Hexamitiasis of Oysters


Category 4 (Negligible Regulatory Significance in Canada)

Common, generally accepted names of the organism or disease agent

Hexamitiasis, Pit disease.

Scientific name or taxonomic affiliation

Hexamita nelsoni, Hexamita inflata. Possibly other species of Hexamita.

Geographic distribution

Cosmopolitan inhabitant of oysters. Reports of associated mortalities in Ostrea conchaphila (=Ostrea lurida) from Puget Sound, Washington and Ostrea edulis in recirculating water basins or pits in the Netherlands.

Host species

Ostrea conchaphila (=Ostrea lurida), Ostrea edulis, Crassostrea gigas, Crassostrea virginica, Saccostrea cucullata, Saccostrea commercialis, and many other species of oysters.

Impact on the host

Commonly found in low intensity in the intestinal tract of oysters with no pathology. Associated mortalities, up to 75% over 2 months, were reported in O. conchaphila from Puget Sound during periods of cold temperatures (below 12°C). Poor environmental condition probably exacerbated the disease in O. edulis in the Netherlands. In such cases, overwhelming systemic infections occurred.

Diagnostic techniques

Wet Mounts: For systemic infections, examine a drop of haemolymph for the many highly motile flagellates. Hexamita sp. associated with nonpathogenic infections are often observed in wet mount squashes of the digestive gland tubules.

Histology: Examine tissue cross-sections for flagellated protozoa in haemolymph sinuses to indicate presence of a systemic infection.

Smears: Stained smears required for identification to species.

Culture: Place soft tissues in 15 ml filtered seawater in a standard bacteriological culture tube. Add an antibiotic to inhibit bacterial growth (bacteria-free cultures are not obtained by this method). Put tubes in a darkened area for 2-3 days. Sample seawater for trophozoites. Note: cultures are short-lived and should not be left longer than a week at room temperature.

Methods of control

No known control methods. However, avoid temperatures below 10°C for prolonged periods for warm-water oyster species, increase water circulation over oyster beds, and reduce crowding to alleviate the disease.


Feng, S.Y. and L.A. Stauber. 1968. Experimental hexamitiasis in the oyster Crassostrea virginica. Journal of Invertebrate Pathology 10: 94-110.

Mackin, J.G., P. Korringa and S.H. Hopkins. 1952. Hexamitiasis of Ostrea edulis L. and Crassostrea virginica (Gmelin). Bulletin of Marine Science of the Gulf and Caribbean 1: 266-277.

Scheltema, R.S. 1962. The relationship between the flagellate protozoon Hexamita and the oyster Crassostrea virginica. The Journal of Parasitology 48: 137-141.

Schlicht, F.G. and J.G. Mackin. 1968. Hexamita nelsoni sp.n. (Polymastigina: Hexamitidae) parasitic in oysters. Journal of Invertebrate Pathology 11: 35-39.

Stein, J.E., J.G. Denison and J.G. Mackin. 1959. Hexamita sp. and an infectious disease in the commercial oyster Ostrea lurida. Proceedings of the National Shellfisheries Association 50: 67-81.

Citation Information

Bower, S.M., McGladdery, S.E., Price, I.M. (1994): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Hexamitiasis of Oysters.

Date last revised: Fall 1994
Comments to Susan Bower

Date modified: