5 February 2024

When was QX found in Port Stephens?

QX was first confirmed in Sydney Rock Oysters in Port Stephens on 27 August 2021. Thanks to the diligence of farmers, we have been able to support industry in taking preparatory steps in response to QX disease, such as discussion of options for disease resistant spat.

When and where was QX reconfirmed in Port Stephens?

In February 2022, QX was reconfirmed in both the Karuah River and Tilligerry Creek growing regions following reports of unexplained mortalities and oysters showing clinical signs suggestive of QX disease. During a March 2022 surveillance sampling program run by DPI to determine presence / absence of QX in various regions of the Port, QX was again detected in multiple western growing areas. QX was detected in both wild and cultivated oysters across multiple size classes. The results of the surveillance sampling are summarised in the below map.

How will QX be managed in Port Stephens?

In August 2021, the QX status of Port Stephens was changed from a ‘low QX risk area’ to ‘under investigation.’ Following the February and March 2022 reconfirmations of QX disease in the estuary associated with high levels of mortality, DPI analysed this data and determined that the Port Stephens QX risk classification met the criteria for a high QX risk area.

Whilst there is active QX underway in Port Stephens, DPI is encouraging farmers to remain vigilant and call 1800 675 888 to report oyster mortalities as well as observations of diseased or unwell oysters.

Oyster movement restrictions

Movements of oysters and oyster cultivation material within Port Stephens will not be restricted.

Outward movements of oysters and oyster cultivation material were initially managed by the Biosecurity (QX Disease) Control Order 2021, which was revoked in February 2023. The estuary has now been reclassified as a high risk area in the QX Biosecurity Zone under clause 49 of the Biosecurity Regulation 2017.

This means that oysters may not be moved from the Port Stephens estuary to ‘low’ or ‘medium’ QX risk areas. Oyster cultivation equipment also must not be moved from Port Stephens estuary to ‘low’ or ‘medium’ QX risk areas without prior treatment.

Permits were issued to allow movements of spat produced by the two hatcheries out of Port Stephens to ‘low’ and ‘medium’ QX risk areas. Protocols for this permit include water treatment and pre-dispatch health testing requirements. Spat may not be moved out of Port Stephens estuary nurseries to ‘low’ or ‘medium’ QX risk areas.

A map below shows the current QX risk status of all estuaries in NSW.

Surveillance activities

QX was first detected in Port Stephens estuary on 27 August 2021. Surveillance following this initial detection revealed the presence of QX at a low prevalence in three main growing areas (Karuah River, Oyster Cove and Tilligerry Creek).

Sampling and testing

2022

In February 2022, DPI reconfirmed QX through active investigation of reports of unexplained mortality in oysters analysed from Tilligerry Creek on 17 February 2022 and from Karuah River on 24 February 2022.

Surveillance testing in March was undertaken on a total of 300 oysters collected across 10 zones, to allow additional information to be obtained. QX was detected in cultivated oysters at 7 of the 10 zones tested. However, in 2 of the zones (Bundabah and Soldiers Point) QX was detected only in cultivated oysters that were known to have been moved in January from one of the other five locations testing positive for QX. It is therefore unknown where these oysters became infected with QX. Detection of QX disease in wild oysters was confirmed at 4 of the 10 zones tested (Upper Tilligerry, Lower Tilligerry, Upper Karuah and Oyster Cove).

A map depicting the sampling sites and results is included on the following page.

DPI is continuing discussions with funding bodies, seeking support for required research to help fill important information gaps for the longer-term management of this significant disease. Samples that were collected and processed as part of the initial surveillance have been stored for further analysis, such as molecular characterisation research into the potential for QX strain variation, if additional funding sources can be obtained.

2023

The geospatial survey of Port Stephens was repeated in early April of 2023, with 300 oysters again collected across the same 10 zones.  As in 2022, detections of sporulating QX disease were confined to the inner estuary, however in 2023 detections were specifically confined to the Upper and Lower Karuah River and Upper and Lower Tilligerry Creek areas.  Oyster Cove was unaffected in the 2023 survey.

Window of infection study

NSW DPI are currently undertaking research in Port Stephens to determine the start of the window of infection for QX disease.  Naïve oysters (n = 30) are being deployed every two weeks at the sites hardest hit by QX disease earlier in 2022, in Tilligerry Creek and the Karuah River.  Oysters are maintained at these sites for a period of 6 weeks and then retrieved for testing.  Testing is being undertaken using a new qPCR* tool for early Marteilia sydneyi detection as well as cytology to look for sporulating QX disease.  Results to date are shown below and will be updated approximately every 2 weeks:

*qPCR stands for quantitative polymerase chain reaction, and is a technology used for measuring DNA using PCR.

Site 1: Karuah River

Site 2: Tilligerry Creek

Temperature and salinity measurements collected via dataloggers for each of the window of infection study sites are shown below.  Arrows indicate the beginning and end of the window of infection for each site.

The permissive temperature for infection is believed to be 21.5°C (dotted lines).  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.

