Infection with Bonamia exitiosa in the Australian Native Oyster (Ostrea angasi)

Author: Jessica Buss

Buss, Jessica, 2020 Infection with Bonamia exitiosa in the Australian Native Oyster (Ostrea angasi), Flinders University, College of Science and Engineering

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Pathogenic Haplosporidia in Bonamia Pichot, Comps, Tigé, Grizel & Rabouin, 1980 cause epizootics in mollusc populations worldwide. A Bonamia sp. was described in Ostrea angasi Sowerby, 1871 from Port Phillip Bay, Victoria in 1991 and threatens the Australian O. angasi aquaculture industry. Most Bonamia spp. research originates in Europe, North America and New Zealand, leaving Bonamia in Australia poorly understood. A pilot survey of farmed O. angasi using a Bonamia quantitative polymerase chain reaction (qPCR) test detected a Bonamia sp. in South Australia in December 2015 and sequencing identified Bonamia exitiosa Hine, Cochennac & Berthe, 2001.

My study began with investigations of diagnostic performance and prevalence and intensity estimates. Surveys of three O. angasi farms in South Australia by histology, heart smear and qPCR were analysed using a latent class model to assess diagnostic sensitivity (DSe) and specificity (DSp) of single tests and test combinations. Histology had the highest DSe and DSp of any single test, but performance was improved by combining histology and qPCR and defining a case as positive if either test returned a positive result. Farmed O. angasi in South Australia have high B. exitiosa prevalence but low intensity of infection. Understanding diagnostic test performance informs design of better surveillance programs.

European Bonamia ostreae Pichot, Comps, Tigé, Grizel & Rabouin, 1980 infections in Ostrea edulis Linnaeus, 1758 build slowly, but infection dynamics of B. exitiosa-O. angasi were undescribed. An infection model was developed in which naïve recipient O. angasi were cohabited with B. exitiosa infected donor O. angasi. I monitored B. exitiosa prevalence and intensity in recipients over 40 days exposure. Bonamia exitiosa rapidly infects, and causes clinical disease and death in O. angasi. First infection is <10 days, and continuous exposure causes increasing B. exitiosa prevalence and intensity. Host death is not required for transmission. The infection model provides a basis for B. exitiosa-O. angasi studies including selecting Bonamia resistant oysters.

Seasonally variable infections are described for Bonamia spp.-oyster systems, but B. exitiosa-O. angasi infections were undescribed. Naïve O. angasi were placed on four B. exitiosa endemic South Australian farms and prevalence and intensity were monitored seasonally for one year. Bonamia exitiosa prevalence increased to >0.50 after one year. Bonamia exitiosa intensities were <4.0 after the first season but did not increase further. Sites with slower B. exitiosa prevalence increase may be preferable for O. angasi farming. Bonamia exitiosa must be considered in planning industry expansion and reef restoration.

Decontamination guidelines for Bonamia spp. were lacking. I assessed three disinfectants for dose-duration efficacy. An iodine and a chlorine based disinfectant provided 100% efficacy against B. exitiosa. Stability makes iodine based products preferred disinfectants. Effective decontamination guidelines for B. exitiosa can improve farm biosecurity and management decisions.

My research provides the first information on B. exitiosa in O. angasi. Information from my thesis creates a basis for future B. exitiosa research in Australia and provides the oyster industry with a foundation for informed decision making and clear direction for industry expansion.

Keywords: Bonamia exitiosa, Ostrea angasi, Australia, Diagnostic performance, Cohabitation, Farm Trial, Decontamination

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2020
School: College of Science and Engineering
Supervisor: Assoc. Prof. James Harris