Bonamia ostreae and Bonamia exitiosa are significant pathogens of oysters that cause high mortality rates and substantial economic losses to the oyster farming industry globally. As such, both pathogens are listed by the OIE (the World Organisation for Animal Health). In Australia, infection by a Bonamia sp. was responsible for the devastation of experimental aquaculture of the Native Oyster Ostrea angasi and adjacent wild beds in Victoria in the early 1990s. Indeed, surveys have found Bonamia sp. in Native Oysters in Tasmania, WA, and NSW and recent monitoring of apparently healthy stock on Victorian aquaculture sites has determined that prevalence of this Bonamia sp. is high and once again is causing clinical disease. While the pathogen was identified as Bonamia sp. its relationship to the OIE-listed species, B. ostreae and B. exitiosa, is unclear. It is important to determine the identification of the Australian Bonamia to species level, to establish whether or not it is the presumed exotic species B. ostreae, B. exitiosa, or a different species endemic to Australia. Identification of the Australian species will clarify Australia’s international obligations as a member country to the OIE, with respect to reporting and international trade. Moreover, the conditions that trigger clinical disease are unknown. Identification of the risk factors associated with the onset of disease in infected oysters is needed to implement control strategies to minimise the impact on production and industry value. This information is essential for the development of improved biosecurity and farming practices that mitigate against disease caused by Bonamia and provide industry and regulators with management and control strategies.
Project number:
2015-001
Project Status:
Completed
Budget expenditure:
$380,091.21
Principal Investigator:
Tracey Bradley
Organisation:
Agriculture Victoria
Project start/end date:
30 Jun 2015
-
30 Aug 2017
Contact:
FRDC
SPECIES
1. Obtain nucleic acid sequence and compare with other, described Bonamia sp. and determine their taxonomic relationship and ensure that available diagnostic tools are suitable.
2. Improve understanding of Bonamiasis infestations in Native Oysters including the determination, under controlled conditions, of the stressors that induce clinical disease in sub - clinically infected oysters.
3. Develop a biosecurity plan and farm management practices to manage the risk of infestation and the mitigation of clinical infection with Bonamia sp.
ISBN:
978-1-76090-307-7
Authors:
Bradley TL
Mercer JA
Hunnam JC
Moody NJG
Mohr PG
Neave MJ
Williams LM
Crane MStJ
Buss JJ4
Wiltshire KH
Prowse TAA
Tanner JE
Harris JO and Deveney MR
Report
•
2021-05-01
•
3.19 MB
2015-001-DLD.pdf
This project was a collaborative study across three research organisations: Agriculture Victoria, CSIRO (Victoria) and the South Australian Research and Development Institute (SARDI). The overall aim of this collaborative project was to better understand many aspects of infection with the parasite Bonamia exitiosa in Native Oysters (Ostrea angasi). The genesis of this project was the detection of clinical bonamiosis in farmed Native Oysters in Port Phillip Bay, Victoria in 2015. At the time the species of this parasite was unknown, and it had caused extensive mortalities (presumed over 80%) in stock on one farm.
CSIRO demonstrated that current diagnostic PCR assays are sufficient and effective in the detection and identification of Bonamia species from farmed O. angasi in southern Australia. The species of Bonamia parasite present in farmed O. angasi between 2013 and 2017 in Victoria, New South Wales and South Australia was confirmed to be B. exitiosa. Next generation sequencing (NGS) and bioinformatic analysis of nucleotides extracted from B. exitiosa infected and uninfected O. angasi facilitated assembly of the first draft genome of a member of the family Haplosporidiidae, B. exitiosa. For the first time a draft genome was also assembled for O. angasi as a result of the sequencing strategy undertaken to identify the B. exitiosa genome. Unexpectedly, a near complete genome was also assembled for an Epsilon proteobacterium, Poseidonibacter from O. angasi tissues infected with B. exitiosa. This organism may have been an adventitious discovery or potentially proliferated in flat oysters weakened by B. exitiosa infection.
In Victoria, 4 individual tank and field trials were undertaken between 2016 and 2018. All trials were designed to investigate proposed risk factors for the development of clinical bonamiosis under normal farming or controlled laboratory conditions. The tank trials utilised presumed sub-clinically infected and healthy oysters and subjected them to stressors such as heat, starvation and turbulence. Further tank trials examined oyster origin and size as risk factors. The field trials utilised existing farm sites and practices to investigate a number of proposed risk factors of interest to the farmers. The field trials were conducted on a known clinically infected farm and examined the risk factors including basket density, basket depth in the water column, oyster size and level of fouling. Concurrently, the project validated the diagnostic performance of the Bonamia sp. qPCR and established the optimal epidemiological qPCR cycle threshold (CT) value to differentiate between a positive and negative result.
SARDI undertook a range of trials in both the field and under laboratory conditions. Oysters were tested from 3 farms to assess diagnostic sensitivity (DSe) and specificity (DSp) of heart smears, histopathology and qPCR individually or in combination, and to assess prevalence. Tank trials were utilised to develop a cohabitation infection model using uninfected hatchery-reared recipient animals and infected donor animals from farms to better understand infection dynamics. The Pacific oyster, Crassostrea gigas, was shown to be susceptible to Bonamia exitiosa by cohabitation in the laboratory. A decontamination trial was undertaken in the laboratory using heavily infected Native Oysters to assess processes for decontaminating equipment that may have been exposed to Bonamia exitiosa. These 5 approaches were combined in a field trial where O. angasi were deployed at Cowell, Coffin Bay and Streaky Bay to examine the prevalence of B. exitiosa over time related to measured environmental parameters and growth rates. SARDI also assessed 3 different diagnostic tests for detecting Bonamia sp: heart smears, histology and qPCR. In this work the effect of combining tests to maximise overall diagnostic performance was also investigated.
