In vivo host-pathogen dynamics and immune responses to Piscirickettsia salmonis using the zebrafish model.

Research on Piscirickettsia salmonis, a critical pathogen affecting salmon aquaculture, has been hampered by inadequate host models to study the infection process in vivo. Here, we highlight zebrafish as a surrogate host model for in-depth analysis of P. salmonis infection by using fluorescent dyes and protein markers to visualize host-pathogen interactions. Using transgenic zebrafish lines with fluorescent immune cells, we observed a strong increase in neutrophil numbers in the infected larvae, resembling an emergency granulopoiesis response. We observed the presence of P. salmonis in the caudal hematopoietic zone accompanied by an increased expression of immune marker genes, such as the neutrophil marker mpx, interleukins and interferons, both in zebrafish larvae and adult zebrafish kidneys, features of emergency granulopoiesis in response to uncontrolled bacterial burden. Our results underscore the importance of zebrafish as a valuable model for studying P. salmonis infections.