Susceptibility of freshwater rearing Asian seabass (Lates calcarifer) to pathogenic Streptococcus iniae

Asian seabass (Lates calcarifer) has been recognized as an economically important aquaculture species which can be adapted to and cultivated in wide range of salinities. The number of freshwater intensive seabass farms in Thailand is increasing annually. Here, we first describe the susceptibility of Asian seabass, which were cultured in freshwater, to Streptococcus inae (SI) and their pathological changes. Three isolates of putative SI were identified using a combination of standard biochemical assays and species‐specific PCR prior subjected to in vivo challenge. Accumulated mortalities of the fish which received 10⁷ CFU fish^?1 of either SI1J, SI SGSA or SI2J were 90%, 90% and 100% at 7 days‐post infection (dpi), respectively, and mortalities increased sharply between 3 and 5 dpi. Clinical signs such as erratic swimming and opaque eyes were identified from a few infected fish, while most died rapidly without any abnormal signs. Histopathological manifestations were observed in the multiple organs (kidney, liver and brain). Haemorrhage, hyperhemia, cellular degeneration and inflammatory cells infiltration were commonly found within the internal organs. Notably, the formation of numerous encyst‐like lesion aggregated by eosinophilic cells, resembling macrophages, were typically found in the brain of the infected fish. Summarily, this study first revealed that freshwater reared Asian seabass is highly susceptible to SI infection and haemorrhagic septicaemia was a major pathological change that could be found in the infected fish.     

Draft genome sequence of scale drop disease virus (SDDV) retrieved from metagenomic investigation of infected barramundi,Lates calcarifer (Bloch, 1790)

Scale drop disease virus (SDDV) is a novel viral pathogen considered to be distributed in farmed barramundi (Lates calcarifer) in South-East Asia. Despite the severity of the disease, only limited genomic information related to SDDV is available. In this study, samples of SDDV-infected fish collected in 2019 were used. The microbiome of brain tissue was investigated using Illumina HiSeq DNA sequencing. Taxonomic analysis showed that SDDV was the main pathogen contained in the affected barramundi. De novo metagenome assembly recovered the SDDV genome, named isolate TH2019, 131 kb in length, and comprised of 135 ORFs. Comparison between this genome and the Singaporean SDDV reference genome revealed that the nucleotide identity within the aligned region was 99.97%. Missense, frameshift, insertion and deletion mutations were identified in 26 ORFs. Deletion of four deduced amino acid sequence in ORF_030L, identical to the SDDV isolate previously identified in Thailand, would be a potential biomarker for future strain classification. Interestingly, the genome of SDDV TH2019 harboured a unique 7,695-bp-long genomic region containing six hypothetical protein-encoded genes. Collectively, this study demonstrated that the SDDV genome can be sequenced directly, although with limited coverage depth, using metagenomic analysis of barramundi sample with severe infection.     

Outbreaks of ulcerative disease associated with ranavirus infection in barcoo grunter,Scortum barcoo (McCulloch & Waite)

In 2013, an outbreak of ulcerative disease associated with ranavirus infection occurred in barcoo grunter, Scortum barcoo (McCulloch & Waite), farms in Thailand. Affected fish exhibited extensive haemorrhage and ulceration on skin and muscle. Microscopically, the widespread haemorrhagic ulceration and necrosis were noted in gill, spleen and kidney with the presence of intracytoplasmic eosinophilic inclusion bodies. When healthy barcoo grunter were experimentally challenged via intraperitoneal and oral modes with homogenized tissue of naturally infected fish, gross and microscopic lesions occurred with a cumulative mortality of 70–90%. Both naturally and experimentally infected fish yielded positive results to the ranavirus‐specific PCR. The full‐length nucleotide sequences of major capsid protein gene of ranaviral isolates were similar to largemouth bass virus (LMBV) and identical to largemouth bass ulcerative syndrome virus (LBUSV), previously reported in farmed largemouth bass (Micropterus salmoides L.), which also produced lethal ulcerative skin lesions. To the best of our knowledge, this is the first report of a LMBV‐like infection associated with skin lesions in barcoo grunter, adding to the known examples of ranavirus infection associated with skin ulceration in fish.