Steinhausia mytilovum (Mussel Egg Disease)


Category 1 (Not Reported in Canada)

Common, generally accepted names of the organism or disease agent

Steinhausia or mussel egg disease.

Scientific name or taxonomic affiliation

Steinhausia (=Chytridiopsis, =Haplosporidium) mytilovum (Sprague 1965, Sprague et al. 1972) and Steinhausia sp. (Microsporida).

Geographic distribution

  1. Eastern United States (Field 1923, Sprague 1965, Figueras et al. 1991a, Sunila et al. 2004) and California, USA (Hillman 1991).
  2. Europe including Italy (Lauckner 1983), Spain (Figueras et al. 1991b, Robledo et al. 1994, Villalba et al. 1997, Sagristà et al. 1998), France (Comtet et al. 2004) and Greece from the Thermaikos Gulf in northern Aegean Sea (Rayyan and Chintiroglou 2003).
  3. Cockburn Sound, Western Australia (Jones 1997, Jones and Creeper 2006).
  4. Amazon River estuary, Brazil (Matos et al. 2005).

Host species

  1. Mytilus edulis and Mytilus sp.
  2. Mytilus galloprovincialis and Mytilus edulis.
  3. Mytilus galloprovincialis.
  4. Mytella guyanensis.

Steinhausia sp. and Steinhausia-like microsporidia have been reported from other bivalves including oysters and cockles and clams.

Impact on the host

This microsporidian infects the cytoplasm and nucleus of mussel ova and can incite a moderate to severe diffuse-type haemocyte infiltration response (De Vico and Carella 2012). This infiltration of granulocytes and large basophilic haemocytes was associated with resorption of the germinal epithelium and ova. However, infected oocytes (=ovocytes) have also been observed in apparently healthy follicles (Sunila et al. 2004, Jones and Creeper 2006). The effect on the viability of individual ova is unknown; however, infection distorts the nucleus of the ovum and can also cause the destruction of the egg. Also, mussel fecundity is believed to be inversely related to the intensity of infection. In Cockburn Sound, Western Australia, the overall prevalence of infection was 44.4% of the female mussels in August and October 1995 (Jones and Creeper 2006). In M. edulis from Long Island Sound, Connecticut, USA with a relatively high prevalence of infection (23%), the proportion of infected ova versus uninfected ova was generally very low (Sunila et al. 2004). Rayyan and Chintiroglou (2003) reported that in addition to a strong hemocyte infiltration response inside infected gonadal follicles, S. mytilovum affected the condition index of infected M. galloprovincialis in the Thermaikos Gulf. However, Comtet (2004) cautioned that the condition index is influenced by numerous factors, both intrinsic (e.g., gametogenic stage, various parasitic infections, etc.) and environmental (e.g. temperature, food availability, pollution, etc.) and that many of these factors were not taken into consideration by Rayyan and Chintiroglou (2003) before they published their conclusions.

The biology of Steinhausia is poorly understood. Transmission likely occurs when loose spores are released along with intact eggs or through phagocytosis and subsequent diapediasis (Jones and Creeper 2006). Vertical transmission (from one generation to the next via the ova sometimes called transgonadal or transovarian transmission) is suspected but not yet proven (Sagristà et al. 1998). In Western Australia, the prevalence of infection did not increase with the size of the mussel suggesting that infection is annual (Jones and Creeper 2006). Matos et al. (2005) suggested that the rate of infection may show a seasonal pattern that is directly related to water temperature, with lower temperatures resulting in a higher prevalence and a larger number of spores per mussel.

Diagnostic techniques

Gross Observations: Infected ova in the mantle tissues of cooked female M. galloprovincialis result in an uneven surface with depressed creamy white patches and swollen tubercles that form spots against the orange-pink background colour of health female tissue (Jones and Creeper 2006).

