Betanodavirus reassortants replicate and produce mortality in gilthead seabream larvae

Among the pathogens that most affect Mediterranean aquaculture, betanodavirus (NNV), a non-enveloped icosahedral RNA virus with a genome composed of two single-stranded positive-sense RNA segments known as RNA1 and RNA2, stands out. Apart from the traditional NNV genotypes, there are two reassortant genotypes called RGNNV/SJNNV and SJNNV/RGNNV, due to the origin of the RNA genome. It has been confirmed that gilthead seabream, a species resistant to traditional NNV genotypes, is susceptible to developing NNV disease in larval stages when infected by the RGNNV/SJNNV genotype, bringing a new focus to the prevention of infection in aquaculture farms. Therefore, the main objective of this research was to examine the susceptibility of 37 and 86 days post-hatching (dph) seabream larvae against RGNNV/SJNNV an SJNNV/RGNNV reassortants and the infective capacity of both viruses. Our results confirmed that both genotypes induced mortality in seabream larvae of both ages, being higher for RGNNV/SJNNV genotype and in 36 dph larvae. For both ages, the mortality rates were related to the replicative capacity of the virus. In 36 dph larvae, the high mortality can be explained by the high mRNA levels of viral genes from the beginning of the infection, increasing exponentially until the end of the infection, being always higher for RGNNV/SJNNV. In addition, we were able to recover viral particles for both genotypes, again higher for RGNNV/SJNNV. In contrast, the low, but statistically significant, mortality rate for 86 dph larvae might be due to the low replicative capacity of both viruses at this age. This issue together with a low expression level of viral genes and the failure in recovering infective particles seems to indicate that, even when the virus is initially competent to infect cells, its growth is dismissed or even blocked by different immune mechanisms and, therefore, the virus loses its pathogenic capacity. Further studies are needed to understand the host-viral interactions for NNV reassortants since they are threatening marine larviculture. Funded by Ministerio de Ciencia e Innovación-Agencia Estatal de Investigación [MCIN/AEI/10.13039/501100011033, grant PID2019-105522GB-I00 to AC], Ministerio de Economía y Competitividad and FEDER [grant RTI2018-096625-B-C33 to ECP and MA] and Fundación Séneca, Grupo de Excelencia de la Región de Murcia [19883/GERM/15]