The co-infection of Caligus rogercresseyi –a parasitic copepod– and Piscirickettsia salmonis –the causative bacterial agent for salmonid rickettsial septicemia (SRS)– is a recurrent problem in Chilean salmon aquaculture. Clinical diets are valuable in preventing disease in farmed fish and may reduce the use of less sustainable and more aggressive fish health management procedures (e.g., antibiotic and delousing treatments).
We tested the immunostimulatory activity of EWOS-BACTER (Cargill Inc.) in salmon smolts exposed first to P. salmonis (via cohabitation) and subsequently to C. rogercresseyi. On day 41 post-P. salmonis introduction, and day 12 post-C. rogercresseyi exposure, BACTER diet had improved survival, and weight gain in co-infected salmon compared with co-infected salmon fed a control grower diet (CONTROL). Co-infected salmon fed BACTER showed less severe clinical signs (e.g., petechial skin, ascites) and reduced lice load than their CONTROL-fed counterparts.
The head kidney transcriptome response to co-infection and its dietary manipulation were investigated via RNA-Seq. Co-infection caused the differential regulation of 13,823 genes (False Discovery Rate <5%, fold-change >1.5) in the CONTROL-fed salmon. BACTER modulated the transcript levels of 1,857 genes within the co-infected salmon. The conjoint analysis of all differentially expressed genes identified two main clusters: one with 9,679 infection-repressed genes representing primarily tissue development and transforming growth factor beta-mediated processes (Gene Ontology enrichment analysis), another comprising 8,344 infection-induced genes, most of them putatively involved in inflammatory and cytotoxic immune processes. Based on this classification, the head kidney transcriptome of BACTER-fed co-infected salmon appeared more polarized towards tissue restoration than the CONTROL-fed co-infected salmon.
These results stress the importance of clinical feeds to control co-infection in Atlantic salmon farming operations. We estimate this research will improve our understanding of the molecular changes experienced by Atlantic salmon co-infected with P. salmonis and C. rogercresseyi and will provide biomarker genes and pathways leverageable for improving fish health management practices.