This report provides independent documentation and analysis of events related to a White Spot Disease (WSD) outbreak in Black Tiger Prawns (Penaeus monodon) cultured on the Logan River from late November 2016 until February 2017. Disease was first observed on the index farm (1IP) on 22nd Nov 2016 and spread rapidly to affect multiple ponds adjacent to and downwind from the index pond, suggesting on-farm spread via aerobiological means (aerosols) and probably via mechanical vectors (insects, toads, birds). By Monday 5th Dec WSD had spread to a second farm (3IP), 1 km north of the index farm and the White Spot Syndrome Virus (WSSV) was found in wild prawns (Metapenaeus spp., Acetes spp., n = 6) sampled from the Logan River near 3IP. By Thursday 8th Dec WSD had spread to a third farm (4IP) around 2 km downriver, while a separate compartment of the first farm (2IP) recorded clinical disease on 12th Dec. An isolated case of a single clinically normal mud crab (Scylla serrata) positive for WSSV was recorded in the outlet canal of a 7th (then non-infected) farm (7ARP, 7.3 km east of 1IP) on 23rd Dec. Subsequently, disease spread to a 5th farm around 5 km downriver (5IP) with clinical disease recorded on 28th Dec. Spread between these 5 farms did not appear random. In all cases index ponds at each farm were located at the southern ends of intake canals, downwind from the mainly northerly winds at the time. Location of index ponds along intake canals was the only consistent risk factor once other risk factors (PL source, pond stocking date, food source, water quality) were assessed at each site. Index pond location and non-random distribution of crustaceans and vectors within the intake canals suggests that the affected farms bought in the disease agent through their intake canals via unidentified, possibly planktonic, carrier hosts. The sixth farm infected (8IP) was drawing water from southern Moreton Bay and was positive for WSSV in samples taken on 24th Jan 2017 during the later stages of harvest. The final farm (7ARP) remained uninfected until 11th Feb 2017. This farm may have been infected from nearby 8IP where hundreds of birds were observed wading in WSSV positive ponds on 3rd Feb. The WSSV positive P. monodon sampled from the river near 3IP (25th Jan) and on mud flats near the outlet of 8IP on 8th Feb 2017 are considered likely to
be farm escapees, but genetic testing is required to confirm this assumption.
While its possible that introduction of WSSV to farms 4IP, 5IP and 8IP downriver from the 1IP/2IP/3IP farm cluster may be explained by predictable downstream movement of water and/or carriers as per CSIRO modelling, the mode of introduction of the virus into index farm 1IP and the anomalous positive mud crab in the outlet of 7ARP requires thorough investigation. Sources of feed, equipment or hatchery supplies of PL do not appear to explain the emergence of the disease at 1IP. Instead, the epidemiology and chronology of disease spread together with evidence of significant recreational fishing effort in and adjacent to the intake canal at 1IP, strongly suggests, in my professional opinion, that the incursion pathway was most likely introduction of WSSV via the 1IP intake canal. Indeed, surveys by Fisheries officers allegedly found several groups of recreational fishers using imported green prawns as bait within 500 meters of the intake of farm 3IP, and of these 33% of bait samples were positive for WSSV. This pathway is plausible given evidence that; 1. Increasing numbers of recreational fishers are using imported prawns as bait, and 2. Biosecurity breakdowns at the international border resulting in c. 50-54% of imported green prawns sold at the retail counter being WSSV positive in the leadup to Christmas/New Year 2016. The risk profile for this pathway may have increased since the 2009 Import Risk Analysis for prawn products, meaning that the risk analysis needs to be thoroughly reviewed and updated to more accurately reflect the various risk pathways and new emerging diseases prevalent in the world today.
White Spot Disease, WSSV, Black Tiger Prawn, Mud Crab, Logan River, Moreton Bay