Detection of aquareovirus in farmed Tasmanian Atlantic salmon (Salmo salar)

• Main Author: Zainathan, SC
• Format: Thesis
• Language: English
• Published: 2012
• Subjects: detection; diagnostic method; TSVR; Atlantic salmon; Salmo salar
• Online Access: https://eprints.utas.edu.au/15020/; https://eprints.utas.edu.au/15020/2/whole-zainathan-thesis.pdf

This thesis focused on the detection and identification of Tasmanian Atlantic salmon aquareovirus (TSRV), which is one of the few viral agents of Atlantic salmon endemic in Tasmania. Due to the low pathogenicity and ubiquitous nature of TSRV, there has been little interest in the significance of this virus. However, more recently, TSRV infections appeared to be associated with diseased fish and concerns about the negative impact of infection with this virus on aquaculture productivity have increased. Industry’s concerns regarding the significance of TSRV have resurfaced and recent research has indicated that under certain conditions TSRV could cause disease. Thus better management and control is being considered by industry and regulators. Validation of diagnostic methods has been a major focus in this thesis. Intra-laboratory and inter-laboratory comparison of PCR and virus isolation on piscine cell lines were carried out to determine the most sensitive diagnostic method, using tissues of farmed Atlantic salmon from various aquaculture sites around Tasmania. A total of 144 fish from 9 sites (12-33 fish per site) were sampled from two regions (Tamar River & South-east Tasmania) during late spring to early summer of 2009 and the data were analysed using different statistical approaches. This study demonstrated the qPCR assay to be highly sensitive (95.2%) and highly specific (95.2%) for the detection of TSRV. The prevalence of TSRV ranged from 6-22% in both regions. Following this, the use of swabs in preference to organs as the sample collection method was evaluated for individual and pooled samples. The use of swabs was shown to be best for field surveillance and screening purposes when the only concern with the presence and absence of virus in the population. The incidence of TSRV infections was investigated by undertaking a field investigation at farm sites located in South-east Tasmania. Throughout this field investigation, the incidence of TSRV infections was low (6.15%). The findings do not exclude the role of TSRV in influencing the host’s susceptibility to other infections. Non specific gross pathology and histopathological changes were observed in TSRV positive salmon and similar observations were present in TSRV negative salmon. On the basis on archival cases liver pathology has been identified as the predominant pathology caused by TSRV. As a basis for a preliminary characterisation study, fourteen isolates of TSRV originating from various locations in Tasmania, covering a 20-year period obtained from various host species, host tissues and isolated on different cell lines, were selected in an attempt to increase the probability of detecting virus variants. Typical and atypical variants of TSRV were identified based on genotypic and phenotypic characterisation of the different isolates. Electron microscopic examination demonstrated the existence of at least three variants based on viral particle size. This study revealed preliminary evidence of vertical transmission of TSRV from brood-stocks to eggs and horizontal transmission from farmed salmon to wild fish. Finally, this characterisation study demonstrated the existence of at least one variant TSRV isolate other than the more commonly isolated, typical TSRV in farmed Tasmanian Atlantic salmon. The use of different detection/diagnostic methods in this thesis, has improved the scope of the detection of TSRV.