene pool, our existing expertise in molecular biology, PKD biology and fish vaccination.