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Diseases of Aquatic Organisms

    DAO prepress abstract   -  DOI: https://doi.org/10.3354/dao03501

    Bonamia exitiosa in farmed native oysters Ostrea angasi in Australia: optimal qPCR epidemiological cut-point and risk factors for clinical disease

    T. L. Bradley*, J. A. Mercer, J. D. Humphrey, N. J. G. Moody, J. C. Hunnam

    *Corresponding author:

    ABSTRACT: Bonamiosis has developed as a problem in Australian native oysters Ostrea angasi since the parasite Bonamia spp. was first detected in Port Phillip Bay, Victoria in the early 1990s. At that time, large scale mortalities in both farmed and wild oysters saw the demise of the pilot native oyster culture industry. More recent attempts to farm the species resulted in sub-clinical infections that over time progressed to clinical disease. The aim of this work was to establish what environmental factors resulted in the clinical manifestation of disease; to determine the diagnostic sensitivity and diagnostic specificity of histopathological examination and a real-time polymerase chain reaction (qPCR) test for the diagnosis of B. exitiosa infection in clinically diseased farmed native oysters; and to calculate the optimal qPCR threshold cycle (CT) epidemiological cut-point for classification of positive and negative cases. After applying a range of stressors to tank-held oysters, the results indicated a 58% increased risk (95% CI: 16%, 99%) of a Bonamia-infected oyster dying if the oyster was held at a higher temperature (p = 0.05). Starving and tumbling oysters, in isolation, was not significantly associated with clinical bonamiosis, but a Bonamia-infected oyster was at the greatest risk of death when an increased water temperature was combined with both starvation and increased motion (p = 0.02; odds ratio = 3.47). The diagnostic sensitivity and specificity of the OIE qPCR were calculated for increasing CT value cut-points from £25 to £40, with an optimal cut-point identified at £34.5 (Specificity: 92.2; 95% PCI: 76.2, 99.8; Sensitivity: 93.5; 95% PCI: 84.7, 99.1).