Resistance of pearlspot larvae, Etroplus suratensis, to redspotted grouper nervous necrosis virus by immersion challenge

A natural outbreak of infectious spleen and kidney necrosis virus (ISKNV) threatens wild pearlspot, Etroplus suratensis in Peechi Dam in the Western Ghats biodiversity hotspot, India

A large-scale mortality of pearlspot, Etroplus suratensis was reported from Peechi Dam, an artificial tropical lake made for irrigation and drinking water supply in Kerala, India during 2018. This dam is located in the premises of Western Ghats, recognized as one of the biodiversity hotspots of the world. The objective of this study was to identify the aetiological agent of this large-scale mortality of E. suratensis by systematic diagnostic investigation and identification of pathogen. Virus isolation was carried out on a species-specific pearlspot fin (PSF) cell line. Infected PSF cells showed cytopathic effects (CPE) like cell shrinkage, rounding, enlargement, clustering, and subsequent detachment of cells with a high viral titre of 10^6⋅95 TCID₅₀ mL^-1 at 8 days post inoculation (dpi). Histopathological examination of the fish showed the presence of numerous abnormal enlarged basophilic cells and intracytoplasmic eosinophilic inclusions in the liver. Moreover, transmission electron microscopy (TEM) analysis revealed the presence of large numbers of 125-132 nm viral particles in the spleen tissues. PCR amplification and phylogenetic analysis of the major capsid protein (MCP) gene sequence confirmed that the causative agent was Infectious spleen and kidney necrosis virus (ISKNV) of the genus Megalocytivirus. The experimental infection recorded 86.7±2.7% mortality in the E. suratensis (body weight - 11.01±2.7 g; body length 8.01±2.23 cm) injected with 1 × 10^4⋅25 TCID₅₀ mL^-1 ISKNV per fish. Our detailed investigation provided definitive diagnosis of ISKNV in the severe mass mortality event in wild E. suratensis in Peechi Dam, India, adding one more species to expanding host range of ISKNV infection. The high mortality rate of ISKNV infection in pearlspot suggests the perilous nature of this disease, particularly among the wild fish population. This article is protected by copyright. All rights reserved.

Transcriptome comparative analysis of immune tissues from asymptomatic and diseased Epinephelus moara naturally infected with nervous necrosis virus

Epinephelus moara is an economically important fish in Southeast Asian countries but is suffering from nervous necrosis virus (NNV) infection. A deeper understanding of the host-NNV interaction mechanisms makes sense for disease control, however, at present, the pathogenesis of natural NNV infection and the resistance mechanism in host remains poorly understood. In this study, asymptomatic and diseased E. moara with clinical symptoms of viral nervous necrosis (VNN) from a grouper farm were both detected with a positive RT-PCR signal of NNV, then transcriptome sequencing of their immune tissues (liver, spleen and kidney) were performed for comparation analysis. The de novo assemblies yielded 53,789 unigenes which had a length varied from 201 to 19,675 bp and a N50 length of 2115 bp, and 29,451 unigenes were functionally annotated, with 83, 250 and 5632 unigenes being differentially expressed in liver, spleen and kidney respectively. KEGG pathway enrichment analysis of the DEGs showed many DEGs were enriched in immune related pathways. Although the expression of class I major histocompatibility complex (MHC) was significantly higher in three immune tissues of the diseased grouper, many immune related genes, including humoral immune molecules (such as antibodies), the cellular mediated cytotoxic molecules (such as perforin) and some adhesion related genes were down regulated in the diseased grouper. Our results provided many unigenes that might play important roles in NNV resistance for further research. Furthermore, a total of 8666 unigenes containing 11,623 simple sequence repeats (SSRs) were identified, which provided useful information for screening molecular markers associated with NNV resistance in E. moara.