Refinement of pre-exposure programs to reduce the use of fish in proliferative kidney disease monitoring in aquaculture

Emerging infectious disease outbreaks among farmed and wild fish are increasing due to global warming. Sustainable and ethical research to climate-driven disease emergence therefore requires data-driven and widely accessible approaches that explicitly incorporate temperature dependent epidemiological dynamics. The main aim of this proposal is to develop an optimised experimental exposure protocol for researchers to use in fish vaccine development and disease monitoring. The studentship focuses on Proliferative Kidney Disease (PKD), a climate-change driven, severe emerging infectious disease of salmonids worldwide. PKD has emerged as one of the main parasitic diseases threatening the sustainability of trout aquaculture in the UK, and is a major cause in the decline of European brown trout populations. The health consequences of PKD are severe - impacted fish undergo massive proliferation of the interstitial kidney tissue, anaemia, and apathy, becoming more susceptible to secondary infections. Despite the aquaculture industry and wild fisheries suffering significant PKD-associated losses every year, no therapeutics for PKD have been successfully established, and vaccine development efforts are hampered by the lack of best-practise guidelines for experimental infection challenge. The aim of this proposal is to optimise the PKD experimental challenge model to limit the use of fish as sentinels of disease in PKD-related research. We propose to achieve this by adapting methods developed previously 1) to quantify parasite dose by water sampling and detection of environmental DNA and 2) to measure PKD specific immunity in exposed fish using serology. These methods would provide a novel best-practise challenge protocol, and a monitoring toolkit. Therefore, this studentship project has outcomes which impact the Reduction and Refinement aspects of the 3Rs principles. Reduction will be achieved by removing the need for sentinel fish and successively exposed fish cohorts during experimental challenge. Refinement is achieved by developing a non-lethal, serology based method of measuring fish immunity, relying on blood samples instead lethal sampling, providing a much more sensitive measure of infection status, and, for the first time, allowing the collection of temporal data on the response of individual fish. This will also reduce the number of fish needed to achieve acceptable sample size in experiments, allowing future scaling up of large field trials during vaccine development work. The combined eDNA and serology provides a unique opportunity to non-lethally track the dynamics of specific immune activity towards the parasite as a strong proxy of immunological memory and protective immunity development. For the first time, we will be able to understand the anti-PKD antibody dynamics in pre-exposed fish that are consistent with sufficient parasite exposure to induce natural protective immunity when re-exposed the following year. The use of non-lethally sampled fish during preexposure and re-exposure will enable the tracking of antibody responses in individual fish in order to understand the relationship between antibody dynamics of fish when first exposed to parasites and how antibody levels change during reexposure and how this links to efficient natural protective immunity. In addition to research, PKD clearly affects fish health throughout the salmonid aquaculture industry; 4-4.5M trout annually in the UK are exposed to PKD using the challenge model we propose to improve. If successful, our proposed approaches could significantly reduce the numbers of fish suffering severe PKD on farms (up to 300,000 trout per growing season in the UK die of PKD despite pre-exposure programs). To facilitate research uptake of our protocols, we work with fish health professionals, linking the detailed experimental data with the commercial stocks, which allows benchmarking our results to the current best-practise on farms.