Haplosporidian Infection of Mussels


Category 2 (In Canada and of Regional Concern)

Common, generally accepted names of the organism or disease agent

Haplosporidian Infection of Mussels.

Scientific name or taxonomic affiliation

  1. Haplosporidium tumefacientis.
  2. Haplosporidium sp.
  3. Minchinia sp.
  4. Haplosporidian-like parasite

Geographic distribution

  1. Coast of California, USA.
  2. Coast of Maine, USA.
  3. Lagoon of Thau, Mediterranean coast of France.
  4. Atlantic Canada.

Host species

  1. Mytilus californianus.
  2. Mytilus edulis.
  3. Mytilus galloprovincialis
  4. Mytilus edulis.

Impact on the host

  1. Causes tumefactions (varying in size from small discrete nodules to contiguous swellings involving the entire organ) in the digestive gland and kidney. Prevalence low (about 2%) and effect on the host population is believed to be negligible. There is no evidence of infectivity to other species of Mytilus.
  2. No detectable host response to infection.
  3. Two percent of the sampled mussels appeared thin and exhibited an abnormal whitish colour. Wet mounts containing fresh squashes of tissue from the visceral mass indicated that some of them were infected with spores of Minchinia sp. (Comps and Tigé 1997).
  4. Detected in only one mussel that had an unusual macroscopic appearance. This was the first detection of this type of parasite in mussels after 12 years of histological examinations conducted on mussels in Atlantic Canada. Although the infection was massive, there was no associated haemocyte defense response (Stephenson and McGladdery 2002).

Diagnostic techniques

Gross Observations:

  1. The digestive gland of infected mussels is light brown to tan in colour in comparison to the dark greenish-brown of uninfected mussels.
  2. None reported.
  3. Some infected mussels appeared thin and exhibited an abnormal whitish colour.
  4. Unusual macroscopic appearance.

Wet Mount:

  1. Spores (about 10 µm long) operculate, with one or two filaments compactly wrapped around the spore in parallel bands.
  2. Not known.
  3. Spore ovoid, 8 to 10 µm in length and 4 to 5 µm in width with two polar filamentous structures estimated to be 80 to 100 µm in length.
  4. Not known.


  1. Small plasmodia (6-9 µm) are subepithelial in the digestive gland. As the nuclei divide, the plasmodium enlarges and displaces the epithelial cells until the sporocyst (up to 80 µm in diameter and containing over 100 developing spores) bulges into the lumen of the digestive gland tubule.
  2. Plasmodia with 5-60 nuclei (18 ± 1 µm mean diameter) were located in the connective tissue between the digestive gland tubules, at the tips of the gills, and in the external epithelium of the mantle.
  3. Plasmodia not described but sporocysts containing maturing spores were reported to be associated with cellular debris in the connective tissue surrounding the digestive tubules (Comps and Tigé 1997).
  4. Spores were operculate (3-5 µm by 6-8 µm) and showed a diversity of forms from roughly spherical to pyriform with or without filamentous extensions. They resembled the Haplosporidium sp. described from M. edulis in Maine and Haplosporidium tumefacientis from M. californianus in California (Stephenson and McGladdery 2002).

Electron Microscopy:

  1. Not reported.
  2. Not reported.
  3. Comps and Tigé (1997) reported on the ultrastructure of the spore and indicated that the main feature for diagnosis was epispore cytoplasm characterised by two opposite polar extensions supported by bundles of microtubule-like structures. Little information was obtained concerning sporogenesis.
  4. Not reported.

DNA Probes:

  1. Not reported.
  2. Not reported.
  3. Not reported.
  4. In situ hybridization conducted by the OIE Reference Laboratory for Haplosporidiosis at the Virginia Institute for Marine Science using Haplosporidium nelsoni and Haplosporidium costale specific DNA probes did not hybridize with the mussel parasite from Atlantic Canada (Stephenson and McGladdery 2002).

Methods of control

No known methods of prevention or control.


Comps, M. and G. Tigé. 1997. Fine structure of Minchinia sp., a haplosporidan infecting the mussel Mytilus galloprovincialis L. Systematic Parasitology 38: 45-50.

Figueras, A.J., C.F. Jardon and J.R. Caldas. 1991. Diseases and parasites of mussels (Mytilus edulis, Linnaeus, 1758) from two sites on the east coast of the United States. Journal of Shellfish Research 10: 89-94.

Sherburne, S.W. and L.L. Bean. 1986. A synopsis of the most serious diseases occurring in Maine shellfish. Fish Health Section, American Fisheries Society Newsletter 14: 5.

Stephenson, M.F. and S.E. McGladdery. 2002. Detection of a previously undescribed haplosporidian-like infection of a blue mussel (Mytilus edulis) in Atlantic Canada. Journal of Shellfish Research 21: 389. (Abstract).

Taylor, R.L. 1966. Haplosporidium tumefacientis sp. n., the etiologic agent of a disease of the California sea mussel, Mytilus californianus Conrad. Journal of Invertebrate Pathology 8: 109-121.

Citation Information

Bower, S.M. (2007): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Haplosporidian Infection of Mussels.

Date last revised: September 2007
Comments to Susan Bower

Date modified: