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 (i.e. 'single-celled') parasite (Marteilia sydneyi). This parasite belongs to a small group of parasites which mostly affect bivalves (i.e. 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.

What species of oyster does QX affect?

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. The introduced Pacific oyster (Crassostrea gigas), also farmed in some estuaries in NSW, is 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 winter. This organism has a lifecycle which is thought to involve at least two hosts. The oyster is the host in which the parasite forms spores (sporulates). Research undertaken by staff at the Queensland Museum has identified a polychaete worm, Nephtys australiensis, as an intermediate host for the parasite. This multi-host lifecycle is not unusual for parasites. Many species cycle through two or more hosts (some parasitic worms use as many as four hosts in their lifecycles). At this stage it is not known if there are additional intermediate hosts in the lifecycle of M. sydneyi.

The parasite enters the 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’s intestine. There it undergoes further development to produce spores, the end-stage of infection in oysters. In the process of producing spores the oyster’s digestive gland is destroyed and infected oysters are no longer capable of taking up nutrients. Infected oysters may also re-absorb their gonads. QX can cause oysters to lose condition quickly (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, spores are released into the water and there is good evidence that 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. 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.

In which localities does it 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, an area in which it established and caused major stock losses. 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 was found on downstream leases 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 organism, Marteilia sydneyi, is far more widespread than previously thought. It has been isolated 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 options are in place in New South Wales and Queensland to minimise the risks associated with this serious oyster pathogen. One of these management options is the QX risk-based closure in NSW.

QX risk-based closure

Previously in NSW QX was managed by putting in place a closure and/or quarantining an area whenever new outbreaks of QX were detected. This led over time to a series of complex section 8 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.

The current closure ranks NSW estuaries according to their risk:

High risk QX

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

Medium Risk QX

  • Tweed River
  • Brunswick River

Low Risk QX

  • All other NSW estuaries

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

QX risk ranking map

What oyster movements are allowed under this closure?

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

Examples:

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

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

The risk rankings of estuaries can be changed under this closure. A drop in ranking from a higher risk to a lower risk category (i.e. from High QX risk to Medium QX risk or Medium QX risk to Low QX risk) will be supported 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 closure interact with the Pacific oyster closure?

The Pacific oyster closure 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 closure and the Pacific oyster closure. To request a copy of this table please contact the Aquatic Biosecurity Unit using the contact details listed below.  The Pacific oyster closure is due for renewal by December 2013 and NSW DPI aims to develop (in consultation with the NSW oyster industry) a new risk-based closure that is simplified whilst still minimising the risk of spreading Pacific oysters.

Further information on closures

Fishing closures relating to QX and Pacific oysters can be found on the fishing closures page.

For further information

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
Telephone (07) 3362 9471