QX disease

Marteilia sydneyi has only been confirmed to cause disease impacts to the native Sydney rock oyster, Saccostrea glomerata. The Sydney rock oyster is the main species of oyster commercially farmed along the east coast of Australia from the NSW/Victorian border north to the Great Sandy Strait in southern Queensland.

Native flat oysters (Ostrea angasi) and the introduced Pacific oyster  (Crassostrea gigas; also known by Magallana gigas), which are also farmed in some estuaries in NSW, are not known to be affected by QX.

Other Marteilia-like parasites have, however, been reported from other Australian native oyster species including O. angasi and the tropical rock oyster, Saccostrea cuccullata.

Individual spores of QX disease are microscopic and cannot be identified without the use of high-power microscopes and special stains.

Signs of infection in oysters include:

• lack of growth often seen as an absence of growing lip on the oyster shell,
• loss of condition with oysters appearing abnormally thin and translucent,
• pale digestive glands as spores develop in that tissue and prevent digestion of food.

It is important to note  however that these gross signs are NOT specific to oysters with QX and can be the result of other environmental and nutritional conditions.

Diagnosis of QX disease should be undertaken only by laboratories skilled in disease recognition and that have the capacity and certification to undertake diagnostic testing. There are three methods that can be used: histology (microscopic examination of thin tissue sections); cytology (microscopic examination of stained tissue imprints); DNA-based tests (including polymerase chain reaction, or PCR, which tests for evidence of parasite DNA).

The use of these tests is dependent on the stage of infection to be detected and the sensitivity of detection required, both of which have been assessed (FRDC funded projects FRDC2001/630, 2001/214). PCR proved to be the most sensitive, followed by cytology then histology. However, PCR cannot differentiate between the sporulating stages of M. sydneyi and other life stages. Recently, a real time PCR has also been developed which is highly specific and sensitive for M. sydneyi detection.

QX historically occurred repeatedly in the estuaries of southeast Queensland and northern New South Wales. In 1994 it was diagnosed in the Georges River in Sydney for the first time and caused the collapse of the oyster farming in that estuary over the following years. A small remnant of the industry returned to the Georges River to farm Sydney rock oysters but avoid areas still affected by this disease. In 2004 the Hawkesbury River suffered its first recorded outbreak of QX with massive stock losses, initially mostly on upriver leases, but by 2005 QX caused significant impact on oyster leases downstream too. QX was detected for the first time in Port Stephens in August 2021 and reconfirmed in February 2022. In Port Stephens active QX outbreaks appear to be focused towards the inner estuary, particularly Karuah River and Tilligerry Creek.

All high QX risk areas are stated in clause 49 of the Biosecurity Regulation 2017

A major surveillance program of the east coast growing areas funded by the FRDC and conducted by staff at the Queensland Museum and NSW DPIRD, was undertaken between 2001 and 2004.  As a result of that work, molecular genetic (DNA) evidence suggests M. sydneyi could be far more widespread than previously thought. DNA evidence consistent with M. sydneyi has been found from as far south as the NSW/Victorian border through to northern Moreton Bay in Queensland. More recent research conducted from 2023 to 2025 has found no evidence of M. sydneyi in Wallis Lake, Wagonga Inlet or Tuross Lake which is not consistent with previous findings.

QX infection in Sydney rock oysters usually occurs in NSW between January - April, with diseased oysters losing condition and dying through autumn and winter. Studies undertaken in the Hawkesbury River have indicated that temperature is an important factor in determining the end of the window of infection. In that estuary, no new infections occurred at water temperatures below 21.5°C (Rubio et al., 2013). In the Hawkesbury River the window of infection is known to conclude by the end of April.

More recent studies undertaken in Port Stephens from 2022 to 2025 also support these previous findings that infection commenced once the minimum, rather than maximum water temperature exceeded 21.5°C and ceased when the minimum water temperature dropped below 21.5°C. Decreases in salinity appeared to be important at the beginning of the window of infection , but not at the end of the window, which is consistent with the idea that low salinity impacts the immune status of the oyster rather than the infectivity of the parasite. During this study the opening of the window varied, opening as early as November.

Whilst we have an estimation of the expected duration of a QX disease outbreak, the true duration of a QX disease outbreak cannot be accurately predicted. Observations made during QX disease outbreaks in other estuaries have found they can be prolonged with ongoing mortality likely to continue into early spring. The pattern of mortality generally commences with a high mortality rate, tapering off until the disease event concludes. Good field observations and record keeping of patterns of mortality and mortality estimates in stocks can provide valuable information to researchers and farmers as to when an outbreak may subside.

