Partial 18S rRNA sequences of apicomplexan parasite 'X' (APX), associated with flat oysters Ostrea chilensis in New Zealand

A phylogenetically unresolved apicomplexan (APXSc) causing swirl lesions in the Tehuelche scallop, Aequipecten tehuelchus, from the Southwest Atlantic coast

The study reports a previously unknown apicomplexan (APXSc) parasite infecting wild scallops Aequipecten tehuelchus (d’Orbigny, 1842) from two separate areas (La Tapera and Punta Conos) of the San José gulf, in Patagonia Argentina. Histology, transmission electron microscope, molecular analyses and in situ hybridization were performed to describe the morphology of APXSc, and confirm its phylogenetic status. The prevalence of APXSc infection was 24% and 72% in scallops from La Tapera and Punta Conos, respectively. Seasonal variation was observed for scallops from La Tapera, recording highest prevalence in summer. A positive relationship between the presence of the APXSc and the size of the scallops was observed. A SSU rDNA consensus sequence of 1758 base pairs was generated which has a 94.8% identity to sequences obtained from a pathogenic apicomplexan parasite infecting Ostrea chilensis in New Zealand, but not closely related to other apicomplexans. The asexual reproduction, i.e. merogony, occurs in the Tehuelche scallop whilst the gamogonic and sporogonic stages were absent, suggesting a yet unknown definitive host. Severe host inflammation response involving fibroblast-like hemocytes surrounding the APXSc in the form of granuloma-like “swirls” is characteristic for this apicomplexan infection. Further studies are needed to reveal the life cycle, and presumable pathogenicity of APXSc.

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A novel apicomplexan (APXSc) parasite infecting wild scallops Aequipecten tehuelchus.

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Positive relationship between the presence of the APXSc with the colder season and the size of the scallops.

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SSU rDNA sequence matched with 94.8% identity to sequences from an apicomplexan infecting Ostrea chilensis in New Zealand.

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The Tehuelche scallop is the intermediate host for APXSc.

Coccidian Parasite in Sea Cucumber (Apostichopus japonicus) Ovaries

We investigated an unknown ellipsoidal body that is sometimes found in the ovaries of the sea cucumber Apostichopus japonicus. Its external morphology, comprising an ellipsoidal dark central body (about 150 µm in length) and a surrounding transparent layer (about 50 µm in thickness), resembled that of a protozoan cyst, particularly an oocyst. Histological observations of the developing A. japonicus ovaries clarified that a small mass of organisms appeared in the cytoplasm of young oocytes, proliferated in these cells through budding, became rod shaped and arranged radially, and, finally, formed an outer layer. These processes were considered to be the formation of a cyst by a protozoan parasite. The small subunit ribosomal RNA (18S rRNA) gene was amplified from the DNA extracted from unknown ellipsoidal bodies by using polymerase chain reaction with universal primers for eukaryote 18S rRNA. The determined sequence was not identical to any of the known sequences in DNA databases, but it clustered in a clade of coccidian species belonging to Eucoccidiorida in phylogenetic analyses. From these results, we concluded that the unknown ellipsoidal body is a cyst (possibly an oocyst) of a coccidian parasite (order Eucoccidiorida) that is formed in the A. japonicus oocyte, though its lower taxonomic position is uncertain. In a survey of the gonads of wild A. japonicus at Esashi, Hokkaido, during the reproductive season, these cysts were detected in more than 50% of females but were never found in males. We consider that the cysts of this parasite can only be formed in A. japonicus ovaries.

PCR and histology identify new bivalve hosts of Apicomplexan-X (APX), a common parasite of the New Zealand flat oyster Ostrea chilensis

Apicomplexan-X (APX) is a significant pathogen of the flat oyster Ostrea chilensis in New Zealand. The life cycle and host range of this species are poorly known, with only the zoite stage identified. Here, we report the use of molecular approaches and histology to confirm the presence of APX in samples of green-lipped mussels Perna canaliculus, Mediterranean mussels Mytilus galloprovincialis and hairy mussels Modiolus areolatus collected from widely distributed locations in New Zealand. The prevalence of APX infection estimated by PCR was 22.2% (n = 99) and 50% (n = 30) in cultured green-lipped mussels from Nelson and Coromandel, respectively; 0.8% (n = 258), 3.3% (n = 150) and 35.3% (n = 17) in wild Mediterranean mussels from Nelson, Foveaux Strait and Golden Bay, respectively; and 46.7% (n = 30) in wild hairy mussels from Foveaux Strait. Histology detected all cases of PCR that were positive with APX and appeared to be more sensitive. The prevalence of APX estimated by histology in green-lipped mussels from Coromandel was 60% versus 50% by PCR, and 4.3%, 10.7% and 52.9% by histology versus 0.8%, 3.3% and 35.3% by PCR in wild Mediterranean mussels from Nelson, Foveaux Strait and Golden Bay, respectively. The specific identity of the parasite found in mussels was determined by sequencing PCR products for a portion (676 bp) of the 18S rRNA gene; the resulting sequences were 99-100% similar to APX found in flat oysters. Phylogenetic analyses also confirmed that all isolates from green-lipped, Mediterranean and hairy mussels grouped with APX isolates previously identified from flat oysters. This study indicates the wide geographical distribution of APX and highlights the potentially multi-host specific distribution of the parasite in commercially important bivalve shellfish.

PCR test to specifically detect the apicomplexan 'X' (APX) parasite found in flat oysters Ostrea chilensis in New Zealand

Described here is a polymerase chain reaction (PCR) test to detect the apicomplexan-X (APX) parasite of a flat oyster species, Ostrea chilensis, endemic to New Zealand. The test primers target sequences in the in situ hybridisation probes identified to bind specifically to APX 18S rRNA and amplify a 723 bp DNA product. The test did not amplify 18S rRNA gene sequences of other apicomplexan species, including Toxoplasma gondii, Neospora caninum, Selenidium spp., Cephaloidophorida spp., Lecudina spp. and Thiriotia sp. Of 73 flat oysters identified by histology to be infected with APX at different severities, 69 (95%) tested PCR-positive. Failure to amplify an internal control indicated the presence of PCR inhibitors in the 4 PCR-negative samples. The high analytical sensitivity, specificity and speed of the PCR test should make it a useful tool for detecting APX.