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Diseases of Aquatic Organisms

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DAO 140:151-165 (2020)  -  DOI:

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

T. L. Bradley1,*, J. A. Mercer2, J. D. Humphrey3, N. J. G. Moody4, J. C. Hunnam1

1Department of Jobs, Precincts and Regions, Mickleham Road, Attwood, Victoria 3049, Australia
2Victorian Fisheries Authority, Bellarine Highway, Queenscliff, Victoria 3225, Australia
3Panaquatic Health Solutions 3/814 Glenferrie Rd, Hawthorn, Victoria 3122, Australia
4Australian Animal Health Laboratory (AAHL), Commonwealth Scientific and Industrial Research Organisation (CSIRO), Portarlington Road, East Geelong, Victoria 3220, Australia

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 subclinical infections that progressed over time to clinical disease. The aim of this work was to establish what environmental factors result in the clinical manifestation of disease; determine the diagnostic sensitivity and diagnostic specificity of histopathological examination and a quantitative polymerase chain reaction (qPCR) test for the diagnosis of B. exitiosa infection in clinically diseased farmed native oysters; and 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, 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 = 0.048). 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 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 World Organisation for Animal Health qPCR protocol 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% posterior credible intervals [PCI]: 76.2, 99.8; Sensitivity: 93.5; 95% PCI: 84.7, 99.1).

KEY WORDS: Australia · Native oysters · Bonamia exitiosa · Diagnosis · Risk factors

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Cite this article as: Bradley TL, Mercer JA, Humphrey JD, Moody NJG, Hunnam JC (2020) Bonamia exitiosa in farmed native oysters Ostrea angasi in Australia: optimal epidemiological qPCR cut-point and clinical disease risk factors. Dis Aquat Org 140:151-165.

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