Effects of company and season on blood fluke (spp.) infection in ranched Southern Bluefin Tuna: preliminary evidence infection has a negative effect on fish growth.

Abstract

Aporocotylid blood flukesandare an ongoing health concern for Southern Bluefin Tuna (SBT),, 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 ofspp. 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 ofandITS-2 DNA in SBT hearts and gills. SBT Condition Index decreased as intensity ofspp. DNA in SBT gills increased, suggesting blood fluke infection had a negative effect on SBT growth (Spearman's r = -0.2426, d.f. = 138,= 0.0041). Prevalence and intensity of infection indicated PZQ remained highly effective at controllingspp. infection in ranched SBT, 10 years after PZQ administration began in this industry. Company A had the highest prevalence and intensity ofspp. 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.