Oyster Egg Disease


Category 1 (Not Reported in Canada)

Common, generally accepted names of the organism or disease agent

Oyster egg disease, Microsporidiosis of oyster eggs.

Scientific name or taxonomic affiliation

Many reports of parasites within the ova of various bivalves including mussels and clams/cockles occur in the literature. Because many of the reported parasites in the ova of oysters have not been specifically identified, they have been alphabetically listed below. Each reported geographic location and host species was consistently assigned the same letter code.

  1. Undescribed ovarian parasites (Becker and Pauly 1968, Imai et al. 1968, Matsusato and Masumura 1981). Matsusato and Masumura (1981) speculated that the parasite in C. gigas from Japan may belong to the Subclass Coccidiida.
  2. Unidentified protistan parasite in ova was initially thought to be a coccidian (Wolf 1977a, b) but is now believed to be Marteilioides chungmuensis or a closely related species.
  3. Steinhausia ( =Chytridiopsis) ovicola in the Family Chytridiopsidae (Sprague et al. 1972, Lauckner 1983).
  4. Steinhausia-like parasite (Anderson et al. 1995, Green et al. 2008).
  5. Microsporidea occurring in ova (Nascimento et al. 1986).
  6. Steinhausia-type microsporidians (Sabry et al. 2011).
  7. Unknown ovarian parasite (da Silva et al. 2012).
  8. Steinhausia-like microsporidian in the ova but the morphology of the parasite presented in the published image is not typical of Steinhausia (Moss et al. 2007).

Geographic distribution

  1. Humboldt Bay, California; Japan including Matsushima and Hiroshima bays.
  2. Northern Territory, Australia.
  3. Marennes, France.
  4. Queensland, Australia.
  5. Todos os Santos Bay, Bahia, Brazil.
  6. Santa Catarina Island, Brazil.
  7. Santa Catarina Island, Brazil.
  8. Southern China (the exact source location for the sample was unknown).

Host species

  1. Crassostrea gigas
  2. Crassostrea echinata.
  3. Ostrea edulis.
  4. Saccoctrea glomerata (=commercialis).
  5. Crassostrea rhizophorae.
  6. Crassostrea rhizophorae and Crassostrea gigas.
  7. Crassostrea gigas.
  8. Crassostrea hongkongensis.

Impact on the host

All infections occur in the cytoplasm of mature oyster ova. Occasionally surrounding tissue exhibit a marked, intense, haemocyte infiltration response and some parasitized eggs were necrotic (Green et al. 2008). Da Silva et al. (2012) noted that only a slight haemocytic infiltration was observed in infected follicles. The intensity of infection is usually low with generally fewer than 1% of the oocytes (=ovocytes) infected. However, the prevalence of infection in female oysters from some locations was considerably higher. For example, in southern China infection was 69% (18 of 26 female C. hongkongensis examined) (Moss et al. 2007) and in the Pimpama River, southeast Queensland between 13 and 67% of the female S. glomerata were infected (Green et al. 2008). Infection appears to be seasonal, corresponding to the gonadal cycle of the oyster, but the reservoir of infection during the nonproliferative gonadal phase of the oyster is unknown. Also enigmatic is the mechanism by which the oyster becomes infected. Vertical transmission (from one generation to the next via the ova) is suspected but not yet proven.

Diagnostic techniques

Gross Observations: Occurrence of abnormally enlarged ovary with a nodular appearance consisting of round whitish swellings up to 25 mm in diameter in some infected C. in Japan from autumn to winter (mainly in September to October - thereafter nodules were concealed by glycogen storage tissue).

