Transcriptomic analysis of rainbow trout response to single and co-infections by Myxobolus cerebralis and Tetracapsuloides bryosalmonae at the portals of entry

Some parasitic infections such as those caused by the myxozoans Myxobolus cerebralis and Tetracapsuloides bryosalmonae pose a major threat to salmonid aquaculture as they lead to severe tissue damage and compromised host immunity. A deeper understanding of the impact of a prior infection with one myxozoan on the follow-on infection with another myxozoan is essential for our understanding of reciprocal impacts on host immunity and the development of sustainable disease management strategies. This study hence aims to investigate the immune response of rainbow trout (Oncorhynchus mykiss) during single vs. co-infections with M. cerebralis and T. bryosalmonae using RNA-seq. Thirty days post single infections, fish were co-infected with the other parasite. Rainbow trout were sampled 1 day post co-infections (31 days post single infections). Tissues from gills and caudal fins (portals of entry) were collected and subjected to total RNA extraction and Illumina sequencing. Differentially expressed genes (DEGs) were identified, followed by functional annotation through Gene Ontology (GO) and Kyoto Encyclopaedia of Genes and Genomes (KEGG) enrichment analyses. In caudal fins, fish primarily infected with Mc and co-infected with Tb (Mc+) showed mild immune activation mainly via complement C4B upregulation. In fish, primary infected with Tb followed by co-infection with Mc (Tb+), a stronger immune response was evident through overexpression of genes involved in interferon signalling (IFI44, ISG15, and RSAD2) and TLR7 triggered pathways. In gills, Mc+ fish exhibited moderate upregulation of genes involved in cytokine receptor activity, IL-1 receptor binding, and CCR3 chemokine binding. In Tb+, humoral and inflammatory immune activation was evident by induction of genes involved in complement (C3) and immunoglobulin-related pathways. Mc+ depicted mild structural or tissue remodelling response, while in Tb+ fish, strong suppression of B cell development and transcriptional activity was evident, which reflects possible immune evasion. In conclusion, the Mc+ group exhibited a moderate and localized response, while the Tb+ group consistently showed a stronger and broader immune response. The information obtained in this study reveals traits of host-parasite interactions and may promote the development of novel approaches and disease management strategies to control WD and PKD in salmonid aquaculture

• Molecular Biology