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Virus-Like Disease of Mussels

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Category 1 (Not Reported in Canada)

Common, generally accepted names of the organism or disease agent

Virus-like particles.

Scientific name or taxonomic affiliation

Viruses have been reported from various species of mussels (Renault 2021). In some cases, the etiological agent of the disease has not be identified. Following is a list of these reports, clustered according to similarity of disease aetiology. Available information pertaining to each cluster of reports has the same letter code in subsequent subject headings presented below.

  1. Virus-like particles (Jones et al. 1996, Elston 1997).
  2. Virus morphologically similar to Picornaviridae associated with granulocytomas (Rasmussen 1986).
  3. Oyster herpesvirus type 1 (OsHV-1) (Burge et al. 2011, Saulnier et al. 2011, Evans et al. 2017).

Geographic distribution

  1. Marlborough Sounds and west coast of South Island, New Zealand (Jones et al. 1996).
  2. Lillebælt, Denmark and possibly Great Britain (Rasmussen 1986).
  3. Tomales Bay, northern California, USA (Burge et al. 2011), Bay of Marennes-Oléron, west coast of France (Saulnier et al. 2011) and Georges River estuary, New South Wales, Australia (Evans et al. 2017).

Host species

  1. Perna canaliculus and Mytilus galloprovincialis (Jones et al. 1996).
  2. Mytilus edulis (Rasmussen 1986, Elston 1997).
  3. Mytilus galloprovincialis (Burge et al. 2011), Mytilus edulis (Saulnier et al. 2011) and Mytilus spp. and Trichomya hirsute (Evans et al. 2017).

Impact on the host

  1. Virus-like particles were observed in P. canaliculus spat (15 to 30 mm in shell length) that suffered 50 to 100 % mortality following thinning and reseeding of mussel lines by farmers in January to April 1994. Perna canaliculus adult (75 to 110 mm in length) with mortalities of 2 to 5 % that occurred at the same locations from February to May 1994 were also associated with the suspected viral infection. Identical cell pathology (see below) and virus-like particles were also observed in stunted (25 to 47 mm in length) subtidal M. galloprovincialis from the same area. Heavy mortalities of mussel spat in Marlborough Sounds in the summers of the 1980's suggests that the virus may have been present for many years but was not detection because of its small size and lack of paracrystalline arrays (Jones et al. 1996).
  2. In Denmark, the granulocytomas were observed in about 4 % of the 900 mussels examined. The disease seems to be progressive in infected mussels. However, the low prevalence of infection and no coinciding report of mortalities suggests minimal impact on mussel stocks (Rasmussen 1986). In Great Britain, the granulocytomas occurred in mussels from polluted coastal waters. Samples were not examined with an electron microscope thus, viruses were not detected (Lowe and Moore 1979, Elston 1997).
  3. No impact was reported in Mytilus spp. and Trichomya hirsute but prevalence in the mussels was significantly lower than in oysters (Crassostrea gigas) (Burge et al. 2011, Saulnier et al. 2011, Evans et al. 2017). However, mussels may be important to the transmission and/or persistence of OsHV-1 in endemically infected areas (Evans et al. 2017). Novoa et al. (2016) confirmed that constitutively expressed molecules in naive M. galloprovincialis confer resistance in oysters (C. gigas) to ostreid herpesvirus 1 (OsHV-1) when oyster hemocytes are incubated with mussel haemolymph. Myticin C peptides (antimicrobial peptides) were constitutively expressed in a fraction of mussel haemocytes and showed antiviral activity against OsHV-1, suggesting that these molecules could be responsible for the antiviral activity of mussel haemolymph (Novoa et al. 2016).

Diagnostic techniques


  1. Extensive haemocyte infiltration of the digestive gland connective tissue and the occurrence of multifocal progressive liquefaction necrosis of the digestive gland interstitial cells and the digestive gland tubule basal and epithelial cells. The periacinar zone of affected digestive diverticula have acute cell swelling with cytolysis and sloughing of epithelial cells into the lumen of digestive gland tubules. Sloughed epithelial cells were pyknotic or karyolytic and formed characteristic rounded granular bodies 10 to 15 µm in diameter. No viral inclusion bodies nor Feulgen staining, abnormal DNA accumulations were observed (Jones et al. 1996, Renault 2016).
  2. Granulocytomas (chronic inflammatory conditions) of varying sizes in the vesicular connective tissue (haemolymph spaces) of the digestive gland diverticula and mantle (Lowe and Moore 1979, Rasmussen 1986, Elston 1997, Renault 2016). Note that Lowe and Moore (1979) did not undertaken transmission electron microscopy studies to explore the presence of viral particles (Renault 2016).
  3. Not reported.

