Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish

Perkinsus qugwadi (SPX) of Scallops

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

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Category

Category 2 (In Canada and of Regional Concern)

Common, generally accepted names of the organism or disease agent

SPX, Scallop protistan X.

Scientific name or taxonomic affiliation

Perkinsus qugwadi. Because this parasite did not enlarge in fluid thioglycollate medium nor stain blue-black with Lugol's iodine (see Culture section below), and because the DNA sequence of the ITS region of the ribosomal RNA gene is considerably different from that of other species of Perkinsus (see DNA Probes section below), Casas et al. (2002) questioned the classification of this parasite as a species of Perkinsus. Other species of Perkinsus have been described from oysters, clams, abalone and scallops in Atlantic Canada and from the eastern coast of the North Pacific Ocean.

Geographic distribution

Patchy distribution in British Columbia, Canada. To date, detected at 5 of 14 grow-our locations, specifically: Booker Lagoon, Broughton Island; Kanish Bay, Quadra Island; Read Island; Denman Island; and rarely in Cypress Bay. SPX is believed  to be native to British Columbia. Infected Japanese scallop seed from Kanish Bay was inadvertently distributed to several other localities in British Columbia. It is not known if the parasite has now become established in those locations.

Host species

Patinopecten yessonesis (introduced into British Columbia for aquaculture), and probably enzootic in other unknown native species in British Columbia.

Impact on the host

Massive proliferation of protozoan throughout the connective tissue of all organs, overwhelms the scallop and results in death. Many infected scallops have creamy-white pustules up to 5 mm in diameter on all organs but most frequently on the gonad, digestive gland and mantle. Interestingly, the extent of haemocytic response does not necessarily correspond to the level of infection. This disease can cause high mortality in juvenile scallops (often exceeding 90%) but appears to be somewhat less infective to adult P. yessoensis which usually have a lower prevalence of mortality (about 50%). However, SPX is not readily transmitted between scallops except by a low percentage (less than 15%) of heavily infected juvenile scallops (less than 40 mm shell height) in which the zoospores develop. Scallops native to enzootic areas (Chlamys rubida and Chlamys hastata) were resistant to infection and the pathogenic effects of P. qugwadi.

Diagnostic techniques

Gross Observations: Creamy-white pustules up to 5 mm in diameter on various organs are suspect and require histological examination.

Histology: Various organs contain rounds protozoa (trophozoite stage) up to 10 µm in diameter. A large vacuole may displace the centrally located nucleus in some trophozoites to form the "signet ring" stage. Tomonts consisting of clusters of 2-8 small (less than 5 µm diameter) developing trophozoites within a cell membrane may occur. Haemocyte infiltration occurs in some scallops. Zoosporangia and zoospores (tiny flagellated stage about 2-3 µm in body length) have been observed in the tissues of about 10% of the small infected scallops (less than 4 cm in shell height). The flagella of the zoospores are often not evident in histological sections but are evident in tissue imprints that are air dried and stained with most commercial formulations of Wright-Giemsa stain.

Electron Microscopy: The ultrastructure of the zoospore is reminiscent of the zoospores of Perkinsus spp. except that the conoid associated micronemes wrap around the nucleus, some mitochondria are lobulated, an electron-dense body is lacking within the lumen of the flagella kinetosome, and the cortical alveolus-like expansions of the cell membrane occur over the entire surface of the cell. Unlike all other Perkinsus spp., the zoospores develop within the tissues of the scallop.

DNA Probes: A sequence of the ITS region of the ribosomal RNA gene has been deposited in the GenBank database of the National Center for Biotechnology Information with accession number AF151528. A potential DNA probe identified from this sequence has not yet been tested as a diagnostic tool because P. qugwadi has not been detected since the late 1990s. Phylogenetic analyses based on the ITS region of P. qugwadi consistently place this species at the base of a clade containing the other Perkinsus spp. (Coss et al. 2001; Casas et al. 2002a,b; Dungan et al. 2002).

Culture: To date, all attempts to establish P. qugwadi in cultures have been unsuccessful. In addition, P. qugwadi had no prezoosporangia development in fluid thioglycollate medium (FTM) nor subsequent dark staining with Lugol’s iodine as described by Ray (1966) and used to diagnose all other known Perkinsus spp. Also, the prezoosporangia and zoosporangia that developed within the tissues of small scallops did not stain blue-black with Lugol's iodine (Blackbourn et al. 1998).

