Detection of nervous necrosis virus RGNNV genotype in pearl gentian grouper (Epinephelus lanceolatus ♂ × E. fuscoguttatus ♀) fry imported to Thailand

Nervous necrosis virus (NNV) is a deadly virus that affects more than 120 fish species worldwide, but there is little information about it in Thailand. In August 2019, a population of pearl gentian grouper fry imported to Thailand experienced mass mortality. The diseased fish exhibited darkening, floating on the water surface, ‘sleepy behaviour’, and erratic swimming. We received a set of samples for disease diagnosis. PCR analysis revealed that these samples were positive for NNV but negative for Megalocytivirus ISKNV and Ranavirus. Sequencing the virus’s genome and phylogenetic analysis revealed that it is a member of the red-spotted grouper nervous necrosis virus (RGNNV) genotype. In situ hybridization using an NNV-specific probe revealed localization of the virus in the vacuolation lesions of the central nervous system (brain and spinal cord) and the retina of infected fish. The virus was successfully isolated from tissue homogenate from diseased fish using the E-11 cell line. This study identified NNV (RGNNV genotype) as a virus associated with massive die-offs in imported pearl grouper fry in Thailand. Considering the infectious nature of the virus and its broad host range, appropriate biosecurity measures are needed to prevent any loss to the marine aquaculture industry. Highlights This study reports detection of NNV in grouper larvae imported to Thailand The virus was assigned to the RGNNV genotype, which is the most extensively distributed genotype Histopathology revealed a pathognomonic lesion of NNV ISH using an NNV-specific probe revealed localization of NNV in the CNS and retina The virus was successfully propagated from diseased fish using the E-11 cell line