Transcriptional immune response of cage-cultured Pacific bluefin tuna during infection by two Cardicola blood fluke species

Infections by two blood fluke species, Cardicola orientalis and Cardicola opisthorchis, currently present the greatest disease concern for the sea-cage culture of Pacific bluefin tuna (PBT) - a species of high global economic importance and ecological concern. In this presentation, we describe our methods for rapidly, quantitatively, and differentially identifying infections by these two parasite species in cultured PBT as well as associated host immune responses. Using real-time qPCR, we were successful in quantitatively detecting parasite specific DNA from within host blood, gill, and heart tissues; positively identifying parasitic infections 44 days earlier than microscopy methods previously employed. Parasite-specific DNA of C. orientalis and C. opisthorchis became prevalent in both heart and gill of PBT within two months of sea-cage culture, which was only mitigated by the administration of anthelmintic Praziquantel. Nevertheless, fish were observed to mount an organ specific transcriptive immune response during infection that mirrored the relative quantity of pathogenic DNA load. In heart, significant (3-6 fold) increases in IgM, MHC2, TCRβ, and IL-8 transcription was observed in infected fish relative to uninfected controls; whereas in the gills only IgM transcription was observed to be induced (11 fold) by infection. Interestingly, the relative quantity of IgM transcription in gill was highly correlated to the relative abundance of C. orientalis but not C. opisthorchis DNA, even though this organ showed high prevalence of DNA from both parasite species. Taken together, these findings indicate that although ineffective at combating infection during primary exposure, a cellular immune response is mounted in PBT as a potential rejoinder to future Cardicola exposure, particularly against C. orientalis. Additionally, the preliminary evidence for mechanism of action of Praziquantel as an anthelmintic in bluefin tuna, as well as its role as an immunostimulant and importance in treating primary parasite exposure during tuna culture will be discussed.