Virus-Like Disease of Mussels


Category 1 (Not Reported in Canada)

Common, generally accepted names of the organism or disease agent

Virus-like particles.

Scientific name or taxonomic affiliation

  1. Virus-like particles.
  2. Virus morphologically similar to Picornaviridae associated with granulocytomas.

Geographic distribution

  1. Marlborough Sounds and west coast of South Island, New Zealand.
  2. Lillebælt, Denmark and possibly Great Britain.

Host species

  1. Perna canaliculus and Mytilus galloprovincialis.
  2. Mytilus edulis.

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) 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.
  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. 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.

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.
  2. Granulocytomas of varying sizes in the vesicular connective tissue of the digestive gland diverticula and mantle.

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.
  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.

Methods of control

No known methods of prevention or control.


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

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.

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.

Citation Information

Bower, S.M. (2001): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Virus-Like Disease of Mussels.

Date last revised: July 2001
Comments to Susan Bower

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