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The protozoan parasite Ichthyophthirius multifiliis is an economically important parasite for the aquaculture- and ornamental fish industry. The parasite is abundant worldwide and infects the skin, gills and fins of freshwater fish species. For approximately the last fifty years the innate and protective immune mechanisms induced by I. multifiliis have been in focus in different fish hosts. By utilizing transgenic zebrafish, new tools to investigate this have emerged. The aim of this study was therefore to elucidate early immune responses in zebrafish larvae by using gene expression and in vivo imaging of neutrophil and macrophage behavior during infection. For the first time, zebrafish larvae were infected with the parasite and infection dynamics, parasite size and host-parasite interactions were investigated. Results showed that the larvae responded with mild inflammation and that the 12 compared to 5 days post fertilization larvae were significantly less susceptible. It was furthermore observed that neutrophils and macrophages were attracted to the parasites and that neutrophils reacted with neutrophil extracellular traps (NETs) when fighting the parasite. The parasite was rotating vigorously, presumably to impede the neutrophils and macrophages from attaching to it but on rare occasions, neutrophils and macrophages were able to kill the parasite. Based on these observations, we concluded that the parasite uses the rotation as an immune evasive strategy and that the zebrafish larvae respond with high activity from neutrophils and macrophages locally but systemically only with mild inflammation.

Over the past recent years, there has been a growing interest in applying environmental DNA (eDNA) in various research fields like pollution response, air quality monitoring and invasive species detection. Aquaculture and in particular recirculated aquaculture system (RAS) is a promising food sector relying mostly on monospecific production. Consequently, outbreaks caused by various pathogens (e.g. bacteria, parasite, fungi, virus) represent a serious challenge to animal welfare and economic losses. The present study is part of the EU project RASOPTA which aims to increase knowledge regarding water quality, off-flavor, pathogen detection in RAS and develop a biomonitoring tool based on water sampling. In the present experiment, we investigate the sensitivity of water eDNA and eRNA to predict the onset of enteric redmouth disease (ERM) caused by Yersinia ruckeri, in rainbow trout. One week prior to infection, half of fish were subjected to unpredictable chronic stress to evaluate the effect of stress on fish immune response and mortality rates in relation with eDNA and eRNA levels of the bacteria. The fish were either mock-infected or expose to low (1E5 CFU/mL) or high (1E7 CFU/mL) dose of bacteria by bath infection. Fish spleens were sampled to evaluate immune response and bacterial load. Clinical signs were equally recorded to monitor disease occurrence. Water and fish were sampled regularly before the start of mortality and after the mortality had ceased. Results from the experiment will be compared and discussed to bring forward the optimal methods between eDNA and eRNA to evaluate occurrence of enteric redmouth disease in rainbow trout. Outcome from this study will contribute to extend knowledge which will provide a basis to develop technologies to predict and prevent outbreaks in RAS.

The consumption of aquatic products increases constantly at higher rate than the human world population. According to the Food and Agriculture Organisation, fisheries have reached their limits since 2018. Aquaculture embodies presently the only alternative to meet the existing demand for healthy aquatic products. In the context of demands to reduce environmental impact, conventional farming methods are facing many issues. Recirculated aquaculture systems (RAS) represent a promising and sustainable alternative for fish production and although they witness an on-growing interest, many challenges remain. Therefore, these challenges gave birth to a European project (RASOPTA) in which a novel chip tool for the Aquaculture industry will be developed based on environmental DNA (eDNA) from water samples as a biomonitoring method for multiple purposes (water quality, off-flavour and fish health). Disease management in RAS constitutes a significant lever to avoid losses and reduce the use of treatments. In the present study, we infected zebrafish (Danio rerio) as a model organism for cyprinids with the causative agent of vibriosis (Vibrio anguillarum), under stressful and non-stressful conditions to fit representative farming conditions. Over seven days, water and fish samples were collected to evaluate V. anguillarum eDNA levels, pathology and immune-relevant gene expression. Results will be further discussed. This study constitutes a major input to the development of the chip technology that will represent a novel tool for Aquaculture and RAS productions.