QX disease in oysters

Marteilia sydneyi

QX disease spores Spores of QX disease as seen under a research microscope at a magnification of 1000 times.

'QX' stands for 'Queensland Unknown', the title given to this disease before scientists discovered the parasitic organism that we now know causes it. In 1976 Marteilia sydneyi was formally described as the cause of QX disease in oysters.

Marteilia sydneyi is a protozoan (single-celled) parasite that belongs to a small group of parasites that mostly affect bivalves (animals with two 'shells', such as oysters, mussels and pipis).

QX disease infects the Sydney rock oyster (Saccostrea glomerata), which is the commercial rock oyster grown along the east coast of Australia from the NSW/Victorian border north to the Great Sandy Strait in southern Queensland.

Infections of oysters usually occur between January and April each year, with many diseased oysters losing condition and dying throughout the winter. This parasite has a life cycle which is thought to involve two hosts, the oyster and a marine worm, but the complete life cycle has yet to be confirmed by scientists.

The parasite enters the oyster through its gills and palps (mouthparts) and migrates to the digestive gland which surrounds the intestine. Here the parasite produces spores and in the process destroys the digestive gland so that the infected oyster can no longer take up nutrients. QX can cause oysters to lose condition quickly (within four weeks in severe cases, longer in others) as they re-absorb their gonads and deplete their stored reserves. Oysters then appear thin and watery (not very inviting as a dinner entrée!) and infection often continues until the oyster dies - the oyster effectively starves to death. Commercial losses have reached millions of dollars in some oyster growing estuaries.

Healthy oyster and QX-disease-infected oyster The difference in condition between a healthy oyster (on left) and one infected with QX disease (on right).

Individual spores of QX disease are microscopic and cannot be identified without the use of high power microscopes. However, one indicator of the disease is the light brown to cream colour of the oyster’s digestive gland due to the presence of parasite spores. It should be stressed that these gross signs are NOT specific to oysters with QX disease and can be the result of other environmental and nutritional conditions.

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