Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish

Herpes-Type Virus Disease of Oysters

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

---

Category

Category 3 (Not Reported in Canada)

Common, generally accepted names of the organism or disease agent

Herpes-type virus disease.

Scientific name or taxonomic affiliation

Herpes-type virus or herpes-like viruses. Ostreid Herpes Virus type 1 (OsHV-1) from Crassostrea gigas in France has been described and represents a major class of herpesviruses that is different from those in vertebrates (Davison et al. 2005). However, the apparent lack of host specificity in bivalves and loss of several gene functions in OsHV-1 prompts speculation that this virus may have resulted from interspecies transmission in the context of introduction and intensive culture of non-native bivalve species (Arzul et al. 2001a, 2001b; ICES 2004). It is not known if the herpes-like viruses reported from various species of oysters and other bivalves are the same or different species of virus.

Geographic distribution

a) Coastal waters of Maine and New York, USA.
b) Oyster larvae in hatcheries in New Zealand (Hine et al. 1992) and hatchery-reared oyster larvae and spat in France (Nicolas et al. 1992). A closely related virus was detected in juvenile oysters in Tomales Bay, California, USA (Friedman et al. 2005) and in the eroded gills of adult C. gigas along the northwestern Pacific coast of Baja California, Mexico (Vásquez-Yeomans et al. 2004).
c) Brittany, France and one sample from a commercial hatchery in the United Kingdom.
d) Tasmania and Western Australia.
e) Vicinity of Wellington, New Zealand

Host species

a) Crassostrea virginica.
b) Crassostrea gigas. Ostreid Herpes Virus type 1 (OsHV-1) from C. gigas larvae in France is infective to larvae of other species of oyster (Crassostrea angulata, Crassostrea rivularis and Ostrea edulis) and clam larvae (Ruditapes decussatus and Venerupis (=Ruditapes) philippinarum) under experimental conditions (Arzul et al. 2001a, 2001b)
c) Ostrea edulis.
d) Ostrea angasi.
e) Ostrea (=Tiostrea) chilensis

Impact on the host

The disease quickly spreads among C. gigas larvae in a commercial hatchery in France, indicating a short productive cycle, with cumulative mortalities approaching 100% within a few days. Associated with high mortalities (80 - 90%) among C. gigas and O. edulis spat in France (Renault et al. 1994a, 1994b). Pathology may be related to poor husbandry such as elevated temperatures and crowding and is most prevalent during the summer periods. To date, there has been no indication of impacts at the stock level in Europe. However, during the period 1998-2002, herpes-like viruses were reported from an increasing number of larval bivalve species from around the world, especially in Europe and is now thought to be ubiquitous with associated substantial mortalities in hatcheries (Renault et al. 2000, 2001; ICES 2004). Arzul et al. (2001a , 2001b) have shown that the Ostreid Herpes Virus type 1 (OsHV-1) from C. gigas can infect several bivalve species.

Meyers (1981) reported an apparent herpes virus infection, with no associated disease, in one of 243 adult C. virginica from the south shore of Long Island, New York. Hine and Thorne (1997) detected intranuclear inclusions associated with herpes-like viruses in haemocytes of 23% of the adult O. angasi population that was experiencing low level mortalities associated with Bonamia sp. but with no apparent mortalities attributable to the viral infection. In T. chilensis larvae and spat, the interstitial cells, mantle and digestive tract epithelial cells were infected and infection appeared to be associated with about 95% mortality over 3 to 4 days at 16 to 18 °C among experimentally exposed veligers (Hine et al.1998). On occasion, viral infections may be complicated by bacterial infections that contribute to the severity of morbidity and mortality.

Herpes-like virogenesis begins in the nucleus of infected cells where capsids and nucleocapsids appear. The virions pass into the cytoplasm and are released at the cell surface or by cytolysis.

Diagnostic techniques

Gross Observations: Pale digestive gland in spat and older oysters. Infected larvae stop feeding, swimming and exhibit velar lesions (less extended velum and detached parts of the velum are observed free in the water). Usually significant mortalities occur six days after spawning with peak mortalities (80 to 100%) between day 8 and 12 often during the summer time.

Wet Mounts: Larval velum cells are hypertrophied and detached from the tissue.

