Tactical Research Fund: Aquatic Animal Health Subprogram: Viral presence, prevalence and disease management in wild populations of the Australian Black Tiger prawn (Penaeus monodon)
CSIRO Oceans and Atmosphere Hobart
In 2012, diseased P. monodon from North-QLD farms were investigated by QDAFF, Townsville. All prawns tested negative using an OIE-endorsed YHV-1 specific PCR test but positive to GAV (YHV-2) and to YHV-1 using OIE-endorsed nested PCR tests designed to co-detect and differentiate YHV-1 from GAV. However, consistent with the YHV-1 specific PCR test data, none of the diseased P. monodon displayed histopathology consistent with acute YHV-1 infection. Analyses undertaken at CSIRO-AAHL confirmed the presence of GAV but not YHV-1. In two OIE-endorsed PCR tests, GAV sequences were also amplified by primers supposed to be specific for YHV-1. When an OIE-endorsed PCR test designed to detect all YHV genotypes was applied to tissue from healthy P. monodon broodstock imported into QLD from NT, sequence analyses identified an undescribed unique genotype designated tentatively as YHV-7. YHV-7 was also amplified by a CSIRO in-house real-time PCR test designed to be specific to the YHV-1. Wild QLD broodstock are increasingly being replaced by wild NT broodstock to produce resilient fast growing P. monodon for aquaculture. The finding of YHV-7, with unknown pathogenic potential and distribution, highlights the potential risks of translocated broodstock spreading unwanted new pathogens to regions with substantial aquaculture interest, and supports a need to update decade-old data on what viruses might exist in NT and QLD populations of P. monodon. Collectively these discoveries highlight urgent needs to (i) re-evaluate genotype specificities of OIE-endorsed PCR tests for different YHV genotypes (ii) redesign PCR tests to improve their power to discriminate YHV-1 from less virulent genotypes and (iii) re-assess the prevalence of GAV and related genotypes (YHV-7) and other endemic viruses potentially problematic to aquaculture (MoV, MBV, HPV, SMV, IHHNV) in wild P. monodon broodstock sourced from the NT and QLD.
1. Determine what GAV/YHV genotypes exist and their relative prevalence in wild P. monodon populations in NT/WA/QLD from which broodstock are captured for aquaculture purposes
2. Revise PCR test designs as necessary to ensure their specificity, particularly in discriminating the highly virulent YHV-1 strain that emerged in Thailand in the early 1990’s from GAV and the other known YHV genotypic variants that appear to be far less pathogenic, and make these tests available for publication in the OIE Diagnostic Manual for Yellow head disease
3. Acquire and/or generate appropriate control nucleic acids specific to the various YHV genotypes for use in YHV-1 or other genotype-specific PCR tests so that their diagnostic specificity can be validated at key diagnostic laboratories (eg. CSIRO-AAHL), and so they can be made available to state and international laboratories with needs for equivalent diagnostic capabilities.
4. Determine the existence and prevalence of other endemic viruses [eg. Mourilyan virus (MoV), Monodon baculovirus (MBV), Hepatopancreatic parvovirus (HPV), Spawner-isolated mortality virus (SMV) and Infectious hypodermal and haematopoetic necrosis virus (IHHNV)] in wild P. monodon populations in the NT and QLD. In addition, test samples for the exotic viruses WSSV and IMNV given some broodstock are sourced from waters with a higher than usual likelihood of incursion from these pathogens.
Principal Investigator: Jeff Cowley
Key Words: Black tiger prawns, Penaeus monodon, yellow head virus, gill-associated virus, enzootic viruses, exotic viruses, PCR tests and controls, northern Australia, aquaculture
Summary: The APFA supported the testing for exotic viruses and will be reassured as WSSV, IMNV and YHV1 were not detected in any of the P. monodon tested from any of the study locations. The 2012 discovery of YHV7 in broodstock originating from JBG was confirmed in 2013 samples from QLD hatcheries. However, as YHV7 was also detected and confirmed in QLD grow-out prawns, targeted surveillance of farms in QLD is required to determine the true prevalence and distribution of this genotype. Whether the detection of YHV7 in prawn farms in QLD is due to recent introductions with broodstock sourced from the Northern Territory or is due to YHV7 already being present in QLD prawns is unknown.