Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish

Bonamia (=Mikrocytos) roughleyi (Australian Winter Disease) of Oysters

Category | Common Name | Scientific Name | Distribution | Host Species
Impact on Host | Diagnostic Technique | Methods of Control | References | Citation

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Category

Category 3 (Host Not in Canada)

Common, generally accepted names of the organism or disease agent

Australian winter disease, "Microcell" disease of Sydney rock oysters.

Scientific name or taxonomic affiliation

Bonamia roughleyi (Cochennec-Laureau et al. 2003) originally described as Mikrocytos roughleyi (Farley et al. 1988). Apparently, Bonamia exitiosa and B. roughleyi are genetically very similar (Abollo et al. 2008, Hill et al. 2010). Hill et al. (2010) grouped these two named species as the B. exitiosa/B. roughleyi clade and indicated that justification for drawing species boundaries among the primarily austral microcells with affinities to B. exitiosa and B. roughleyi remains elusive.

Geographic distribution

New South Wales, Australia.

Host species

Saccostrea glomerata (=commercialis).

Impact on the host

A systemic intracellular infection in the haemocytes that is associated with focal abscess-type lesions in the gill, connective, and gonadal tissues and alimentary tract. The disease is associated with low temperatures and high salinities (30-35 ppt). It can kill up to 70% of mature Sydney rock oysters in their third winter (July to September) before marketing (Farley et al. 1988, Hine 1996). Outbreaks are patchy within the range where it occurs and although some mortalities occur in winter, most of the oysters do not die until the warmer spring weather of September or October (Nell 2002).

Diagnostic techniques

Gross Observations: The disease is characterized by pale digestive gland, severe ulcerations and/or abscess lesions especially in the gonad region, mantle (frequently near the adductor muscle) and gills and by impaired adductor muscle contraction (Mackin 1959).

Histology: Diapedesis was marked and often with secondary infections of the ciliate Hexamita sp. (Mackin 1959). Abscesses contain microcells (organisms 1-2 µm in diameter with spherical nucleus greater than 1 µm containing bipolar or eccentric nucleolar structures) within the haemocytes. Some microcells have a cytoplasmic vacuole that displaces the nucleus to the periphery of the cell.

Electron microscopy: Like Bonamia ostreae, B. roughleyi has electron-dense haplosporosomes and mitochondria both organelles which are lacking in Mikrocytos mackini (Cochennec-Laureau et al. 2003).

DNA Probes: Polymerase chain reaction (PCR) diagnosis that identifies B. roughleyi infected oysters by the presence of a short amplicon (ca. 680 bp) was described by R.D. Adlard and R.J.G. Lester (Dept. of Parasitology, Univ. of Queensland, Brisbane, Qld 4067, Australia). Although the assay was able to detect a single B. roughleyi in 400 host cells, further refinement of the technique (by employing primers designed on the nucleotide sequence of the parasite and the use of nested primers) is required (Adlard and Lester 1995). The PCR assay originally developed to detect Bonamia ostreae by Cochennec et al (2000) is now known to amplify the DNA of B. roughleyi. The sequence of the amplicon (951 nucleotides in length) had 95.2% and 98.4% sequence similarities with B. ostreae and Bonamia exitiosa, respectively (Cochennec-Laureau et al. 2003). However, restriction fragment length polymorphism (RFLP) analysis can be applied to the amplicon to distinguish B. roughleyi from other Bonamia spp. (Cochennec-Laureau et al. 2003, Carnegie and Cochennec-Laureau 2004).

Methods of control

Oysters from infected areas (currently or historically) should not be moved to areas with no record of B. roughleyi. High mortalities can be reduced by harvesting large oysters before the austral winter and by overwintering smaller oysters on up-river leases where lower salinities and higher racks protect them from the disease. Nell et al. (1994) reported no differences in mortality between diploid and triploid groups of S. glomerata but triploid oysters can reach market size 6-18 months faster and maintained better meat condition than diploid oysters without an increased risk of winter mortality. Hand et al (1998) found that triploid S. glomerata had a higher survival rate (only 12.2% average cumulative mortality over the second winter and spring period) than diploids (35.0% average cumulative mortality over the same period) during natural exposure to B. roughleyi. They proposed that the reduction in mortality during the second year of culture combined with the growth and condition advantages of triploidy could provide the Sydney rock oyster industry in New South Wales, Australia with a significant improvement in profitability. Smith et al. (2000) indicated that considerable protection against M. roughleyi could be gained by raising the tidal height for growing oysters over winter in southern New South Wales, Australia growing areas, and that the growth advantages of triploid S. glomerata could be utilized without an increased mortality level caused by this disease. In an attempt to control the disease, a breeding program was established in 1997 (Nell et al. 2000). Nell and Perkins (2006) reported that progeny of third-generation selected S. glomerata breeding lines had evaluated resistance to disease caused by both Bonamia roughleyi and M. sydneyi compared to a non-selected control. But, selection for resistance to B. roughleyi did not appear to confer resistance to M. sydneyi and the converse also applied (Nell and Perkins 2006).

References

Abollo, E., A. Ramilo, S.M. Casas, P. Comesaña, A. Cao, M.J. Carballal and A. Villalba. 2008. First detection of the protozoan parasite Bonamia exitiosa (Haplosporidia) infecting flat oyster Ostrea edulis grown in European waters. Aquaculture 274: 201–207.

Adlard, R.D. and R.J.G. Lester. 1995. Development of a diagnostic test for Mikrocytos roughleyi, the aetiological agent of Australian winter mortality of the commercial rock oyster, Saccostrea commercialis (Iredale & Roughley). Journal of Fish Diseases 18: 609-614.

Anderson, I.G. 1990. Diseases in Australian invertebrate aquaculture. In: Proceedings and Abstracts, Fifth International Colloquium on Invertebrate Pathology and Microbial Control, Society for Invertebrate Pathology, 20­24 August 1990, Adelaide, Australia, p. 38-48.

Carnegie, R.B. and N. Cochennec-Laureau. 2004. Microcell parasites of oysters: recent insights and future trends. Aquatic Living Resources 17: 519-528.

Cochennec, N., F. LeRoux, F. Berthe and A. Gerard. 2000. Detection of Bonamia ostreae based on small subunit ribosomal probe. Journal of Invertebrate Pathology 76: 26-32.

Cochennec-Laureau, N., K.S. Reece, F.C.J. Berthe and P.M. Hine. 2003. Mikrocytos roughleyi taxonomic affiliation leads to the genus Bonamia (Haplosporidia). Diseases of Aquatic Organisms 54: 209-217.

Farley, C.A., P.H. Wolf and R.A. Elston. 1988. A long-term study of "microcell" disease in oysters with a description of a new genus, Mikrocytos (g.n.) and two new species Mikrocytos mackini (sp.n.) and Mikrocytos roughleyi (sp.n.). U.S. National Marine Fish Service Bulletin 86: 581-593.

Hand, R.E., J.A. Nell, I.R. Smith and G.B. Maguire. 1998. Studies on triploid oysters in Australia. XI. Survival of diploid and triploid Sydney rock oysters (Saccostrea commercialis (Iredale and Roughley)) through outbreaks of winter mortality caused by Mikrocytos roughleyi infestation. Journal of Shellfish Research 17: 1129-1135.

Hill, K.M., R.B. Carnegie, N. Aloui-Bejaoui, R.E. Gharsalli, D.M. White, N.A. Stokes and E.M. Burreson. 2010. Observation of a Bonamia sp. infecting the oyster Ostrea stentina in Tunisia, and a consideration of its phylogenetic affinities. Journal of Invertebrate Pathology 103: 179–185.

Hine, P.M. 1996. Southern hemisphere mollusc diseases and an overview of associated risk assessment problems. Revue Scientifique et Technique de l'Office International des Epizooties 15: 563-577.

Mortensen, S., I. Arzul, L. Miossec, C. Paillard, S. Feist, G. Stentiford, T. Renault, D. Saulnier and A. Gregory. 2007. Molluscs and crustaceans, 5.3.11 Bonamiosis due to Mikrocytos roughleyi. In: Raynard, R., T. Wahli, I. Vatsos, S. Mortensen (eds.) Review of disease interactions and pathogen exchange between farmed and wild finfish and shellfish in Europe. VESO on behalf of DIPNET, Oslo. p. 370-371. (For electronic publication see www.dipnet.info under "Documents", subgroup "Reports and project deliverables").

Mackin, J.G. 1959. Mortalities in oysters. Proceedings of the National Shellfisheries Association 50: 21-40.

Nell, J. 2002. The Australian oyster industry. World Aquaculture 33: 8-10.

Nell, J.A. and B. Perkins. 2006. Evaluation of the progeny of third-generation Sydney rock oyster Saccostrea glomerata (Gould, 1850) breeding lines for resistance to QX disease Marteilia sydneyi and winter mortality Bonamia roughleyi. Aquaculture Research 37: 693-700.

Nell, J.A., E. Cox, I.R. Smith and G.B. Maguire. 1994. Studies on triploid oysters in Australia. 1. The farming potential of triploid Sydney rock oysters Saccostrea commercialis (Iredale and Roughley). Aquaculture 126: 243-255.

Nell, J.A., I.R. Smith and C.C. McPhee. 2000. The Sydney rock oyster Saccostrea glomerata (Gould 1850) breeding program: progress and goals. Aquaculture Research 31: 45-49.

Smith, I.R., J.A. Nell and R. Adlard. 2000. The effect of growing level and growing method on winter mortality, Mikrocytos roughleyi, in diploid and triploid Sydney rock oysters, Saccostrea glomerata. Aquaculture 185: 197-205.

Citation Information

Bower, S.M. (2012): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Bonamia (=Mikrocytos) roughleyi (Australian Winter Disease) of Oysters.

URL: http://www.pac.dfo-mpo.gc.ca/science/species-especes/shellfish-coquillages/diseases-maladies/pages/mikrouoy-eng.htm

Date last revised:  January 2012
Comments to Susan Bower