Histology: Presumptive diagnosis can be made on the histological observations of intranuclear inclusion bodies that are Feulgen positive, abnormal chromatin pattern (usually emarginated) and hypertrophied (enlarged) nuclei in various cells of oysters (including interstitial cells, connective tissue cells or fibroblasts and epithelium) as tabulated by Hine et al. (1998). The main histological change in spat is the presence of abnormal nuclei (enlarged nuclei associated with marginated chromatin in fibroblast-like cells and highly condensed nuclei in ovoid cells interpreted as haemocytes) throughout the connective tissue especially in the mantle, labial palps, gills and digestive gland. Cellular abnormalities are not associated with massive haemocytic accumulations (Renault and Novoa 2004).

Electron Microscopy: A confirmatory diagnosis is based on the following electron microscopic features in infected cells from bivalves because bivalve viruses have not been cultured. Intranuclear inclusion bodies consisting of icosahedral (6- and 5-sided particles) virions 70-100 nm in diameter, a single coat and, occasionally, a dense nucleoid (or toroidal core). In C. gigas larvae, viral particles (capsid size of 72-97 nm) occur in the nuclei and cytoplasm of connective tissue cells of the velum. Enveloped virons are about 97 nm in diameter. The C. gigas viruses are most similar to the Betaherpesvirinae according to Hine et al. (1992) but are antigenically cross-reactive with channel catfish virus, which belongs to the Alphaherpesvirinae according to Le Deuff et al. (1995).  Intranuclear particles 80 nm in diameter and enveloped cytoplasmic particles 160 to 180 nm in diameter occurred in infiltrating cells with enlarged nuclei (tentatively identified as haemocytes and fibroblasts) in the digestive gland of O. edulis. A tailed envelope extensions (250-350 nm) has been reported on cytoplasmic mature viral particles found in some of the oyster species but not others. Hine et al. (1998), Renault and Novoa (2004) and Davison et al. (2005) presented morphological comparison of herpes-like viruses described from various species of oysters and comparison to other herpesviruses.

DNA Probes: DNA has been extracted from virions purified from fresh, heavily infected C. gigas larvae (Le Deuff and Renault 1999). Sequence data demonstrate that the Ostreid Herpes Virus type 1 (OsHV-1) is not closely related to herpes viruses from vertebrate hosts (including fish) (Renault and Novoa 2004, Davison et al. 2005). The OsHV-1 genes are unspliced whereas those in herpes viruses of mammals and birds contain one interon and fish and amphibians contain two interons. Polymerase Chain Reaction (PCR) and in situ hybridisation (ISH) procedures have been developed for the detection of herpes-like viruses of oysters in France (Renault and Lipart 1998, Renault et al. 2000b, Renault and Arzul 2001, Renault and Novoa 2004). Application of PCR, ISH, and immunochemistry suggested that herpes-like virus occurred in a high prevalence (>75%) of apparently normal C. gigas adults and the presence of viral DNA and viral proteins in the gonad supported the hypothesis of vertical transmission from adults to larvae (Arzul and Renault 2001). However, the detection of viral DNA in parental oysters did not systematically correspond to infection or result in successful transmission to the progeny although the infection status of the parents appeared to influence infection and survival rates of the progeny (Renault and Novoa 2004).

Methods of control

No known methods of prevention or control. The case in Maine was reported following transfer of oysters to thermal effluent from a power station, for grow-out. The cases in New Zealand and France occurred in mid-summer at a time of elevated water temperatures. Moving oysters into cooler waters may reduce the pathogenicity of the virus. Le Deuff et al. (1996) reported 80 to 90 % mortality in C. gigas larvae reared at 25 to 26 °C but not in larvae reared at 22 to 23 °C. However, morbidity and mortality can occur below temperatures at which viral replication is completed and latent infections will persist. In T. chilensis, the opposite appeared to occur because the virus was observed to replicate at ambient temperatures (!6 ° to 18 °C) but not at higher temperatures (Hine et al. 1998). Results of experimental transmission studies using infected C. gigas larvae indicate that the virus remained infectious after several months of storage at -20 °C (Le Deuff et al. 1994). For areas in which the virus is not enzootic, the most sensible management is to prevent its introduction with contaminated broodstock, larvae or seed (juveniles for grow-out).

References

Arzul, I. and T. Renault. 2001. Detection of oyster herpes virus DNA and proteins in Crassostrea gigas adult oysters. Book of Abstracts, European Association of Fish Pathologists, Tenth International Conference "Diseases of Fish and Shellfish". Trinity College Dublin, Ireland, 9 - 14 September 2001. pg. P-207.

Arzul, I., T. Renault, C. Lipart and A.J. Davison. 2001a. Evidence for interspecies transmission of oyster herpesvirus in marine bivalves. Journal of General Virology 82: 865-870.

Arzul, I., T. Renault and C. Lipart. 2001b. Experimental herpes-like viral infections in marine bivalves: demonstration of interspecies transmission. Diseases of Aquatic Organisms 46: 1-6.

Comps, M. and N. Cochennec. 1993. A Herpes-like virus from the European oyster Ostrea edulis L. Journal of Invertebrate Pathology 62: 201-203.

Davison, A.J., B.L. Trus, N. Cheng, A.C. Steven, M.S. Watson, C. Cunningham, R.M. Le Deuff and T. Renault. 2005. A novel class of herpervirus with bivalve hosts. Journal of General Virology 86: 41-53.

Elston, R.A. 1993. Infectious diseases of the Pacific oyster, Crassostrea gigas. Annual Review of Fish Diseases 3: 259-276.

Elston, R. 1997. Special topic review: bivalve mollusc viruses. World Journal of Microbiology and Biotechnology 13: 393-403.

Elston, R.A. 1999. Health management, development and histology of seed oysters. Chapter 10. Herpes-like virus infection of larval and juvenile oysters. pp. 71-74. World Aquaculture Society, Baton Rouge, Louisiana, USA. 110 pp.

Farley, C.A. 1976. Epizootic neoplasia in bivalve molluscs. Progress Experimental Tumor Research 20: 283-294.

Farley, C.A. 1978. Viruses and viruslike lesions in marine mollusks. Marine Fisheries Review 40: 18-20.

Farley, C.A., W.G. Banfield, G. Kasnic Jr. and W.S. Foster. 1972. Oyster Herpes-type virus. Science 178: 759-760.

Friedman, C.S., R.M. Estes, N.A. Stokes, C.A. Burge, J.S. Hargrove, B.J. Barber, R.A. Elston, E.M. Burreson and K.S. Reece. 2005. Herpes virus in juvenile Pacific oysters Crassostrea gigas from Tomales Bay, California, coincides with summer mortality episodes. Diseases of Aquatic Organisms 63: 33-41.

Hine, P.M. and T. Thorne. 1997. Replication of herpes-like viruses in haemocytes of adult flat oysters Ostrea angasi: an ultrastructural study. Diseases of Aquatic Organisms 29: 189-196.

Hine, P.M., B. Wesney and B.E. Hay. 1992. Herpes virus associated with mortalities among hatchery-reared larval Pacific oysters Crassostrea gigas. Diseases of Aquatic Organisms 12: 135-142.

Hine, P.M., B. Wesney and P. Besant. 1998. Replication of a herpes-like virus in larvae of the flat oyster Tiostrea chilensis at ambient temperatures. Diseases of Aquatic Organisms 32: 161-171.

ICES. 2004. Trends in important diseases affecting fish and molluscs in the ICES area 1998-2002. International Council for the Exploration of the Sea, Copenhagen, Denmark. Prepared and edited by the Working Group on Pathology and Diseases of Marine Organisms. For electronic publication see: http://www.ices.dk/pubs/crr/crr265/crr265.pdf.

Le Deuff, R.-M. and T. Renault. 1999. Purification and partial genome characterization of a herpes-like virus infecting the Japanese oyster, Crassostrea gigas. Journal of General Virology 80: 1317-1322.

Le Deuff, R.-M., J.L. Nicolas, T. Renault and N. Cochennec. 1994. Experimental transmission of a Herpes-like virus to axenic larvae of Pacific oyster, Crassostrea gigas. Bulletin of the European Association of Fish Pathologists 14: 69-72.

Le Deuff, R.-M., T. Renault and N. Cochennec. 1995. Antibodies specific for channel catfish virus cross-react with Pacific oyster, Crassostrea gigas, herpes-like virus. Veterinary Research 26: 526-529.

Le Deuff, R.-M., T. Renault and A. Gérard. 1996. Effects of temperature on herpes-like virus detection among hatchery-reared larval Pacific oyster Crassostrea gigas. Diseases of Aquatic Organisms 24: 149-157.

McGladdery, S.E. 1999. Shellfish diseases (viral, bacterial and fungal). In: Woo, P.T.K., D.W. Bruno (eds.) Fish Diseases and Disorders, Volume 3: Viral, Bacterial and Fungal Infections, Vol. 3. CABI Publishing, Wallingford, UK. pp. 723-842.

Meyers, T.R. 1981. Endemic diseases of cultured shellfish of Long Island, New York: adult and juvenile American oysters (Crassostrea virginica) and hard clams (Mercenaria mercenaria). Aquaculture 22: 305-330.

Mortensen, S., I. Arzul, L. Miossec, C. Paillard, S. Feist, G. Stentiford, T. Renault, D. Saulnier and A. Gregory. 2007. Molluscs and crustaceans. In: Raynard, R., T. Wahli, I. Vatsos, S. Mortensen (eds.) Review of disease interactions and pathogen exchange between farmed and wild finfish and shellfish in Europe. VESO on behalf of DIPNET, Oslo. Chapter 5.3.5, pp. 330-333. For electronic version see www.dipnet.info under "Documents" subgroup "Reports and project deliverables".

Nicolas, J.L., M. Comps and N. Cochennec. 1992. Herpes-like virus infecting Pacific oyster larvae, Crassostrea gigas. Bulletin of the European Association of Fish Pathologists 12: 11-13.

Renault, T. and C. Lipart. 1998. Diagnosis of herpes-like virus infections in oysters using molecular techniques. European Aquaculture Society, Special Publication 26: 235-236.

Renault, T. and I. Arzul. 2001. Herpes-like virus infections in hatchery-reared bivalve larvae in Europe: specific viral DNA detection by PCR. Journal of Fish Diseases 24: 161-167.

Renault, T. and B. Novoa. 2004. Viruses infecting bivalve molluscs. Aquatic Living Resources 17: 397-409.

Renault, T., N. Cochennec, R.-M. Le Deuff and B. Chollet. 1994a. Herpes-like virus infecting Japanese oyster (Crassostrea gigas) spat. Bulletin of the European Association of Fish Pathologists 14: 64-66.

Renault, T., R.-M. Le Deuff, N. Cochennec and P. Maffart. 1994b. Herpesviruses associated with mortalities among Pacific oyster, Crassostrea gigas, in France - comparative study. Revue de Médicine Vétérinaire 145: 735-742.

Renault, T., R.-M. Le Deuff, N. Cochennec, B. Chollet and P. Maffart. 1995. Herpes-like viruses associated with high mortality levels in larvae and spat of Pacific oysters, Crassostrea gigas: a comparative study, the thermal effects on virus detection in hatchery-reared larvae. Veterinary Research 26: 539-543.

Renault, T., R.-M. Le Deuff, B. Chollet, N. Cochennec and A. Gérard. 2000a. Concomitant herpes-like virus interactions in hatchery-reared larvae and nursery-cultured spat Crassostrea gigas and Ostrea edulis. Diseases of Aquatic Organisms 42: 173-183.

Renault, T., R.-M. Le Deuff, C. Lipart and C. Delsert. 2000b. Development of a PCR procedure for the detection of a herpes-like virus infecting oysters in France. Journal of Virological Methods 88: 41-50.

Renault, T., C. Lipart and I. Arzul. 2001. A herpes-like virus infecting Crassostrea gigas and Ruditapes philippinarum larvae in France. Journal of Fish Diseases 24: 369-376.

Vásquez-Yeomans, R., J. Cáceres Martínez and A. Figueras Huerta. 2004. Herpes-like virus associated with eroded gills of the Pacific oyster Crassostrea gigas in Mexico. Journal of Shellfish Research 23: 417-419.

Citation Information

Bower, S.M. (2007): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Herpes-Type Virus Disease of Oysters.

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

Date last revised:   February 9, 2009
Comments to Susan Bower