Age dependency of RGNNV/SJNNV viral encephalo-retinopathy in Gilthead Sea Bream (Sparus aurata)

Abstract

Gilthead sea bream (GSB) (Sparus aurata) have been generally considered to be resistant to viral encephalo-retinopathy. However, the recent increase of the number of outbreaks in sea bream hatcheries caused by nervous necrosis virus (NNV) called RGNNV/SJNNV, a reassortant virus, has encouraged researchers to investigate the disease pathogenesis in this species, with particular emphasis on fish age. For this purpose, experimental challenges of juveniles and larvae were carried out.Four experimental trials were performed by infecting GSB of different ages with the RGNNV/SJNNV virus: juveniles of 7 g of weight and larvae of 70, 35 and 21 days post hatch (dph).Virological, histological and immunohistochemical analysis, as well as qualitative and quantitative real time PCR at different time points post infection (pi) were performed. ELISA test for antibody detection was also implemented, where applicable.Overall results showed that the gilthead seabream larvae are susceptible to RGNNV/SJNNV infection irrespective to the age, as proved by the massive viral replication detected by quantitative RT-PCR and the massive presence of immunoprecipitates in the nervous tissues evidenced by immunohistochemistry. However, clear clinical signs and mortality were observed only in the youngest group of 21-dph larvae. The viral kinetics investigations led to think that the efficiency of RGNNV/SJNNV replication was higher in 21dph larvae compared to the older ones, which in turn correlated with the disease outcome. Most likely the latter was related to the developmental stage of the immune system. Noteworthy survivors were always tested negative for the presence of specific antibodies. The identification of the factors governing the host ability to control viral replication, or conversely those inhibiting the virus in older fish will be crucial to better understand the disease pathogenesis in this species. Notably, despite the age and the disease outcome of the infection, GSB remained persistently infected for a long time, even up to one year, becoming asymptomatic carriers.