Electron microscopy

  1. Electron-dense, uncoated virus-like particles 25 to 47 nm in diameter occur in the cytoplasm, usually adjacent to the cisternae of highly modified endoplasmic reticulum, in sloughed necrotic digestive gland cells. No occlusion bodies or paracrystalline arrays were observed. Extracts from infected material, that were purified by isopycnic centrifugation in CsCl and viewed with an electron microscope after negative staining, revealed large numbers of 25 nm, non-enveloped, virus-like particles with a density of 1.364 g per ml (Jones et al. 1996, Elston 1997, Renault 2016).
  2. Presence of picorna-like viruses within the cytoplasm of the granulocytes making up the granulocytomas. The viron was nonenveloped, the capsid was apparently icosahedral (27 nm in diameter) and no viral DNA was detected by the Feulgen reaction (Rasmussen 1986, Elston 1997). These virus-like particles were enclosed in vesicles, arranged singly or in paracrystalline arrays (Renault 2016).
  3. Not reported.

DNA probes

  1. Not reported.
  2. Not reported.
  3. DNA of the ostreid herpesvirus-1 (OsHV-1), was purified, described, and fully sequenced from oyster larvae in France (Davison et al. 2005). A real-time PCR (quantitative PCR (qPCR)) that is a sensitive assay which quantifies the target (copy number of target DNA sequence) was developed based on the C-region of OsHV-1, a region represented twice in the OsHV-1 genome (Pepin et al. 2008, Burge et al. 2011, Saulnier et al. 2011) or variations of this assay (Evans et al. 2017). The sequence analysis of qPCR products from M. galloprovincialis was identical (100%) to other previously reported OsHV-1 (Burge et al. 2011). OsHV-I viral DNA was detected in a few M. edulis (7 of 120 samples), without the associated viral loads being a sign of replication (<10+2 copies/mg) (Saulnier et al. 2011). Evans et al. (2017) also found that viral quantities in mussels (Mytilus spp., T. hirsuta) were consistently very low (below the quantification limit of the assay; <12 DNA copies per PCR reaction). Saulnier et al. (2001) speculated that M. edulis may be 'healthy carriers' of OsHV-1 and that they could act as a vector species. However, in situ hybridization approaches to depict OsHV-l in tissue sections of M. edulis, in which OsHV-1 DNA was detected by PCR, would be need to be conducted to clarify its tissue trophism at the cellular level (Saulnier et al. 2011).

Methods of control

No known methods of prevention or control.


Burge, C.A., R.E. Strenge and C.S. Friedman. 2011. Detection of the oyster herpesvirus in commercial bivalves in northern California, USA: conventional and quantitative PCR. Diseases of Aquatic Organisms 94: 107-116.

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

Evans, O., I. Paul-Pont and R.J. Whittington. 2017. Detection of ostreid herpesvirus 1 microvariant DNA in aquatic invertebrate species, sediment and other samples collected from the Georges River estuary, New South Wales, Australia. Diseases of Aquatic Organisms 122: 247-255.

Jones, J.B., P.D. Scotti, S.C. Dearing and B. Wesney. 1996. Virus-like particles associated with marine mussel mortalities in New Zealand. Diseases of Aquatic Organisms 25: 143-149.

Lowe, D.M. and M.N. Moore. 1979. The cytology and occurrence of granulocytomas in mussels. Marine Pollution Bulletin 10: 137-141.

Novoa, B., A. Romero, Á.L. Álvarez, R. Moreira, P. Pereiro, M.M. Costa, S. Dios, A. Estepa, F. Parra and A. Figueras. 2016. Antiviral activity of myticin C peptide from mussel: an ancient defense against herpesviruses. Journal Of Virology 90: 7692-7702.

Pepin, J.F., A. Riou and T. Renault. 2008. Rapid and sensitive detection of ostreid herpesvirus 1 in oyster samples by real-time PCR. Journal of Virological Methods 149: 269-276.

Rasmussen, L.P.D. 1986. Virus-associated granulocytomas in the marine mussel, Mytilus edulis, from three sites in Denmark. Journal of Invertebrate Pathology 48: 117-123.

Renault, T. 2016. Chapter 39 - Picornalike Viruses of Mollusks. In: Kibenge, F.S.B., M.G. Godoy (eds.) Aquaculture Virology. Academic Press, San Diego, pp. 529-531.

Renault, T. 2021. Chapter 8. Viruses infecting marine mollusks. In: Hurst, J.C. (ed.) Studies in Viral Ecology, second edition. Wiley Online Library, pp. 275-303.

Saulnier, D., J.-F. Pepin, S. Guesdon, L. Degremont, N. Faury, P. Haffner, T. Renault, M.-A. Travers, D. Tourbiez, P. Geairon, O. Le Moine, J.-L. Seugnet and P. Soletchnik. 2011. Mortalités massives de l'huître creuse - Rapport final du programme de recherche sur l'étude de la cinétique de détection d'agents infectieux associés à des épisodes de mortalités de naissains d'huîtres creuses Crassostrea gigas sur un site ostréicole de Marennes-Oléron (CIDAGINF 2009-2010). Contrat de projets Etat-Région Poitou-Charentes 2007-2013. Ifremer, La Tremblade.

Citation Information

Bower, S.M. (2022): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Virus-Like Disease of Mussels.

Date last revised: November 2022
Comments to Susan Bower

Date modified: