Microsporidiosis of Scallops


Category 3

(Host Not in Canada)

Common, generally accepted names of the organism or disease agent

Microsporidiosis of queen scallops.

Scientific name or taxonomic affiliation

Unidentified microsporidian. The overall dimensions of mature spores with evidence of a diplokaryon and polar tubules consisting of eight coils in a single row, and the presence of this microsporidian in an invertebrate host suggested affiliation with the genus Pseudopleistophora (Lohrmann et al. 2000).

Geographic distribution

Several coastal sites around the United Kingdom.

Host species

Aequipecten opercularis.

Impact on the host

Pathogenicity on wild population of A. opercularis remains unknown despite high prevalences of infection (up to 20%) in some locations. No host reaction to this microsporidian was observed and infected scallops showed no evidence of being in poor condition. Only spore stages (immature and mature) were found. The effect of this parasite on cultured scallops is unknown.

Diagnostic techniques

Histology: Observed only in the connective tissue of the digestive gland. Aggregates of immature spores (2.3 by 1.3 µm) within extremely hypertrophied host cell (up to about 300 µm in diameter) and surrounded by a thin layer of fibroblast-like cells. Mature spores (2.3 by 1.2 µm) clustered in lumen of blood vessels.

Electron microscopy: Immature spores appeared to be in direct contact with host cell cytoplasm. Mature spores were limited by an outer exospore and an electron-lucent endospore covering the plasma membrane. The polar tubule, inserted into the anterior anchoring disc, passed through the center of the spore and in the posterior two thirds of the spore was wound in most cases into 7 to 9 coils aligned in a single row. Two spherical nuclei, located in the central third of the spore between the polaroplast and the posterior vacuole, were closely apposed and flattened in the zone of contact with each other forming a diplokaryon.

Methods of control

Scallops from infected populations should not be introduced into unaffected areas.


Lohrmann, K.B., S.W. Feist and A.R. Brand. 2000. Microsporidiosis in queen scallops (Aequipecten opercularis L.) form U.K. waters. Journal of Shellfish Research 19: 71-75.

Citation Information

Bower, S.M. (2003): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Microsporidiosis of scallops.

Date last revised: June 2003
Comments to Susan Bower

Date modified: