QX oyster disease

QX stands for Queensland Unknown and is the title which was given to this disease prior to the discovery of the organism that we now know causes it.

In 1976 Marteilia sydneyi was formally described as the cause of QX in Sydney rock oysters.

What is QX?

QX is a disease of Sydney rock oysters (Saccostrea glomerata) caused by a protozoan ('single-celled') parasite (Marteilia sydneyi). This parasite belongs to a small group of parasites which mostly affect bivalves (animals with two 'shells' including oysters, mussels and pipis). A closely related species, Marteilia refringens, causes 'Aber' disease in European flat oysters.

Marteilia sydneyi is NOT a virus or bacteria.

Does QX affect any other species of oyster?

Marteilia sydneyi has only been confirmed as a disease causing agent, or pathogen, of the native Sydney rock oyster. 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 farmed in some estuaries in NSW, are not known to be affected by QX.

How does QX affect oysters?

QX is seasonal. QX infection in Sydney rock oysters usually occurs between January - April, with diseased oysters losing condition and dying through autumn and winter. M. sydneyi has a lifecycle which is  involves at least two hosts. The oyster is the host in which the parasite forms spores (sporulates), and a polychaete worm, Nephtys australiensis, has been identified as an intermediate host for the parasite (Adlard and Nolan, 2015). This multi-host lifecycle is not unusual for parasites. At this stage it is not known if there are additional intermediate hosts in the lifecycle of M. sydneyi.

The parasite enters into the soft tissue of Sydney rock oysters 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 to produce spores, the end-stage of infection in oysters. As the parasite  is producing spores the oyster digestive gland is destroyed making it incapable of absorbing nutrients. To try and survive QX infection, oysters may also re-absorb their gonad, which causes  a rapid loss in their condition (within four weeks in severe cases, longer in others). Mortality of the oyster follows when stored reserves within the oyster are exhausted and the oyster effectively starves to death.

During the end-stages of development, the parasite releases spores into the water and evidence suggests  these are taken up by an alternate host(s) in which the parasite overwinters. Research undertaken by staff at the Queensland Museum has identified a polychaete worm, Nephtys australiensis, as an intermediate host for the parasite (Adlard and Nolan, 2015). It is not known if there are additional intermediate hosts in the lifecycle of M. sydneyi.

Development occurs in the alternate host(s) giving rise to stages infective to the oyster which then start the next annual cycle of infection.

Our current understanding is that healthy oysters do not become infected by association with infected oysters. Repeated laboratory trials and field studies on infection periods indicate that this parasite does not directly transmit from oyster to oyster.

The seasonal cycle of QX involves two or more hosts as illustrated below.Seasonal cycle of QX

What does QX look like?

QX microscope

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

QX infection

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

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.

Diagnosis of the presence of QX

Diagnosis of QX disease should be undertaken only by laboratories skilled in disease recognition and that have the capacity and certification to undertake diagnostic tests.  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 (probing tissue for the presence 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). DNA-based tests proved to be the most sensitive, followed by cytology then histology.

Where does QX occur?

QX disease historically occurred repeatedly in the estuaries of southeast Queensland and northern New South Wales.

In 1994 it was first diagnosed in the Georges River in Sydney and caused cessation of oyster farming 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 disease with massive stock losses, initially mostly on upriver leases, but by 2005 QX caused significant impact on oyster leases downstream  too.

A major surveillance program of the east coast growing areas funded by the FRDC and conducted by staff at the Queensland Museum and NSW DPI, was undertaken between 2001 and 2004.  As a result of that work, it became clear that the QX organism, Marteilia sydneyi, is far more widespread than previously thought. Marteilia sydneyi has been found from as far south as the NSW/Victorian border through to northern Moreton Bay in Queensland.

What drives disease outbreaks?

A parasite with a multiple-host lifecycle that infects oysters in open system aquaculture paints a complex biological picture.

The short answer to what drives disease outbreaks is that we don't know, but we can be assured that there are three interactions of central importance:

  • elements of the parasites lifecycle, for example, the abundance and density of the hosts and the dose of infective stages that reach the oyster
  • the susceptibility of the oyster to the disease, for example, can its immune systems fight off the disease?
  • environmental and nutritional factors which themselves will impact on both the parasites ability to infect the oyster, and on the oysters ability to defend itself from the parasite.

