Perkinsus sp. of European Oysters
Category 1 (Not Reported in Canada)
Common, generally accepted names of the organism or disease agent
Perkinsus of European flat oysters.
Scientific name or taxonomic affiliation
Perkinsus mediterraneus. Phylogenetic analysis showed that this parasite was most closely related, but a distinct sister clade to the Perkinsus olseni (=atlanticus) group that occurs in clams from the same area.
Mediterranean coasts of Spain (including Balearic Islands) and France. If the Perkinsus-like parasite reported by Alderman and Gras (1969) was P. mediterraneus then the distribution would also include the coast of Portugal and the northern Atlantic coast (Cancale) of France.
Ostrea edulis and possibly Crassostrea angulata. Also detected in the clam Venus verrucosa from Menorca (Balearic Islands, Spain) and sometimes in mixed infections with P. olseni (Cao et al. 2008).
Impact on the host
A Perkinsus-like parasite was first encountered in Portuguese oysters (C. angulata) being examined for the cause of of high mortalities attributed to "gill disease" and was also detected in one O. edulis from Cancale, France (Alderman and Gras 1969 - Note: in this publication the parasite was identified as Labyrinthomyxa-like, which was the genus name for this group of parasites at that time). Infection evoked haemocytic infiltration of the connective tissue in the proximity of the parasite and trophozoites were frequently observed in the cytoplasm of haemocytes. The infection seems to be seasonal with peak prevalance (up to 70% of the oysters infected) in the late summer and autumn however, associated mortalities were not reported (Casas et al. 2004).
Histology: Trophozoites (spherical cells, 7.9 ± 0.34 µm in diameter), each contained a large vacuole, containing a vacuoplast, that displaced the nucleus to the periphery of the cell conferring a "signet-ring" appearance, were located in the connective tissue of various organs (visceral mass, gills, labial palps, mantle lobes). The trophozoites were covered in patches of a wall-like structure. Tomonts (= multicellular stages) contain daughter cells in a rosette-like arrangement inside the wall-like structure of the mother cell. Trophozoite morphology does not have taxonomic value because it can be influenced by the host, the time of the year, and nutrient availability (Villalba et al. 2004).
Electron Microscopy: Prezoosporangium has a thick wall with external structures that resembles a glycocalyx. A marked characteristic of all species of Perkinsus is the ultrastructure of the zoospore. In P. mediterraneus, the nucleus is positioned in the posterior part of the zoospore and both flagella and the apical ribbon are situated at the anterior end. Ribbon-associated vesicles, toxicysts and rhoptries extend from anterior to posterior areas of the zoospore. Cortical alveoli are present on the anterior part of the cellular surface.
DNA Probes: A Perkinsus genus-specific DNA probe effective in labeling other species of Perkinsus via in situ hybridisation also hybridised with this parasite in O. edulis. Parsimony analysis of DNA sequences from the small subunit ribosomal RNA gene and the internal transcribed spacer (ITS) region of the ribosomal gene unit confirmed that this parasite was distinct from any Perkinsus ITS sequence previously published and that P. mediterraneus formed a monophyletic group comprising a related but distinct sister clade to the P. olseni (=atlanticus) group. Abollo et al. (2006) developed a species-specific polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assay of the rRNA ITS region that used a combination of two endonucleases (Rsa I plus Hinf I) to discriminate P. olseni and P. mediterraneus. More recently, Casas et al. (2008) reported that phylogenetic analysis on three different data sets indicated that P. mediterraneus has a close genetic relationship to Perkinsus honshuensis.
Culture: After incubation in Fluid Thioglycollate Medium (FTM) supplemented as described by Ray (1966) for 1 week followed by staining with Lugol's iodine stain, blue-black prezoosporangia were 97.4 ± 1.99 µm in diameter and almost doubled is size (167.1 ± 8.09 µm in diameter) if incubated in FTM for 2 weeks. Although not true propagating cultures, this procedure is used for the diagnosis of many species of Perkinsus but may also detect other organisms (Villalba et al. 2004). Zoosporulation occurred in prezoosporangia transferred from FTM to sterile filtered (0.22 µm) seawater containing antibiotics. The discharge tube was one sixth the diameter of the zoosporangium, the lowest ratio observed for any Perkinsus species. Biflagellated zoospores (ellipsoid to ovoid in shape, 4.4 ± 0.18 µm in length) were liberated through the discharge tube 5 to 6 days after the prezoosporangia were transferred to seawater and incubated at 22°C in the dark.
Casas et al. (2008) described a procedure for the continuous in vitro culture of P. mediterraneus in a protein free medium. Although parasite viability was high (greater than 90%) in vitro, the rate of proliferation was low with densities increasing 2- to 6-fold between subcultures at 6 week intervals.
Methods of control
No known method of prevention or control.
Abollo, E., S.M. Casas, G. Ceschia and A. Villalba. 2006. Differential diagnosis of Perkinsus species by polymerase chain reaction-restriction fragment length polymorphism assay. Molecular and Cellular Probes 20: 323-329.
Alderman, D.J. and P. Gras. 1969. "Gill Disease" of Portuguese oysters. Nature 224: 616-617.
Apakupakul, K., A. Villalba, S. Casas and K.S. Reese. 2001. Molecular analysis of a Perkinsus species in the European flat oyster Ostrea edulis. Journal of Shellfish Research 20: 537-538. (Abstract).
Cao, A., E. Abollo, B.G. Pardo, P. Martinez and A. Villalba. 2008. Identification of the Perkinsus spp. occurring in the Spanish coast and evaluation of their intraspecific variability. Journal of Shellfish Research 27: 994. (Abstract).
Casas, S.M., A. Grau, K.S. Reece, K. Apakupakul, C. Azevado and A. Villalba. 2004. Perkinsus mediterraneus n. sp., a protistan parasite of the European flat oyster Ostrea edulis from the Balearic Islands, Mediterranean Sea. Diseases of Aquatic Organisms 58: 231-244.
Casas, S.M., K.S. Reece, Y. Li, J.A. Moss, A. Villalba and J.F. La Peyer. 2008. Continuous culture of Perkinsus mediterraneus, a parasite of the European flat oyster Ostrea edulis, and characterization of its morphology, propagation, and extracellular proteins in vitro. Journal of Eukaryotic Microbiology 55: 34-43.
Ray, S.M. 1966. A review of the culture method for detecting Dermocystidium marinum, with suggested modifications and precautions. Proceedings of the National Shellfisheries Association 54: 55-69.
Villalba, A., K.S. Reece, M.C. Ordás, S.M. Casas and A. Figueras. 2004. Perkinsosis in molluscs: A review. Aquatic Living Resources 17: 411-432.
Bower, S.M. (2007): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Perkinsus sp. of European oysters.
Date last revised: November 2010
Comments to Susan Bower
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