Despite many years of research, the complete lifecycle of the QX parasite remains unknown. The parasite is believed to have a multi host lifecycle, transitioning through one or more other species before it infects the oyster. The identity of the alternate host(s) has not been confirmed, but there is evidence suggesting that a polychaete worm (Nephtys australiensis) could play a role as a host in the development of the parasite (Adlard and Nolan, 2015). Research completed in another Marteilia species overseas has found copepods (Paracartia grani), a type of zooplankton to contain the parasite (Audemard et al 2002)(Carrasco et al 2008).

It is important to note that oysters infected during the window of infection can continue to die slowly even at lower water temperatures and mortality may persist at different areas within an estuary for up to nine months. Alternatively, the presence alone of Marteilia sydneyi does not necessarily result in outbreaks of QX.

To summarise, there are many factors that can influence development of QX outbreaks:

• Host factors such as the immune response of the oyster
• Parasite factors such as abundance and availability of the parasite(s) to infect the oyster
• Environmental factors such as decreased salinity or temperature which may affect parasite development or host factors (e.g. immune response).

Due to the complex multiple-host lifecycle of the QX parasite in open waterways where oysters are grown there are many factors that can influence development of QX outbreaks.

The short answer is that we don't fully know what drives disease outbreaks, However, major things to consider are:

• Host factors such as the immune response of the oyster
• Parasite factors: such as abundance and availability of the parasite to infect the oyster
• Environmental factors which can affect either the host such as immunosuppression resulting from decreased salinity or temperature affecting development of the parasite.

It is important to note that the presence alone of Marteilia sydneyi does not necessarily result in outbreaks of QX.

To address the many unknowns surrounding QX disease NSW DPIRD has conducted extensive research into the disease and undertaken surveillance throughout NSW. Prior to the 2021 outbreak of QX disease, the Port Stephens estuary was the 2^nd largest producer of Sydney rock oysters in NSW. Following the outbreak NSW DPIRD sought to understand both the extent of disease in the estuary and more about factors contributing to the disease. A geospatial and a window of infection  study were conducted in Port Stephens between 2022 and 2025. The window of infection study explored environmental conditions which may contribute to disease and builds on previous window of infection studies undertaken in the Hawkesbury River by Rubio et al 2013. The Port Stephens research aimed not only to support local oyster growers but also to provide information to the broader NSW oyster industry to assist in development of management strategies alongside the presence of QX in an estuary. To learn more about Port Stephens estuary wide surveillance, the window of infection study and genetic typing of Marteilia sydneyi please visit our QX surveillance and research webpage.

The NSW DPIRD Sydney rock oyster breeding program has commercially available oyster families that have been bred to have greater than 70% survival through a QX disease event compared to non-selected (wild) oysters that can experience losses of up to 90% in a QX disease outbreak. Further details regarding the overall performance of these families can be obtained from DPIRD Research Scientist Laura Parker (email: laura.parker@dpird.nsw.gov.au or phone: 4916 3900).

Adlard, R.D., Nolan, M.J., Elucidating the life cycle of Marteilia sydneyi, the aetiological agent of QX disease in the Sydney rock oyster (Saccostrea glomerata), International Journal for Parasitology (2015), doi: http://dx.doi.org/10.1016/j.ijpara.2015.02.002

Audemard C, Le Roux F, Barnaud A, Collins C, Sautour B, Sauriau PG, De Montaudouin X, Coustau C, Combes C, Berthe F (2002) Needle in a haystack: involvement of the copepod Paracartia grani in the life-cycle of the oyster pathogen Marteilia refringens. Parasitology 124:315-323 https://doi.org/10.1017/s0031182001001111

Carrasco N, Arzul I, Chollet B, Robert M, Joly JP, Furones MD, Berthe FCJ (2008) Comparative experimental infection of the copepod Paracartia grani with Marteilia refringens and Marteilia maurini. Journal of Fish Diseases 31:497-504 https://doi.org/10.1111/j.1365-2761.2008.00910.x

Rubio A; Frances J; Coad P; Stubbs J; Guise K, 2013. The onset and termination of the QX disease window of infection in Sydney rock oyster (Saccostrea glomerata) cultivated in the Hawkesbury River, NSW, Australia. Journal of Shellfish Research, 32(2):483-496. http://www.bioone.org/doi/abs/10.2983/035.032.0228