Viral Infections of Clams


Category 1 (Not Reported in Canada)

Common, generally accepted names of the organism or disease agent

Viruses and virus-like infections of clams.

Scientific name or taxonomic affiliation

  1. Virus-like particles with characteristics of the Papovaviridae and with resemblance to Polyomavirus (Farley 1976, 1978).
  2. Virus-like inclusions and lesions reminiscent of those caused by Paramyxoviridae (Harshbarger et al. 1979)
  3. Virus with characteristics of the Reoviridae especially with strong morphological resemblance to the fish pathogen, infectious pancreatic necrosis virus (IPN) (Hill 1976).
  4. Herpes-like virus infection (Renault et al. 2001).
  5. Birnavirus with serological and biochemical properties similar to infectious pancreatic necrosis virus (AB IPNV) (Lo et al. 1988, Chou et al. 1994).

Geographic distribution

  1. North eastern coast of the United States.
  2. North eastern coast of the United States.
  3. Coast of Great Britain.
  4. Normandy coast of France.
  5. Taiwan.

Host species

  1. Mya arenaria. Other bivalves including Crassostrea virginica and Pinctada maxima were reported to be infected by papova-like viruses.
  2. Mya arenaria.
  3. Tellina tenuis and experimentally infective to Crassostrea gigas.
  4. Venerupis (= Ruditapes, = Tapes) philippinarum larvae. A similar viral infection was associated with mortalities of larval and spat oysters in France and interspecies transmission appears possible.
  5. Meretrix lusoria.

Impact on the host

For most viral infections, the impact on hosts have not been specifically described. However, in a sample of 50 M. arenaria from an area in Massachusetts, U.S. where paralytic shellfish poisoning had occurred and 20 % of the clams had histological characteristics typical of papova-like viral infection in gill epithelial cells, 10 % of the clams had gill hyperplasia (Farley 1976). The herpes-like viral infection in larval V. philippinarum was associated with sporadic high mortalities in a commercial hatchery (Renault et al. 2001). The birnavirus was associated with mass mortalities among farmed M. lusoria and high mortalities could be replicated in the laboratory in small M. lusoria (4 months old with a mean weight of 1.2 grams) especially when the temperature was increased by 8 °C (from 25 to 33 °C) after viral infection (Chou et al. 1994).

Diagnostic techniques


  1. Finely granular, Feulgen-positive, intranuclear inclusions in the connective tissue cells, haemocytes and gill epithelium. Affected cells may have some hypertrophy and marginated chromatin in the particle-filled nucleus.
  2. Feulgen-negative, intranuclear and cytoplasmic inclusions characteristic of measles or distemper lesions of teratomatous glandular tissue.
  3. Inclusion bodies in the cells of the digestive gland.
  4. Presence of abnormal nuclei (condensed or with chromatin margination) in fibroblast-like cells and/or haemocytes throughout the connective tissue and possibly a few epithelial cells of moribund larvae.
  5. None reported but the gills of diseased M. lusoria were described as being abnormally dark grey (usually white) in colour.

Electron Microscopy:

  1. Icosahedral (6- and 5-sided) non-enveloped virons, 40 to 55 nm in diameter, in the nucleus of affected cells.
  2. Not described.
  3. Para-crystalline arrays of virus particles, of hexagonal profile and 50 to 60 nm in diameter, in the cytoplasm of cells of the digestive gland.
  4. The virus appears to induce apoptosis characterised as nuclear changes of chromatin condensation to form dense crescent-shaped aggregates lining the nuclear membrane followed by nuclear collapse. Concurrently, the cell cytoplasm condenses and the cell rounds up but the morphology of the mitochondria and ribosomes is preserved. Nuclei of infected fibroblast-like cells and rarely haemocytes usually contain circular to polygonal empty capsids (82 ± 4 nm in diameter) and nucleocapsids (74 ± 4 nm in diameter). Extracellular viruses were usually enveloped (111 ± 5 nm in diameter) and a tail was rarely observed.
  5. Virus-like particles in membrane-bound structures (1.0 to 1.5 µm in diameter) in the cytoplasm of necrotic gill cells. The unenveloped, hexagonal virus particles (average diameter of 62 nm) were uniform electron dense without cores.


  1. Not reported.
  2. Not reported.
  3. The virus grew best in a bluegill fibroblast cell line but also produced cytopathic effect in Atlantic salmon embryo, fathead minnow and grunt fin cell lines. For details on culture procedures see Hill (1976).
  4. Not reported.
  5. The virus was isolated from diseased clams using the tilapia ovary cell line (To-2) and also replicated in chinook salmon (CHSE-214), rainbow trout gonad (RTG-2) and eel ovary (EO) cell lines at 20 °C (Lo et al. 1988, Chou et al. 1994).

Methods of control

No known methods of prevention or control. Potential interspecies transmission (between larvae of V. philippinarum, Crassostrea gigas, and Ostrea edulis) of the herpes-like virus within a commercial hatchery indicates that precautions should be taken to avoid the spread of the virus between batches of bivalve larvae.


Chou, H.Y., H.J. Li and C.F. Lo. 1994. Pathogenicity of a birnavirus to hard clam (Meretrix lusoria) and effect of temperature stress on its virulence. Fish Pathology (Tokyo) 29: 171-175.

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

Farley, C.A. 1976. Proliferative disorders in bivalve mollusks. Marine Fisheries Review 38 (10): 30-33.

Farley, C.A. 1978. Viruses and viruslike lesions in marine molluscs. Marine Fisheries Review 40 (10): 18-20.

Harshbarger, J.C., S.V. Otto and S.C. Chang 1977 (1979). Proliferative disorders in Crassostrea virginica and Mya arenaria from the Chesapeake Bay and intranuclear virus-like inclusions in Mya arenaria with germinomas from a Maine oil spill site. Haliotis 8: 243-248.

Hill, B.J. 1976. Properties of a virus isolated from the bivalve mollusc Tellina tenuis (da Costa). In: Page, L.A. (ed.), Wildlife Diseases. Plenum Press, New York and London, pp. 445-452.

Lo, C.F., Y.W. Hong, S.Y. Huang and C.H. Wang. 1988. The characteristics of the virus isolated from the gill of clam Meretrix lusoria. Fish Pathology (Tokyo) 23: 147-154.

Renault, T., C. Lipart and I. Arzul. 2001. A herpes-like virus infects a non-ostreid bivalve species: virus replication in Ruditapes philippinarum larvae. Diseases of Aquatic Organisms 45: 1-7.

Citation Information

Bower, S.M. (2010): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Virus Infection of Clams.

Date last revised: February 2010
Comments to Susan Bower

Date modified: