Insights into the connection between pathogen water eDNA and disease progression in zebrafish infected with Vibrio anguillarum

The host-pathogen interactions are tripartite and include the host, the pathogen, and the environment. For a long time, attention has mainly been addressed to the host's immune mechanisms and the pathogen's virulence factors while the environmental factor has received less interest. About a decade ago, studies on environmental DNA (eDNA) started to emerge as this approach is versatile for numerous research fields and interest is persistently growing. In the present study, the relationship between disease progression and pathogen water environmental DNA level was investigated. To perform this investigation, an infection model was setup using zebrafish and the gram-negative bacteria Vibrio anguillarum. Two different fish densities and bacterial concentrations were used to determine different patterns of pathogen eDNA in the fish tanks. Water sampling to collect eDNA was conducted alongside with observation of the fish Clinical Signs, mortalities, and immune responses. Water eDNA relative abundance of the pathogen could be determined directly after fish challenge, when Clinical Signs occurred, and when mortality started. Moreover, immune-relevant gene expression in zebrafish gills showed that il-1β, il-8, il-10, nf-κb and saa correlated to the pathogen eDNA signal, depending on fish density and bacterial concentration. In conclusion, this study demonstrated that water eDNA is a promising non-invasive tool to state disease progression and immune response in zebrafish infected with V. anguillarum.