Transcription of immune genes in ranched southern bluefin tuna show correlations to Cardicola spp. (Digenea: Aporocotylidae) infection

Cardicola forsteri and C. orientalis (Digenea: Aporocotylidae) are significant pathogens affecting the Southern bluefin tuna (SBT), Thunnus maccoyii , aquaculture industry. The aims of this research were to identify a relationship between immune gene expression of SBT and C. forsteri and C. orientalis infection burdens; and to understand how immune gene expression changes over time during commercial ranching. SBT were collected during commercial operations in 2022 after 4, 8 and 10 weeks of ranching and the transcription of interleukin 1 beta (il1b), interleukin 8 (il8), tumour necrosis factor 2 (tnf2), immunoglobulin m (igm), major histocompatibility complex 2 (mhc2) and t cell receptor beta (tcrb) were measured from the anterior kidney and gills using reverse transcription quantitative polymerase chain reaction (RT-qPCR). Cardicola forsteri and C. orientalis infection was quantified as number of eggs/mm of SBT gill filament, number of adult flukes in SBT heart and C. forsteri and C. orientalis ITS-2 rDNA copy number/mg in SBT gills. Expression of il8 and igm in anterior kidney was positively correlated to C. forsteri and C. orientalis eggs/mm of SBT gill filament and to the copy number of C. forsteri ITS-2 rDNA in SBT gills. Expression of igm and mhc2 was higher in SBT positive for adult flukes and the expression of igm, mhc2 and tcrb in the anterior kidney was positively correlated to the number of adult flukes in SBT heart. Expression of tcrb in gills was negatively correlated to the number of adult flukes in SBT heart and negatively correlated to the copy number of C. orientalis ITS-2 rDNA. Expression of the adaptive immune genes igm , mhc2 and tcrb increased over the ranching duration in the kidney of SBT and the innate immune genes il1b and il8 decreased over the ranching duration in SBT gills. Despite evidence of an immune response to C. forsteri and C. orientalis, SBT were not able to clear infections during the standard industry ranching timeline. • Gene expression of SBT was related to the different life stages of Cardicola spp. • Expression of il8 and igm in SBT kidney and gill was correlated to Cardicola spp. egg life stage. • Expression of igm, mhc2 and tcrb was correlated to Cardicola spp. adult life stage. • Expression of tcrb was negatively correlated to C. orientalis DNA in SBT gill. • Ranching time increased expression of igm, mhc2 and tcrb in kidney and decreased il1b and il8 in gill.