Comparative genomics inferred two distinct populations of piscine pathogenic Streptococcus agalactiae, serotype Ia ST7 and serotype III ST283, in Thailand and Vietnam


The genomes of Streptococcus agalactiae (group B streptococcus; GBS) collected from diseased fish in Thailand and Vietnam over a nine-year period (2008–2016) were sequenced and compared (n = 21). Based on capsular serotype and multilocus sequence typing (MLST), GBS isolates are divided into 2 groups comprised of i) serotype Ia; sequence type (ST)7 and ii) serotype III; ST283. Population structureinferred by core genome (cg)MLST and Bayesian clustering analysis also strongly indicated distribution of two GBS populations in both Thailand and Vietnam. Deep phylogenetic analysis implied by CRISPR array’s spacer diversity was able to cluster GBS isolates according to their temporal and geographic origins, though ST7 has varying CRISPR1-spacer profiles when compared to ST283 strains. Based on overall genotypic features, Thai ST283 strains were closely related to the Singaporean ST283 strain causing foodborne illness in humans in 2015, thus, signifying zoonotic potential of this GBS population in the country.

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Viral infections in tilapines: More than just tilapia lake virus


There is an increasing number of emerging infectious agents, although not zoonotic, that affect farmed tilapines worldwide. Most recently, the emergence of a new virus, tilapia lake virus (TiLV), has diverted global attention to it while little or no research is being done on the other viruses of possible equal importance. These other viruses have reportedly been associated with relatively high mortalities (20–100%) in several cases of natural disease outbreaks or laboratory challenges. The objective of this review is to highlight viral infections of tilapines that may have been neglected by the scientific community but are also requiring systematic investigation. The viral infections in this review include infectious pancreatic necrosis virus (IPNV, Aquabirnavirus), nervous necrosis virus (VNN, Betanodavirus), tilapia larvae encephalitis virus (TLEV, Herpesvirus), and the Iridovirus infections namely Bohle iridovirus (BIV, Ranavirus) infectious spleen and kidney necrosis virus (ISKNV, Megalocytivirus), Lymphocystivirus and another Iridovirus-like infection. This review summarizes current scientific knowledge on viral infections that affect tilapines and highlights diagnosis methods for disease surveillance and discusses the future visions for disease control.

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Tilapia lake virus (TiLV) from Peru is genetically close to the Israeli isolates


Tilapia is reportedly affected by two RNA viruses; tilapia lake virus (TiLV) and nervous necrosis virus (NNV), both causing up to 90% mortality in acute infections. Random investigation was performed with samples from 4 farms located in the department (province) of San Martín, a region with the highest concentrated tilapia farms in Peru. Fish tissues of the eyes, brain, and liver were subjected to molecular detection and histopathological examination. The results showed that all of the samples were tested negative for NNV but 4 samples from 3 farms were TiLV positive. Syncytial hepatitis lesion typically found in TiLV-infected tilapia was also observed. TiLV genome sequence analysis of the Peruvian isolate showed 96.89–97.13% and 95.38–95.68% identity to that of the Israeli and Thai isolates, respectively. Multilocus sequence phylogenetic analysis based on 8305 nucleotides of five TiLV isolates indicated that the Peruvian isolate is genetically close to the Israeli isolates, possibly suggesting the same origin.

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A validated semi-nested PCR for rapid detection of scale drop disease virus (SDDV) in Asian sea bass (Lates calcarifer)


Scale drop diseases virus (SDDV), a newly characterized virus of farmed Asian sea bass (Lates calcarifer), has been reported in several countries in Southeast Asia. However, no fully validated detection method is publicly available for disease diagnosis and surveillance. Here, we described a newly developed semi-nested PCR (snPCR) method for detection of the virus from field samples. The designed primers targeting a gene encoding ATPase generated amplicons of 738 bp and 412 bp in the first and second step PCR, respectively. The established protocol could detect down to 100 viral copies/μL template and was 100-fold more sensitive than single step PCR. A Specificity test against extracted DNA from ten bacterial pathogens, tissues from viral infected specimens and fish host revealed no cross amplification. The SDDV snPCR method could detect the virus from all clinical samples showing symptoms of scale drop disease (n = 25) and all samples from outbreaks of an unknown disease (n = 6) whereas all clinically healthy fish sea bass (n = 161) and grouper (n = 45) collected from different provinces tested negative. The newly established protocol might be useful for Asian sea bass farming countries to initiate disease diagnosis and surveillance.

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Mortality from scale drop disease in farmed Lates calcarifer in Southeast Asia


Clinically diseased adult Lates calcarifer from a natural outbreak showing (a) fin rot and loss of scales; (b) scales along the lateral line detached easily upon gently rubbing; (c) ventral surface of some fish showing loss of scales and skin haemorrhage


In Southeast Asia, a new disease called scale drop disease (SDD) caused by a novel Megalocytivirus (SDDV) has emerged in farmed Asian sea bass (Lates calcarifer) in Singapore, Malaysia and Indonesia. We received samples from an Eastern Thai province that also showed gross signs of SDD (loss of scales). Clinical samples of 0.2-1.1 kg L. calcarifer collected between 2016 and 2018 were examined for evidence of SDDV infection. Histopathology was similar to that in the first report of SDDV from Singapore including necrosis, inflammation and nuclear pyknosis and karyorrhexis in the multiple organs. Intracytoplasmic inclusion bodies were also observed in the muscle tissue. In a density-gradient fraction from muscle extracts, TEM revealed enveloped, hexagonal megalocytiviral-like particles (~100-180 nm). By PCR using primers derived from the Singaporean SDDV genome sequence, four different genes were amplified and sequenced from the Thai isolate revealing 98.7%-99.9% identity between the two isolates. Since viral inclusions were rarely observed, clinical signs and histopathology could not be used to easily distinguish between SDD caused by bacteria or SDDV. We therefore recommend that PCR screening be used to monitor broodstock, fry and grow-out fish to estimate the current impact of SDDV in Southeast Asia and to prevent its spread.


Lates calcarifer ; PCR; SDDV; scale drop disease

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Natural occurrence of edwardsiellosis caused by Edwardsiella ictaluri in farmed hybrid red tilapia (Oreochromis sp.) in Southeast Asia



Infections caused by Edwardsiella ictaluri bacteria have mainly affected the catfish aquaculture industry in Asia and America, while few reports reveal E. ictaluri-associated mortality in non-catfish species. Here, we report a natural occurrence of E. ictaluri in hybrid red tilapia juveniles farmed in a floating cage system in Southeast Asia. Cumulative mortality reached 40–50% within the first month after stocking. Diseased fish exhibited the presence of numerous white spots in the swollen spleen and head kidney. Pure cultures of pinpoint bacterial colonies were recovered from internal organs of all naturally diseased fish (n = 10) and subsequently four representative isolates were chosen for further identification and analyses. The bacteria were later confirmed as E. ictaluri based on biochemical characteristics, specific PCR for both genus and species levels, partial 16S rRNA and gyrB sequencing. Artificial infections using Nile tilapia juveniles produced Edwardsiellosis with typical signs of visceral white spots as observed in naturally diseased fish. Discovery of E. ictaluri infection in farmed red tilapia adds to the growing list of emerging pathogens in tilapia aquaculture in the region of which better awareness needs to be made.

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TiLV histopathology

TiLV 2018

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