Site 3 - Cromarty Bay

Additional sentinel oysters that had been deployed at Cromarty Bay since the close of the 2022 QX season were also collected for analysis.

What is QX?

QX stands for ‘Queensland unknown’. It is a disease of Sydney Rock Oysters (Saccostrea glomerata) caused by a protozoan ('single-celled') parasite (Marteilia sydneyi). A closely related species, Marteilia refringens, causes 'Aber' disease in European flat oysters. This group of parasites is only known to infect invertebrate (animals without a backbone) hosts and does not have any implications for human health. Marteilia sydneyi is not a virus or bacteria.

Does QX impact other species of oyster?

Marteilia sydneyi has only been confirmed as a disease-causing agent, or pathogen, of the native Sydney Rock Oyster. Sydney Rock Oysters are 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) are also farmed in some estuaries in NSW and are not known to be affected by QX.

How does QX affect oysters?

QX infection in Sydney Rock Oysters usually occurs in NSW where it is known to occur 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. 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.

Martelia sydneyi has a lifecycle that involves at least two hosts and cannot be passed directly from one oyster to another.

The parasite enters the soft tissue of Sydney Rock Oyster through its gills and palps (near the oyster's mouth). If it progresses to cause disease, the parasite divides and proliferates. It then migrates to the digestive gland which surrounds the oyster intestine. There it undergoes further development and multiplication to produce spores (sporulation), the end-stage of infection in oysters. Sporulation damages the digestive gland of the oyster, resulting in starvation and eventual death of the oyster. Time from infection until death can vary between several weeks to several months. Affected oysters can be in poor condition and appear translucent or “watery”. The digestive gland can also appear to be a light tan colour instead of the usual dark brown. Prior to death of the oyster, the spores are released into the environment where they are taken up by alternate host(s) required in the life cycle of QX. 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).

Duration of QX outbreaks

The expected 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 in Port Stephens estuary during this present disease event can provide valuable information as to when an outbreak may subside.

What causes outbreaks and how does QX spread?

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 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 that can affect either the host, such as immunosuppression resulting from decreased salinity or temperature affecting development of the parasites.

It is important to note that the presence of Marteilia sydneyi alone does not necessarily result in outbreaks of QX disease. QX cannot be spread directly from one oyster to another.

The spores released by the oyster need to enter an intermediate host(s) for further development. However, infected oysters should not be moved to other waterways to avoid spreading the parasite to other areas where it may not yet be established and be causing disease.

Food Authority advice for harvesting and sales:

• All seafood on the market remains safe to eat
• QX disease does not affect humans and is not a food safety risk
• Oysters in the marketplace must comply with stringent food safety standards of the NSW Shellfish Program
• Oysters that are affected by QX will not depurate as effectively and are likely to exhibit shorter shelf life while being stored live.

Available assistance

For a summary of assistance options see: https://www.dpi.nsw.gov.au/emergencies/floods/natural-disaster-assistance-guide

A free Rural Financial Counsellors service is available as is wellbeing support.

Growers are advised to visit the Rural Assistance Authority website www.raa.nsw.gov.au/disaster-assistance/storm-and-flood-programs. 

QX resistant Sydney Rock Oysters

The DPI 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 DPI Research Scientist Michael Dove.

(Email: michael.dove@dpi.nsw.gov.au, Phone: 02 4916 3807).

Considerations for re-stocking spat and stock availability 

Some important considerations for restocking in a QX affected estuary are:

• QX disease resistance of the stock
• QX infections have ceased
• Water quality in the estuary, particularly if restocking spat
• Avoiding areas of the estuary that experience recurrent QX outbreaks, and
• Selling marketable oysters before QX exposure or ensuring that QX disease resistant oysters are sold prior to a second season of exposure.

For more information on hatchery and nursery spat availability visit: https://www.nswoysters.com.au/spat-availability.html

Reporting oyster mortalities

If you observe unexplained mortalities in your oysters, please call the DPI Aquatic Biosecurity Programs team on the Emergency Animal Disease Hotline: 1800 675 888 or the Port Stephens Fisheries Institute switch: (02) 4916 3900.

Where can I get further information?

The DPI website contains a range of information on QX. Please visit: https://www.dpi.nsw.gov.au/fishing/aquatic-biosecurity/aquaculture/aquaculture/qx- oyster-disease

Important Contact details

Aquatic Biosecurity: report unexplained mortality via the 24-hour Emergency Animal Disease hotline: 1800 675 888 (preferred method). For non-urgent enquires call the DPI Port Stephens Fisheries switch: (02) 4916 3900 (during business hours only).

NSW Food Authority: (status of harvest areas and sale of stock): 1300 552 406.

Aquaculture Management and Administration: Ian Lyall (02) 4916 3856 or Emma Wilkie 0428 764 310 or Aquaculture Administration 0407 693 244.

Fisheries Compliance: Report illegal activity via the Fishers Watch Hotline: 1800 043 536.

Scientific references

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

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 (Saccostreaglomerata) 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