Unidentified Protistan Parasites of Pearl Oysters

Category

Category 3 (Host Not in Canada)

Common, generally accepted names of the organism or disease agent

Various unidentified protistan parasites of pearl oysters.

Scientific name or taxonomic affiliation

  1. Unidentified protistan parasite possibly a sprorozoan in digestive gland tubules.
  2. Unidentified protistan parasite possibly a Thraustochytridea in connective tissue under surface lesions of the palps and mantle.
  3. Unidentified protistan parasite possibly a ciliate attached to digestive gland epithelial cells.
  4. Intracellular ciliate in the epithelial cells of the digestive gland similar to that described from mussels but the relationship to these organisms in mussels from the northern hemisphere is unknown.

Geographic distribution

Western Australia.

Host species

Pinctada maxima.

Impact on the host

  1. Putative sporoblasts develop within the epithelial cells of the digestive gland and fill the lumen of the tubules. Associated with a basophilic haemocyte infiltration. Prevalence of less than 0.005% has limited further investigations into this parasite (Jones and Creeper 2006).
  2. The thraustochytrid-like parasite was identified in moribund, gaping P. maxima from a farm that had experienced losses after a cyanobacterial bloom. Affected oysters showed extensive necrosis of the epithelium lining the palps and mantle with invasion of the underlying connective (leydig) tissues by brown pigmented and eosinophilic, segmented unicellular organisms and smaller dense basophilic cells.
  3. A large elongate ciliate-like organism intimately associated with the digestive tract epithelium to which it appears to have a sessile attachment. The tubule epithelium below the site of attachment consisted of a small focal patch of multinucleate host cells. The prevalence of infection was low in P. maxima from the Exmouth Gulf, Western Australia.
  4. The intracellular ciliate is often associated with haemocyte infiltration in P. maxima less that 70 mm shell length. In smaller pearl oyster spat (20-40 mm in shell length) the ciliate can penetrate the mucosal basal lamina and reside within the haemolymph sinuses or free within the interstitial connective tissue. This ciliate has only been detected in pearl oyster spat from the Exmouth Gulf, and outlying islands of Western Australia since 2001.

Diagnostic techniques

Histology:

  1. Putative sporoblasts (about 20 µm in diameter) occur within the epithelium and lumen of digestive gland tubules and gut. Haemocyte infiltration into the connective tissues between the tubules was associated with infection (Fig. 1).
  2. Figure 1. Clusters of putitive sporoblasts of an unidentified protistan parasite within the lumen of the gut (Pg) and tubules of the digestive gland (Pt). Numerous haemocytes (H) occur in the connective tissue surrounding infected tubules of Pinctada maxima. Haematoxylin and eosin stain.

  3. The brown pigmented and eosinophilic, segmented unicellular organisms (10 to 15 µm in diameter) and smaller dense basophilic cells (5 µm in diameter) within the connective tissue of the palps and mantle seem to be embedded in a mucinous matrix (Fig. 2).
  4. Figure 2. Abundant large eosinophilic thraustochytrid-like organisms (E) some containing brown pigments (P) and smaller basophilic cells (B) in the connective tissue of the mantle of Pinctada maxima. Haematoxylin and eosin stain.

  5. A large elongate ciliate-like organism (30 µm by 20 µm) attached to the epithelium of the digestive gland tubule associated with hyperplasia of the tubule nuclei at the site of attachment (Fig. 3).
  6. Figure 3. Cluster of multinucleate cells (arrow) at the attachment site of a large protist in a digestive gland tubule of Pinctada maxima. Haematoxylin and eosin stain. The black and white image may proved better contrast for viewing details.

  7. Teardrop shaped basophilic ciliate (10 to 15 µm long by about 5 µm wide) with a macronucleus containing dense basophilic spheres. This ciliate normally occupies an intraluminal or intraepithelial location within the digestive gland tubule of spat (Fig. 4).

Figure 4. Intracellular ciliates (arrows) in the epithelial cells of a digestive gland tubule of Pinctada maxima. Haematoxylin and eosin stain.

Methods of control

No known methods of prevention or control.

References

Jones, J.B. and J. Creeper. 2006. Diseases of pearl oysters and other molluscs: a Western Australian perspective. Journal of Shellfish Research 25: 233-238.

Citation Information

Bower, S.M., Jones J.B. (2007): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Unidentified Protistan Parasites of Pearl Oysters.

Date last revised: May 2007
Comments to Susan Bower

Date modified: