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

Category

Category 3 (Host Not in Canada)

Common, generally accepted names of the organism or disease agent

Australian winter disease or winter mortality, "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. 2010b). Hill et al. (2010b) 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. Although Hill et al. (2010a) suggested that M. roughleyi could be distinguished from B. exitiosa based on characteristics in histopathology, ultrastructure and epizootic data, they indicated that the ultrastructural observations were based on poorly fixed material and that further work was required to confirm the placement of M. roughleyi in the genus Bonamia. More recently, Carnegie et al. (2014) evaluated B. roughleyi in field collections of the type host (Saccostrea glomerata) and other oysters in New South Wales, Australia but were not able to identify a Bonamia sp. small-subunit ribosomal DNA sequence in association with the B. roughleyi observed histologically and a fluorescent in situ hybridization assay designed to detect Mikrocytos mackini was negative. Thus, they suggested that this parasite was not a Bonamia sp. or a Mikrocytos sp. and should be regarded as B. roughleyi nomen dubium (Carnegie et al. 2014, Carnegie and Engelsma 2014). In agreement, Abbott and Meyer (2014) indicated that the identity of this microcell remains unknown and nothing genetically identifiable as B. roughleyi was found in Australia or elsewhere by Hill et al. (2014). Spiers et al. (2014) also concluded that another aetiological agent and a confluence of environmental factors are a more likely explanation for winter mortality in Sydney rock oysters.

Geographic distribution

New South Wales, Australia but outbreaks are patchy within this range. Nell (2001) speculated that there is a northern limit to the distribution of this parasite because the transfer of large numbers of oysters from infested estuaries, for “fattening” during winter, to northern rivers of New South Wales from the mid 1960’s to the mid 1980’s has not spread the disease north of Port Stephens.

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 80% of mature Sydney rock oysters in their third winter (July to September) before marketing (Farley et al. 1988, Hine 1996, Nell 2001). Dry autumns (which cause high salinities), early winters, and low temperatures increase the likelihood of a severe kill (Nell 2001). In severe outbreaks, small spat may also be affected (Nell, 2001). The severity of the kill can vary markedly between years, estuaries, adjacent leases, and even within leases. 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). Overall, the disease is most severe in late winter and early spring (Roughley 1926; Nell 2001, 2007; Spiers et al. 2014).

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). Histological changes in diseased oysters from Georges River examined by Spiers et al. (2014) were reported to be mostly internal, with the presence of hyperplasia, ulceration or erosion of the stomach and intestinal epithelium and hypercellularity of this tissue with activated haemocytes (abundant eosinophilic cytoplasm with eccentric nucleus). Similar changes were occasionally noted in the palps and gills. These characteristic changes began to appear in April and May but were most marked and prevalent in the oysters from August to late September, with milder changes still evident in October (Spiers et al. 2014).  In some diseased oysters, the 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. Similar microcells were observed by Carnegie et al. (2014) and Spiers et al. (2014) in diseased S. glomerata from estuaries in New South Wales, Australia. These authors agreed that the parasite was incorrectly named as a species of Bonamia or Mikrocytos (Carnegie et al. 2014, Spiers et al. 2014).

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).  However, the description of B. roughleyi by Cochennec-Laureau et al. (2003) was apparently based on few observations of poorly fixed material (Hill et al. 2010a) and Engelsma et al (2014) and Hine et al. (2014) expressed doubt that Cochennec-Laureau et al. (2003) actually visualized a Bonamia sp.

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). Also, Carnegie et al. (2014) indicated that it is not clear what Adlard and Lester (1995) detected, as their PCR assay was not Bonamia-specific.The PCR assay originally developed to detect Bonamia ostreae by Cochennec et al. (2000) was thought 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 was applied to the amplicon to distinguish B. roughleyi from other Bonamia spp. (Cochennec-Laureau et al. 2003, Carnegie and Cochennec-Laureau 2004). Nevertheless, Carnegie et al (2014) indicated that the parasite identified as M. roughleyi (=B. roughleyi) by Farley et al. (1988) is not the same parasite genetically detected and identified as B. roughleyi by Cochennec- Laureau et al. (2003) and  that the parasite detected by Cochennec- Laureau et al. (2003) must be considered to be B. exitiosa. Spiers et al. (2014) reported that the microcell-like structures that they observed associated with winter mortality were not detected by an in situ hybridisation (ISH) assay designed to detect organisms in the Bonamia exitiosa−B. roughleyi clade.

Methods of control

Oysters from infected areas (currently or historically) should not be moved to areas with no record of the disease. High mortalities can be reduced by harvesting large oysters before the austral winter (i.e., before the onset of mortality) and by overwintering smaller oysters on up-river leases where lower salinities and higher racks (15 cm above normal elevation) protect them from the disease (Nell 2001). 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

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Citation Information

Bower, S.M. (2015): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Bonamia (=Mikrocytos) roughleyi (Australian Winter Disease) of Oysters.

Date last revised: April 2015
Comments to Susan Bower

Date modified: