Abstract
Blood flukes cause serious health issues for farmed bluefin tuna worldwide. Their infection severity has been assessed by counting adult flukes in heart flushes. However, Cardicola orientalis adults reside in blood vessels in the gills. The aim of this study was to assess species composition, prevalence and intensity of adult blood fluke infection in gill blood vessels of ranched Southern Bluefin Tuna, Thunnus maccoyii . Based on molecular analysis, out of 41 adult blood flukes found in the gills of the tuna, 32 flukes were confirmed to be C. forsteri , while only 1 fluke was C. orientalis . There was a significant relationship between the number of adult C. forsteri in the heart and in the gills, with the heart having significantly greater prevalence. The presence of adult C. forsteri in the gills can cause blockage of the gill blood vessels and have a significant effect on the host. Furthermore, the presence of adult C. forsteri in gill blood vessels may have implications for the diagnosis of the infection and in particular identification of a negative host. The presence of adult C. forsteri in gill blood vessels should be recorded in the species description and considered when assessing the effects of this parasite on the host.