FISH FACTS: A history of white spot disease in Australia
A RECENT outbreak of the devastating white spot disease (WSD) caused by White Spot Syndrome Virus (WSSV) in prawn farms along the Logan River in the southern part of Moreton Bay has raised many questions regarding how the virus got into our country, whether it will establish or fizzle out, what needs to be done to prevent its spread into other States, and what can be done to prevent similar incursions in the future.
White Spot Syndrome Virus is an internationally notifiable disease agent that infects all decapod crustaceans as well as several other carrier species. So not only prawns become infected, but also crabs, freshwater crayfish, lobsters, copepods and various different types of plankton. The white spot virus that infects crustaceans should not be confused with white spot disease of fish, which is caused by ubiquitous ciliate protozoans.
White spot disease in crustaceans was first described in Taiwan in 1992 and in Japan in 1993 after they received diseased prawns imported from China. Since then WSSV was spread with movements of live prawns and frozen prawn products throughout Asia, Central and South America, and the Middle East during the 1990s and 2000s. Because of its massive impacts on the culture of prawns overseas, WSD is one of several diseases of aquatic animals that Federal biosecurity authorities have tried to exclude from Australia by implementing quarantine controls on crustacean products imported into the country.
Compared to most other countries, Australia has stringent import quarantine regulations that have managed to exclude many serious diseases. The magnitude of the risk was confirmed in December 2000 when an incursion of WSSV was detected in Darwin after imported frozen prawns were accidentally used as feed for broodstock at a crustacean hatchery. In that case, imported prawns were considered to be of poor quality insufficient for use for human consumption (based on smell), and were illegally repackaged, and diverted into the bait market, after which they were purchased and used to feed hatchery broodstock black tiger prawns and mud crabs, which became infected.
This is not surprising, as the scientific community has known for a long time that viruses like WSSV (and several other viruses of prawns and crabs), are well preserved in frozen tissues and remain viable and infective upon thawing out. In the Darwin case, once the mistake was realized, the broodstock were tested and found to be infected with WSSV, after which all of the affected stock was destroyed and the affected premises decontaminated.
Subsequent testing of wild mud crabs and prawns adjacent to the hatchery outlet found some were also infected with WSSV, but over time this infection fizzled out and subsequent testing over several years showed the virus did not become established in Darwin Harbour.
Since 2000, Australia had remained white spot free, until detection of the disease in a prawn farm on the Logan River in late November 2016. Biosecurity Queensland responded to the discovery by beginning destruction and decontamination activities at the affected farm, as well as by placing restrictions on commercial and recreational fishers making it illegal for them to capture or move crustaceans from the Logan and Albert River systems.
Despite these precautions, as was the case in the Darwin Harbour incursion, a small number of prawns (six) and a mudcrab from the Logan River have tested positive for the virus, and sure enough several other nearby prawn farms also became infected. At the time of writing (mid January), WSSV had been detected on five of eight prawn farms in the area, all of which have had their stock destroyed and are in the process of decontamination, at the cost of many tens of millions of dollars of destroyed product, job losses, not to mention the multi-million dollar cost of the cleanup which will go on for several months.
While the exact route of entry of the virus remains to be determined at the time of writing, revelations of recreational anglers being found fishing with WSSV infected imported prawns within 500 meters of the affected farms, as well as evidence of regular fishing activity within the intake canals of the farms themselves, have again highlighted the dangers of using imported prawns as bait.
Of additional concern is evidence on some fishing forums of some so called “keyboard experts” encouraging other anglers to use prawns sold for human consumption as bait, seemingly oblivious of the fact that if that product is imported, it may even be illegal to do so and highly dangerous for our fisheries and aquaculture industries as well as the environment due to risks from not only WSSV, but several other diseases of crustaceans that we do not have in Australia at present.
What makes things difficult for recreational anglers is the fact that labeling of imported, frozen uncooked prawn products sold by the kilogram over the counter usually extends only to the country of origin information. The consumer is seldom, if ever, informed that the product is being sold only for human consumption and that it shouldn’t be used as bait.
If readers of this article can take home one message from all of this, it is please do not use imported green prawns as bait. Recent moves by the Federal Government to ban further importation of frozen green prawns until a full investigation is made of the risks they pose are understandable, given the history surrounding the product. What is important now is to try to ensure that the lessons of Darwin 2000 and Logan River 2016 are not repeated anywhere else in Australia, as we watch and wait with fingers crossed to see whether the disease establishes, or fizzles out in the Logan, as it did in Darwin 16 years earlier.