Methods of control

The extreme pathogenicity of this parasite to P. yessoensis requires that caution be taken to prevent the spread of this disease. Progeny of P. yessoensis that survived an epizootic outbreak of P. qugwadi had a significant increase in resistance to infection and resulting mortalities. Hybrid scallops, resulting from a cross between P. yessoensis females (from the same group of scallops that survived an epizootic outbreak of P. qugwadi) and Patinopecten caurinus males (native to British Columbia), had similar resistance to P. qugwadi (Bower et al. 1999). The identification of scallop stocks that are resistant to P. qugwadi has facilitated the development of a scallop culture industry in British Columbia.

 

References

Blackbourn, J., S.M. Bower and G.R. Meyer. 1998. Perkinsus qugwadi sp.nov. (incertae sedis), a pathogenic protozoan parasite of Japanese scallops, Patinopecten yessoensis, cultured in British Columbia, Canada. Canadian Journal of Zoology 76: 942-953.

Bower, S.M. and G.R. Meyer. 1994. Causes of mortalities among cultured Japanese scallops, Patinopecten yessoensis, in British Columbia, Canada. In: Bourne, N.F., B.L. Bunting and L.D. Townsend (eds.). Proceedings of the 9th International Pectinid Workshop, Nanaimo, B.C., Canada, April 22-27, 1993. Canadian Technical Reports of Fisheries and Aquatic Science 1994: 85-94.

Bower, S.M., J. Blackbourn, D.J.H. Nishimura and G.R. Meyer. 1990. Diseases of cultured Japanese scallops (Patinopecten yessoensis) in British Columbia, Canada. In: A. Figueras (ed.). Abstracts, Fourth International Colloquium on Pathology in Marine Aquaculture. 17­21 September 1990, Vigo (Pontevedra), Spain, Instituto de Investigaciones Marinas-CSIC, Vigo, p. 67-68.

Bower, S.M., J. Blackbourn, G.R. Meyer and D.J.H. Nishimura. 1992. Diseases of cultured Japanese scallops (Patinopecten yessoensis) in British Columbia, Canada. Aquaculture 107: 201-210.

Bower, S.M., J. Blackbourn and G.R. Meyer. 1998. Distribution, prevalence, and pathogenicity of the protozoan Perkinsus qugwadi in Japanese scallops, Patinopecten yessoensis, cultured in British Columbia, Canada. Canadian Journal of Zoology 76: 954-959.

Bower, S.M., J. Blackbourn, G.R. Meyer and D.W. Welch. 1999. Effect of Perkinsus qugwadi on various species and strains of scallops. Diseases of Aquatic Organisms 36: 143-151.

Bower, S., E. Burreson and K. Reece. 2003. Annex 10: Review of molecular techniques used to differentiate the various species/isolates of Perkinsus. Report of the Working Group on Pathology and Diseases of Marine Organisms, Aberdeen, UK, 11-15 March 2003. Mariculture Committee, ICES CM 2003/F:03, Ref. ACME, pg. 54-60 ( for electronic version see: http://www.ices.dk/products/CMdocs/2003/F/F0303.PDF (pg 60 of 101)).

Casas, S.M., A. Villalba and K.S. Reece. 2002. Study of perkinsosis in the carpet shell clam Tapes decussatus in Galicia (NW Spain). I. Identification of the aetiological agent and in vitro modulation of zoosporulation by temperature and salinity. Diseases of Aquatic Organisms 50: 51-65.

Casas, S.M., J.F. La Peyre, K.S. Reece, C. Azevedo and A. Villalba. 2002b. Continuous in vitro culture of the carpet shell clam Tapes decussatus protozoan parasite Perkinsus atlanticus. Diseases of Aquatic Organisms 52: 217-231.

Coss, C.A., J.A.F. Robledo and G.R. Vasta. 2001. Fine structure of clonally propagated in vitro life stages of a Perkinsus sp. isolated from the baltic clam Macoma balthica. The Journal of Eukaryotic Microbiology 48: 38-51.

Dungan, C.F., R.M. Hamilton, K.L. Hudson, C.B. McCollough and K.S. Reece. 2002. Two epizootic diseases in Chesapeake Bay commercial clams, Mya arenaria and Tagelus pledius. Diseases of Aquatic Organisms 50: 67-78.

 

Citation Information

Bower, S.M. (2003): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Perkinsus qugwadi (SPX) of Scallops.

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

Date last revised:  Fall 1994
Comments to Susan Bower