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

Abstract

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.

A highly sensitive and specific SYBR Green quantitative polymerase chain reaction (qPCR) method for rapid detection of scale drop disease virus in Asian sea bass, Lates calcarifer

“…The disease has been reported in a number of countries in Southeast Asia recently including Malaysia, Singapore, Indonesia and Thailand (de Groof et al, 2015; Senapin et al, 2019). However, presence of the virus in the region was linked to mortalities since 1992 with similar clinical signs but probably misdiagnosed as other pathogen infection(s) (de Groof et al, 2015).…”

Development of a SYBR Green quantitative PCR assay for detection of Lates calcarifer herpesvirus (LCHV) in farmed barramundi

“…Detection of LCHV in multiple organs of the clinically sick fish suggests that the virus caused systemic infection, similar to that of SDDV (Senapin et al, 2019; Charoenwai et al, 2019; Sriisan et al, 2020). Interestingly, the liver and kidney tissues which are normally used for PCR diagnostics of fish viruses appeared to be unsuitable for LCHV detection while the fin seemed to be the best targeted tissue due to its highest viral loads, followed by gills, muscle, spleen and brain.…”

“…The present study focused primarily on the development and validation of a sensitive qPCR detection method for LCHV. Apart from LCHV, several pathogens have been reported to cause similar clinical signs of “scale drop” including SDDV (Gibson-Kueh et al, 2012; de Groof et al, 2015; Senapin et al, 2019), a pathogenic strain of Vibrio harveyi , and Tenacibaculum maritimum (Dong et al, 2017a; Gibson-Kueh et al, 2012). Relatively low viral loads present in the clinically sick fish with scale drop disease-like symptoms suggests that LCHV might be an opportunistic pathogen rather than the true causative agent of the diseased fish investigated in this study.…”

“…Therefore, molecular detection methods are required for diagnostic and screening purposes. Several DNA-based detection methods for SDDV have been freely available such as single PCR (Senapin et al, 2019), semi-nested PCR (Charoenwai et al, 2019), loop-mediated isothermal amplification (LAMP) (Dangtip et al, 2019), probe-based qPCR (de Groof et al, 2015), and SYBR Green-based qPCR (Sriisan et al, 2020). The latter one is the most sensitive method with a detection limit of 2 copies of DNA template per reaction.…”

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 an emerging viral pathogen that has caused outbreaks in farmed barramundi ( Lates calcarifer Bloch, 1,790) in several South‐East Asian countries including Singapore, Indonesia, Malaysia and Thailand recently (de Groof et al., 2015; Gibson‐Kueh et al., 2012; Senapin et al., 2019). This double‐stranded DNA virus has been classified as a novel species within the genus Megalocytivirus , belonging to the family Iridoviridae .…”