Cardicola forsteri is a blood fluke that is highly prevalent among farmed southern bluefin tuna (SBT), in South Australia. The role of C. forsteri in annual SBT mortality outbreaks, which peak 6 to 12 weeks post transfer into farms, is unknown. The objective of this study was to identify lesions unique to cultured SBT that died during a mortality event in 2009 and to determine the significance of C forsteri-associated lesions. In this study, all farmed SBT had branchitis and myocarditis due to C forsteri, whereas no life stages of C forsteri were histologically identified in any wild-caught SBT. Mortality was associated with the presence of severe branchitis (P<.005), and the odds of severe branchitis were 90 times greater for SBT that died than for SBT that were live caught during peak mortality (95% confidence interval, 5 to 1,684). In SBT that had died, no lesions other than those associated with C forsteri were of sufficient severity or physiologic significance to account for death. This study shows for the first time that a substantial proportion of post stocking mortality in cultured SBT is strongly associated with severe branchitis caused by C forsteri.
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