Phylogeography of betanodavirus genotypes circulating in Tunisian aquaculture sites, 2012-2019

The purpose of this study was to determine the phylogenetic relationships among the primary betanodavirus strains circulating in Tunisian coastal waters. A survey was conducted to investigate nodavirus infections at 15 European sea bass Dicentrarchus labrax and gilthead sea bream Sparus aurata farming sites located along the northern and eastern coasts of Tunisia. The primary objective of the study was to create epidemiological awareness of these infections by determining phylogenetic relationships between the main betanodavirus strains circulating during the period 2012-2019, using RNA1 and/or RNA2 genome segments. Approximately 40% (118 of 294) tissue pools tested were positive for betanodavirus. Positive pools were distributed across all of the sampling sites. While fish mortalities were always correlated with the presence of virus in sea bass, a severe outbreak was also identified in sea bream larvae in 2019. Phylogenetic analysis revealed that almost all Tunisian strains from both sea bass and sea bream irrespective of outbreaks clustered within the RGNNV genotype. It is noteworthy that samples collected during the 2019 outbreak from sea bream contained both RNA1 and RNA2 fragments belonging to the RGNNV and SJNNV genotype, respectively, an indication of viral genome reassortment. To our knowledge, this is the first report of reassortant betanodavirus in Tunisia.