Genomic heterogeneity and prevalence of hepandensovirus in Penaeus esculentus from Western Australia, and P. merguiensis from the Gulf of Carpentaria, Australia

Decapod Hepandensovirus 1 (HDV), formerly known as hepatopancreatic parvovirus, has been associated with stunting, lowered production and outright mortalities in prawns in aquaculture. Despite the fact that broodstock are sourced and aquaculture farms are planned in the regions of northern and Western Australia, data on these parvoviruses from this region are limited. The prevalence of HDV in Penaeus esculentus and Penaeus merguiensis is moderate (34–51%) in southern Western Australia, Exmouth Gulf and the Gulf of Carpentaria but statistically higher (P < 0.05) in Shark Bay (82%). We speculate this is due to the topography of Shark Bay combined with the currents of the Indian Ocean gyre (IOG). Despite an on average 8–12% genomic heterogeneity, the nucleotide sequences of HDV in WA most closely align with HDV in regions associated with the IOG; Thailand, India, Tanzania, Madagascar; eastern Asia, Korea and less commonly, with sequences from the eastern coast of Australia. This potentially changes the paradigm of a single strain of HDV being ubiquitous in Australia and there was little risk in moving broodstock from WA to the eastern states, so there was no testing of broodstock for HDV. There is no strong evidence to clarify whether the strain of HDV in WA P. esculentus came from either its’ nearest genetic relatives, P. monodon or P. semisulcatus or from P. merguiensis from the Solanderian province of Australia. P. esculentus HDV appears to be most related to strains within the IOG. The HDV nucleotide heterogeneity of wild prawns contrasts strongly to studies undertaken with prawns from aquaculture where genetic selection may have occurred

The effect of Penaeus merguiensis densovirus on Penaeus merguiensis production in Queensland, Australia

Penaeus merguiensis densovirus (PmergDNV) is currently present on several Queensland prawn farms culturing Penaeus merguiensis. Densoviruses have been linked to mortality and stunting that has caused significant financial loss to prawn farms in Asia. A histopathological study for PmergDNV was initially undertaken to compare broodstock to grow out factors from 60 broodstock animals from each of 22 ponds. There was a significant negative correlation (r = −0.61) between the number of animals with PmergDNV lesions and healthy animals. Furthermore, a higher number of septic hepatopancreatic tubules was correlated (r = 0.48) to high PmergDNV loads. Hence, a polymerase chain reaction analysis of 10-day-old post-larvae (PL) was conducted to determine whether PmergDNV infection was resulting in production losses. An attributable risk analysis of PL from 190 ponds over a 2-year period revealed that 28–29% of ponds with below average survival will have at least average survival following the removal of or decreased levels of PmergDNV. P. merguiensis culture facilities in Queensland should have at least a 14.5% increase in production, equating to an increase of $2.25 million within the first year alone, following the removal or reduction of PmergDNV in their ponds. Hence, focussing efforts on prevention, better management practices and maintaining healthy stock should be of top priority

Experimental infection of redclaw crayfish (Cherax quadricarinatus) with Macrobrachium rosenbergii nodavirus, the aetiological agent of white tail disease

Macrobrachium rosenbergii nodavirus (MrNV) or white tail disease has been reported as a new disease of crustacea in western Queensland, Australia. In Australia, Macrobrachium can be hard to source due to their need for a saltwater environment for breeding. No alternative animal experimental model for MrNV has been identified, so redclaw crayfish (Cherax quadricarinatus) were tested as a potential experimental animal in order to study MrNV infection. The highest mortality (35%) was in the groups injected with MrNV and the lowest mortality (0%) was in the control groups. Necrotic muscle and muscle degeneration with haemocytic infiltration were observed in infected crayfish. For the first time, a quantitative real-time polymerase chain reaction (qPCR) on clinical material was developed and it confirmed MrNV infection in infected animals. The mean viral titres (2.73 × 102 copies) and cycle times (Ct = 31.33) lead us to hypothesize that MrNV only poorly replicates in juvenile C. quadricarinatus. However, C. quadricarinatus may be a less than perfect but useable experimental animal model for MrNV infection in the future because of clinical signs, gross lesions, histopathological changes and qPCR titres present in experimentally infected C. quadricarinatus. This study determined that redclaw crayfish (C. quadricarinatus) had low susceptibility and were limited carriers of white tail disease

Intranuclear bacilliform virus and hepatopancreatic parvovirus (PmergDNV) in the mud crab Scylla serrata (Forskal) of Australia

The culture of mud crabs Scylla serrata (Forskal) is largely experimental in Australia but some operators are producing marketable quantities. In an effort to understand the bottlenecks to production in hatcheries, a number of disease studies were initiated. In one study, basophilic to particularly magenta, intranuclear inclusions consistent with intranuclear bacilliform virus were discovered in the hepatopancreas of ~ 9% of wild broodstock and ~ 13% of batches of their progeny larvae. The inclusions were highly focused in hepatopancreatic tubules with 13-42% of tubules infected. Concurrently, using quantitative real-time Taqman PCR, ~ 74% of haemolymph samples of adults and 50% of batches of larvae were positive for Penaeus merguiensis Densovirus (PmergDNV) with copy numbers ranging from 6 × 102 to 1.5 × 105. Sequencing of 2475 base pairs of viral genome confirmed that the virus was 99% similar to PmergDNV. This is the first record of one of the members of the hepatopancreatic parvovirus group of viruses to be found outside penaeid prawns. This study demonstrates that there are at least two viruses circulating in mud crab populations in northern Australia that might impact on hatchery production

Influence of temperature and exposure time on the infectivity of Bohle iridovirus, a ranavirus

This study examined the functional temperature range of a ranavirus outside host cells over increasing temperatures and exposure times and subsequently tested infectivity in cell culture. Initially, cell susceptibility was determined by incubating Bohle iridovirus (BIV) at 30°C, 40°C, 50°C and 60°C for 5, 30 and 60 min and subsequently titrating samples in one epithelioma papulosum cyprinid (EPC) and two Bluegill fry 2 (BF2) lineages at 28°C. Titres obtained in the three cell lines were similar and EPC cells were subsequently used to further investigate ranavirus infectivity with two degree increments in temperature between 40°C and 60°C for 5, 30 and 60 min. The rate of inactivation was found to be dependent on temperature and time of exposure. Bohle iridovirus could replicate in EPC cells following exposure to most temperatures and prolonged time, but titers were reduced as temperature and time of exposure increased. Viral titres were greatest (10⁸ TCID50/ml) after exposure to 30°C and declined with increasing time of exposure and increasing temperature. Declines in BIV infectivity were largely between 40°C (10⁸ TCID₅₀/ml) and 44°C (10⁵ TCID₅₀/ml at 5 and 30 min and 10³•⁵ TCID₅₀/ml at 60 min) and secondly at temperatures greater than 52°C (from 10³•⁵ TCID₅₀/ml and approaching zero with increasing temperature and time). Treatment at 58°C for 60 min and 60°C for 30 and 60 min resulted in complete loss of BIV infectivity. The results from this study show that ranavirus can withstand much higher temperatures than previously thought, which is fundamental for understanding ranavirus epidemiology, indirect transmission dynamics and for biosecurity purposes