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

Management and reporting of QX disease

Any unexplained and significant mortality of oysters should be reported to the appropriate authorities.  Management in place in New South Wales to minimise the risks associated with this serious oyster pathogen includes the QX biosecurity zone under Part 3 of the Biosecurity Regulation 2017.

QX risk-based biosecurity zone

Previously in NSW, QX was managed by putting in place a closure and/or quarantining an area whenever new outbreaks of QX were detected to prevent movement of oysters and equipment to other estuaries. This led over time to a series of complex fishing closures. In an attempt to simplify the QX closures this system was replaced in August 2008 by a single closure which takes a risk-based approach to managing QX. This risk-based approach has been carried over to the new biosecurity legislation, implemented on1 July 2017 and there is now a QX biosecurity zone for all of NSW. Within the biosecurity zone there are 3 QX Risk Areas, high, medium and low. . The movement of equipment out of high and medium QX risk areas  is permitted only if it has been treated in accordance in the provisions of the Biosecurity Order Permitted Activities.

The current biosecurity zone places NSW estuaries into categories according to their risk:

High QX risk areas

  • Bellinger River
  • Kalang River
  • Richmond River
  • Clarence River
  • Macleay River
  • Hawkesbury River
  • Georges River

Medium QX risk areas

  • Tweed River
  • Brunswick River

Low QX risk areas

  • All other NSW estuaries

The current risk ranking of NSW oyster producing estuaries is illustrated in the map below.

QX risk ranking map

Can cultivated oysters be  moved under the biosecurity zone?

Under this biosecurity zone, oysters can be traded freely between estuaries with the same QX risk ranking.  Estuaries of lower QX risk can move product to higher QX risk estuaries. Higher QX risk estuaries cannot move product to lower QX risk estuaries.

Examples:

Oysters can me moved from Brunswick River to Georges River as this is going from a medium QX risk to high QX risk ranked estuary.

Oysters cannot be moved from Richmond River to Tweed River as this is moving oysters from a higher QX risk estuary to a lower QX risk estuary.

The risk rankings of estuaries can be changed under this biosecurity zone. A drop in ranking from a higher QX risk to a lower QX risk category (i.e. from High QX risk to Medium QX risk or Medium QX risk to Low QX risk) will be considered when there is three consecutive years of surveillance testing with negative results.  For example, QX was identified in Wooli River in 2006 and surveillance testing in 2007, 2008 and 2009 provided negative results. Therefore in May 2009 the QX risk ranking of Wooli River was changed from Medium QX risk to Low QX risk.

How does the QX biosecurity zone interact with the Pacific oyster mortality syndrome (POMS)biosecurity zone?

The POMS biosecurity zone has some complex rules that still restrict inter-estuarine movements of oysters even though those movements may be permitted under the QX closure. Permit holders have been provided a table outlining the permitted oyster movements according to the rules of both the QX biosecurity zone and the POMS biosecurity zone. To request a copy of this table please contact the Aquatic Biosecurity Unit using the contact details listed below.

Further information on closures

Information on biosecurity zones relating to QX, Sydney rock oysters and Pacific oysters can be found on the Biosecurity Regulation 2017 webpage.

Under Division 3 Clause 10 of the Fisheries Management (Aquaculture) Regulation 2017, all oyster movement in NSW must be documented in the Oyster shipment logbook system.

In New South Wales

NSW Department of Primary Industries Aquatic Biosecurity Unit:
Phone: (02) 4982 1232
Address:  Locked Bag 1,  Nelson Bay  NSW 2315

Email: aquatic.pests@dpi.nsw.gov.au

Fax: 02 4982 1107

In Queensland

Queensland Department of Primary Industries and Fisheries
Phone: (07) 3362 9471

Adlard, R.D. & Nolan, M.J. (2015). Elucidating the life cycle of Marteilia sydneyi, the aetiological agent of QX disease in the Sydney rock oyster (Saccostrea glomerata). International Journal for Parasitology, 45:419-426.