Expansion of the Betanodavirus epinepheli (RGNNV)/B. pseudocarangis (SJNNV) Reassortant Strain (Family Nodaviridae) Range to the Suez Canal Region

Viral nervous necrosis (VNN), caused by betanodaviruses, is a major constraint to marine finfish aquaculture worldwide. This study reports the first detection and molecular characterization of an RGNNV/SJNNV reassortant betanodavirus in farmed gilthead seabream (Sparus aurata) in the Suez Canal basin, Egypt. Four mortality episodes in two adjacent marine hatcheries affected fry from 12 days post-hatching through juveniles of seven months. Affected fish showed nervous signs, and brain histopathology revealed spongiform vacuolation and neurodegeneration. No parasitic or bacterial pathogens were detected. RT-qPCR and conventional RT-PCR detected betanodavirus RNA in clinically affected fry and juveniles and in brains of apparently healthy broodstock. In one hatchery, viral RNA was also detected in incoming well water. Partial RNA1 and RNA2 sequencing of a virus from a single adult fish identified a reassortant strain, designated EGY_Waf25, comprising RGNNV RNA1 and SJNNV RNA2. EGY_Waf25 clustered with Mediterranean and North African reassortant betanodaviruses but formed a distinct branch in RNA1-, RNA2-, and concatenated RNA1/RNA2-based analyses. Genetic distance analyses supported this placement while indicating local differentiation. These findings extend the recognized geographic occurrence of RGNNV/SJNNV reassortants to the Suez Canal basin and identify a potential regional biosecurity risk for Egyptian mariculture. Enhanced surveillance, routine broodstock screening, and water monitoring are recommended.