Two-year surveillance of tilapia lake virus (TiLV) reveals its wide circulation in tilapia farms and hatcheries from multiple districts of Bangladesh

Tilapia lake virus (TiLV) is an emerging pathogen in aquaculture, reportedly affecting farmed tilapia in 16 countries across multiple continents. Following an early warning in 2017 that TiLV might be widespread, we executed a surveillance programme on tilapia grow-out farms and hatcheries from 10 districts of Bangladesh in 2017 and 2019. Among farms experiencing unusual mortality, eight out of 11 farms tested positive for TiLV in 2017, and two out of seven tested positive in 2019. Investigation of asymptomatic broodstock collected from 16 tilapia hatcheries revealed that six hatcheries tested positive for TiLV. Representative samples subjected to histopathology confirmed pathognomonic lesions of syncytial hepatitis. We recovered three complete genomes of TiLV from infected fish, one from 2017 and two from 2019. Phylogenetic analyses based on both the concatenated coding sequences of 10 segments and only segment 1 consistently revealed that Bangladeshi TiLV isolates formed a unique cluster within Thai clade, suggesting a close genetic relation. In summary, this study revealed the circulation of TiLV in 10 farms and six hatcheries located in eight districts of Bangladesh. We recommend continuing TiLV-targeted surveillance efforts to identify contaminated sources to minimize the countrywide spread and severity of TiLV infection.     

Development of primers and a procedure for specific identification of the diatom <Emphasis Type="Italic">Thalassiosira weissflogii</Emphasis>

A recent study showed Thalassiosira weissflogii to be a diatom containing suitable nutrition for larviculture of the black tiger shrimp, Penaeus monodon. Accurate and practical identification of this diatom species is therefore important for commercial hatcheries. The purpose of this study was to establish a DNA-based method of identification to supplement morphological examination, avoiding confusion with other Thalassiosira sp. Primers, 18SF/28SR1, specific for ribosomal DNA genes (3′-end of 18S rDNA through 5′-end of 28S rDNA, covering two internal transcribed spacers), were employed as a first-step polymerase chain reaction, followed by a second nested amplification using specifically designed primers, ITS1-F-D/ITS1-R-D. The nested-PCR result revealed specificity in the detection, distinguishing T. weissflogii from T. pseudonana, Cyclotella meneghiniana, and Chaetoceros sp., and the PCR fragment of the amplified region had a sequence that was 99% identical to the T. weissflogii sequence held by GenBank.     

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.