Haplosporidian Parasite of Rock Oysters
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Category
Category 3 (Host Not in Canada)
Common, generally accepted names of the organism or disease agent
Haplosporidian of rock oysters.
Scientific name or taxonomic affiliation
Originally identified as Haplosporidium sp. (Hine and Thorne 2000, 2002), the parasite was subsequently described as Minchinia occulta (Bearham et al. 2008b). However, as indicated by Hine et al. (2009) the taxonomy of the haplosporidians needs a thorough revision.
Geographic distribution
Various locations in northern areas of Western Australia.
Host species
Saccostrea cuccullata.
Impact on the host
This parasite was associated with epizootics (up to 80% mortality) among S. cuccullata from a few locations in north Western Australia and subsequent investigations detected prevalences of infection ranging from 27.1% (n = 332) to 3.2% (n = 31) in near-by locations (Hine and Thorn 2000). In heavily infected oysters, plasmodia and sporulation stages occupied greater than 50% of the connective tissue. There was little if any haemocyte infiltration in response to sporulation but brown cells were abundant at the sites of infection (Hine and Thorne 2002). Jones and Creeper (2006) and Bearham et al. (2008b) speculated that this parasite would be a major impediment to the development of a tropical rock oyster aquaculture industry and threatens the viability of any rock oyster farms located in northern Western Australia.
Diagnostic techniques
Histology
Minchinia occulta was observed as focal infections in the connective tissue of the gills and as disseminated infections in the mantle, reproductive follicles, and around the digestive diverticulae. Minchinia occulta was also observed in the epithelial cells of the gut but not in the epithelium of the digestive diverticulae with sporulation confined to the connective tissue of the digestive gland. The non-spore stages were unusual in that they consisted primarily of uni- and bi-nucleate stages reminiscent of Bonamia spp. Multi-nucleate plasmodia were round or ovoid (6.3 to 15.4 µm in diameter) and sporulation stages consisted of sporonts (17.0 to 22.5 µm in diameter), sporont syncytium (20.0 to 31.0 µm in diameter), and spore clusters (28.0 to 31.8 µm in diameter). Spores were ovoid to circular or flask-shaped and measured 4.5-6.7 µm by 3.3-4.1 µm in size (Hine and Thorne 2002, Bearham et al. 2008b).
Electron Microscopy
Multi-nucleate plasmodia were irregular in outline (amoeboid), contained haplosporosomes, had lipid droplets, and the smooth and/or anastomosing endoplasmic reticulum was sparse. These plasmodia were diplokaryotic, with the two nuclei separated by osmophilic material about 50 nm thick. Sporonts had few or no haplosporosomes and contained 2 to 13 spherical nuclei per section that were usually grouped as diplo-, tetre- or poly-karya. Dense material also occurred between closely apposed surfaces of the tightly grouped nuclei. In multi-nucleate sporonts, the nuclei were separated and distributed throughout the cytoplasm. Sporoblasts (sporocysts) were uni-nucleate or large bi-nucleate forms that could be distinguished from sporonts by the lack of a thickened plasma membrane. Spores were ovoid to circular in shape (4.5-5.0 µm by 3.5-4.1 µm) with a single equatorial nucleus (1.5-2.3 µm in diameter) and had a characteristic operculum, a few filamentous wrappings and a rod-like structures in the posterior sporoplasm. The wall of mature spores comprised of inner (90 nm thick), middle (30 nm thick) and outer (130 nm thick) layers and a surface fibrous/filamentous coat of microtubular-like structures giving them a furry appearance that may degrade as the spore matures (Hine and Thorne 2002, Bearham et al. 2008b). A ridge is also present on one side of the spore beginning just below the operculum and ending approximately 2 µm from the aboral end of the spore (Bearham et al. 2008b). In addition to these structures, Hine et al. (2009) indicated that: the uni- and bi-nucleate stages have indentations of the nuclear surface near the nuclear membrane-bound Golgi; in the spore, formative bodies from the spherulosome (spherule) develope into haplosporosomes; the sporoplasm plasma membrane was tight against the spore wall; and exsporulation occurs in the host.
DNA Probes
Molecular probes provide a means to detect the cryptic life stages of haplosporidian parasites and to identify them. The small ribosomal subunit (SSU) region of the rRNA gene was used for the development of molecular tools because it has both conserved and variable regions. An in situ hybridisation (ISH) assay consisting of a oligonucleotide probe was constructed from the SSU rRNA gene and a polymerase chain reaction (PCR) assay was developed. The phylogenetic analysis of the SSU region indicated that the parasite belonged to the genus Minchinia (Bearham et al. 2007, 2008a). Bearham et al. (2008a) determined that the the oligonucleotide ISH and PCR assays were significantly more sensitive than histology for detecting the parasite. Molecular analysis of archived samples indicate that this parasite was the same as the one reported as Haplosporidium sp. by Hine and Thorn (2002) (Bearham et al. 2007).
Methods of control
No known methods of prevention or control.
References
Bearham, D., Z. Spiers, S. Raidal, J.B. Jones and P.K. Nicholls. 2007. Molecular characterisation of a haplosporidian parasite infecting rock oysters Saccostrea cuccullata in north Western Australia. Journal of Invertebrate Pathology 95: 33-40.
Bearham, D., Z. Spiers, S. Raidal, J.B. Jones and P.K. Nicholls. 2008a. Detection of Minchinia sp., in rock oysters Saccostrea cuccullata (Born, 1778) using DNA probes. Journal of Invertebrate Pathology 97: 50-60.
Bearham, D., Z. Spiers, S.R. Raidal, J.B. Jones and P.K. Nicholls. 2008b. Spore ornamentation of Minchinia occulta n. sp. (Haplosporidia) in rock oysters Saccostrea cuccullata (Born, 1778). Parasitology 135: 1271-1280.
Hine, P.M. and T. Thorne. 2000. A survey of some parasites and diseases of several species of bivalve mollusc in northern Western Australia. Diseases of Aquatic Organisms 40: 67-78.
Hine, P.M. and T. Thorne. 2002. Haplosporidium sp. (Alveolata: Haplosporidia) associated with mortalities among rock oysters Saccostrea cuccullata in north Western Australia. Diseases of Aquatic Organisms 51: 123-133.
Hine, P.M., R.B. Carnegie, E.M. Burreson and M.Y. Engelsma. 2009. Inter-relationships of haplosporidians deduced from ultrastructural studies. Diseases of Aquatic Organisms 83: 247-256.
Jones, J.B. and J. Creeper. 2006. Diseases of pearl oysters and other molluscs: a Western Australian perspective. Journal of Shellfish Research 25: 233-238.
Citation Information
Bower, S.M. (2014): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Haplosporidium sp. of Rock Oysters.
Date last revised: April 2014
Comments to Susan Bower
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