Histology: All developmental stages occur within a spherical or subspherical vacuole (about 11 µm in diameter) in the cytoplasm and occasionally in the nucleus of a well developed oocyte or mature ovum. Sporocysts (spherical or ovoid cysts, spherical inclusions - uni-nucleate or multi-nucleate) within the parasitophorous vacuole contain a variable number of developing spores. Infection may distort the nuclear membrane, cell membrane or both, of the oyster oocyte. Usually only one parasitophorous vacuole in each oocyte however, up to five have been observed in a single oocyte of C. commercialis (Anderson et al. 1995). Sporocysts (18 × 20 µm in diameter) of S. ovicola contain 40-60 spherical spores (2-3 µm in diameter). Infection can be accompanied by an intense infiltration of haemocytes throughout the entire gonad. Generally, the morphology of the Microsporida in oyster eggs resembles that of Steinhausia mytilovum in Mytilus edulis and Steinhausia sp. in Cerastoderma edule. However, the unknown ovarian parasite reported by da Silva et al. (2012) in C. gigas from southern Brazil was somewhat reminiscent of early life stages of Marteilioides chungmuensis but did not give positive results to the nested-polymerase chain reaction (PCR) assays for this parasite (da Silva et al. 2012).

Methods of control

Oysters from infected populations, or oysters grown in proximity to infected populations, should not be introduced into unaffected areas.


Anderson, T.J., P.M. Hine and R.J.G. Lester. 1995. A Steinhausia-like infection in the ovocytes of Sydney rock oyster Saccostrea commercialis. Diseases of Aquatic Organisms 22: 143-146.

Becker, C.D. and G.B. Pauley. 1968. An ovarian parasite (Protista incertae sedis) from the Pacific oyster, Crassostrea gigas. Journal of Invertebrate Pathology 12: 425-437.

da Silva, P.M., A.R.M. Magalhães and M.A. Barracco. 2012. Pathologies in commercial bivalve species from Santa Catarina State, southern Brazil. Journal of the Marine Biological Association of the United Kingdom 92: 571-579.

Green, T.J., B.J. Jones, R.D. Adlard and A.C. Barnes. 2008. Parasites, pathological conditions and mortality in QX-resistant and wild-caught Sydney rock oysters, Saccostrea glomerata. Aquaculture 280: 35–38.

Imai, T., K. Mori, Y. Sugawara, H. Tamate, J. Oizumi and O. Itakawa. 1968. Studies on the mass mortality of oysters in Matsushima Bay VII. Pathogenetic investigation. Tohoku Journal of Agricultural Research 19: 250-265.

Lauckner, G. 1983. Diseases of Mollusca: Bivalvia. In: O. Kinne [ed.]. Diseases of Marine Animals. Volume II: Introduction, Bivalvia to Scaphopoda. Biologische Anstalt Helgoland, Hamburg, p. 550-553.

Matsusato, T. and K. Masumura. 1981. Abnormal enlargement of the ovary of oyster, Crassostrea gigas (Thunberg) by an unidentified parasite. Fish Pathology (Tokyo) 15: 207-212.

Maurand, J. and C. Loubès. 1979. Les microsporidies parasites de mollusques. Haliotis 8 (1977): 39-48.

Moss, J.A., E.M. Burreson, J.F. Cordes, C.F. Dungan, G.D. Brown, A. Wang, X. Wu and K.S. Reece. 2007. Pathogens in Crassostrea ariakensis and other Asian oyster species: implications for non-native oyster introduction to Chesapeake Bay. Diseases of Aquatic Organisms 77: 207-223.

Nascimento, I.A., D.H. Smith, F.I. Kern and S.A. Pereira. 1986. Pathological findings in Crassostrea rhizophorae from Todos os Santos Bay, Bahia, Brazil. Journal of Invertebrate Pathology 47: 340-349.

Sabry, R.C., P.M. da Silva, T.C.V. Gesteira, V.D.A. Pontinha and A.R.M. Magalhães. 2011. Pathological study of oysters Crassostrea gigas from culture and C. rhizophorae from natural stock of Santa Catarina Island, SC, Brazil. Aquaculture 320: 43-50.

Sprague, V., R. Ormières and J.F. Manier. 1972. Creation of a new genus and a new family in the Microsporida. Journal of Invertebrate Pathology 20: 228-231.

Wolf, P.H. 1977a. An unidentified protistan parasite in the ova of the blacklipped oyster, Crassostrea echinata, from northern Australia. Journal of Invertebrate Pathology 29: 244-246.

Wolf, P.H. 1977b (printed in 1979). Diseases and parasites in Australian commercial shellfish. Haliotis 8: 75-83.

Citation Information

Bower, S.M. (2012): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Oyster Egg Disease.

Date last revised: November 2012
Comments to Susan Bower

Date modified: