ABSTRACT
Aporocotylid blood flukes Cardicola forsteri and C. orientalis are an ongoing health concern for the Southern Bluefin Tuna (Thunnus maccoyii, SBT) industry, where infections can lead to morbidity and mortality in ranched SBT populations. This study compared blood fluke infection in SBT from two companies during the 2021 ranching season. Both companies administered the same dosage of praziquantel approximately 5 weeks after transfer, feeding with frozen baitfish daily; the only difference in the company's practices was that the pontoons were located 2.5 km apart. Infection severity was measured as prevalence and intensity by quantifying adult C. forsteri in SBT heart and copy numbers of C. forsteri and C. orientalis ITS-2 DNA in SBT heart and gills. Data from the 2018 and 2019 harvests of SBT were used to make comparisons with 2021 harvest data. Cardicola orientalis was detected at transfer and no longer detected after treatment with praziquantel. Cardicola spp. were present in 83% of sampled SBT in 2021. Both companies demonstrated similar patterns of infection, and Company A had higher prevalence and intensity of Cardicola spp. infection. Based on C. forsteri ITS-2 DNA, infection intensity at harvest was significantly greater for both companies in 2021 when compared to 2018 and 2019. Continued monitoring of Cardicola spp. in SBT and improvements in diagnostics contribute to our understanding of Cardicola spp. epizootiology and the detection of changes in treatment efficacy.
ABSTRACT
Aporocotylid blood flukes Cardicola forsteri and C. orientalis are an ongoing health concern for Southern Bluefin Tuna (SBT), Thunnus maccoyii, ranched in Australia. Therapeutic application of praziquantel (PZQ) has reduced SBT mortalities, however PZQ is not a residual treatment therefore reinfection can occur after the single treatment application. This study documents the epidemiology of Cardicola spp. infection in ranched SBT post treatment over three ranching seasons (2018, 2019 and 2021). Infection prevalence (percentage of SBT affected) and intensity (parasite load) was determined by adult fluke counts from heart, egg counts from gill filaments and the use of specific quantitative polymerase chain reaction (qPCR) for detection of C. forsteri and C. orientalis ITS-2 DNA in SBT hearts and gills. SBT Condition Index decreased as intensity of Cardicola spp. DNA in SBT gills increased, suggesting blood fluke infection had a negative effect on SBT growth (Spearman's r = -0.2426, d.f. = 138, p = 0.0041). Prevalence and intensity of infection indicated PZQ remained highly effective at controlling Cardicola spp. infection in ranched SBT, 10 years after PZQ administration began in this industry. Company A had the highest prevalence and intensity of Cardicola spp. infection in 2018, and Company G had the highest in 2019. No consistent pattern was seen in 2021. Overall, intensity of infection did not increase as ranching duration increased post treatment. Results from this study improve our knowledge of the biology of blood flukes and helps the SBT industry to modify or design new blood fluke management strategies to reduce health risks and improve performance of SBT.