Histology: Sporocysts (parasitophorous cysts, spherical inclusions, uni-nucleate or multi-nucleate) within the cytoplasm and nucleus of the oocyte or ovum (Sagristà et al. 1998) and may be associated with haemocyte infiltration of the gonadal tissue (inside affected gonadal follicles and in the connective tissue surrounding those follicles). Sporocysts spherical in shape (9-18 µm in diameter) and may contain 10 to 41 spherical spores (about 1-2 µm in diameter). Although there is usually only one sporocyst per ovum, up to three sporocysts have been observed in histological sections of the gonad of M. galloprovincialis. This may be associated with indentation of the ovum nucleus. Peterson et al. (2011) reported that S. mytilovum was easier to detect in histological samples stained with Gram stain (AccustainTM Gram stain for tissue kit, HT90T, Sigma-Aldrich) than in samples stained with other tested stains such as a conventional haematoxylin and eosin method, May-Grunwald Giemsa method, Luna’s method for erythrocytes and eosinophil granules, and Fite’s method for acid fast organisms.

Figure 1. Steinhausia mytilovum (arrows) in Mytilus galloprovincialis from Cockburn Sound, Western Australia. Note the accumulation of haemocytes in the connective tissue adjacent to the infected ova. Image provided by J. B. Jones, Department of Fisheries, Government of Western Australia,

Electron microscopy: The ultrastructure of S. mytilovum was described and illustrated by Sagristà et al. (1998) as follows. The merogonial plasmodium showed endomitosis with a centriolar plaque and intranuclear microtubules. Sporoblasts were diplokaryotic, and round or oval. There was an incipient polar filament in the central region. The prespores, also diplokaryotic, were oval. They showed an electron dense tubular system connected to the polar filament in formation. Mature spores had diplokarya, an electron dense cytoplasm and a coiled isofilar polar filament in the posterior region. Matos et al. (2005) found that the ultrastructure of S. mytilovum in Mytella guyanensis from Brazil was very similar to that described by Sagristà et al. (1998).

Methods of control

Mussels from areas known to be infected (currently or historically) should not be introduced to unaffected areas.


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Comtet, T., C. Garcia, Y. Le Coguic and J.P. Joly. 2004. First record of the microsporidian parasite Steinhausia mytilovum in Mytilus sp. (Bivalvia: Mytilidae) from France. Diseases of Aquatic Organisms 58: 261-264.

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Matos, E., P. Matos and C. Azevedo. 2005. Observations on the intracytoplasmic microsporidian, Steinhausia mytilovum, a parasite of mussel (Mytella guyanensis) ovocytes from the Amazon River Estuary. Brazilian Journal of Morphological Science 22: 183-186.

Peterson, T.S., J.M. Spitsbergen, S.W. Feist and M.L. Kent. 2011. Luna stain, an improved selective stain for detection of microsporidian spores in histologic sections. Diseases of Aquatic Organisms 95: 175-180.

Rayyan, A. and C.C. Chintiroglou. 2003. Steinhausia mytilovum in cultured mussels Mytilus galloprovincialis in the Thermaikos Gulf (northern Aegean Sea, Greece). Diseases of Aquatic Organisms 57: 271-273.

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Sagristà, E., M.G. Bozzo, M. Bigas, M. Poquet and M. Durfort. 1998. Developmental cycle and ultrastructure of Steinhausia mytilovum, a microsporidian parasite of oocytes of the mussel, Mytilus galloprovincialis (Mollusca, Bivalvia). European Journal of Protistology 34: 58-68.

Sprague, V. 1965. Observations on Chytridiopsis mytilovum (Field), formerly Haplosporidium mytilovum Field (Microsporidia?). Journal of Protozoology 12: 385-389.

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.

Sunila, I., L. Williams, S. Russo and T. Getchis. 2004. Production and pathology of blue mussels, Mytilus edulis (L.) in an experimental longline in Long Island Sound, Connecticut. Journal of Shellfish Research 23: 731-740.

Villalba, A., S.G. Mourelle, M.J. Carballal and C. López. 1997. Symbionts and diseases of farmed mussels Mytilus galloprovincialis throughout the culture process in the Rías of Galicia (NW Spain). Diseases of Aquatic Organisms 31: 127-139.

Citation Information

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

Date last revised: November 2012
Comments to Susan Bower

Date modified: