Preliminary evidence of a life stage specific antibody response to Cardicola spp. (Trematoda: Aporocotylidae) in ranched Southern bluefin tuna, Thunnus maccoyii

Cardicola forsteri (Trematoda: Aporocotylidae) is a significant pathogen for the Australian Southern bluefin tuna (SBT, Thunnus maccoyii) aquaculture industry infecting up to 100 % of fish. This study investigated the relationship between the SBT C. forsteri antibody response and Cardicola spp. infection during an extended ranching period. SBT were sampled at 11 time points during commercial harvest in Port Lincoln, South Australia ranging from 13 to 40 weeks post treatment (PT) with praziquantel. C. forsteri specific serum antibodies were measured using an enzyme linked immunosorbent assay (ELISA) and Cardicola spp. infection was quantified using adult fluke counts from SBT heart and quantitative polymerase chain reaction (qPCR) of C. forsteri or C. orientalis ITS-2 rDNA in SBT gills. C. forsteri specific IgM was significantly higher in SBT positive for C. forsteri ITS-2 rDNA compared to SBT that were negative. C. forsteri specific antibody levels remained elevated throughout the study duration and were significantly higher at week 30 and week 40 PT than week 17 PT. A significant negative correlation between adult fluke infection intensity and C. forsteri specific IgM was identified at week 25 PT and a significant positive correlation was identified at week 40 for C. forsteri ITS-2 rDNA infection intensity and C. forsteri specific IgM. A positive correlation was identified between C. forsteri specific IgM and C. forsteri ITS-2 rDNA infection intensity from 4 to 5 weeks earlier. Cardicola spp. infection intensity peaked at 25 weeks PT before significantly decreasing for the remainder of the study despite 100 % infection prevalence of C. forsteri ITS-2 rDNA from SBT gills.