In this study, viral replication and transcription of innate immunogenes have been
determined by RT-real time-PCR (RT-qPCR) in the course of an experimental
infection. Three different experimental conditions were considered: i) RGNNVinoculated
animals; ii) SJNNV-inoculated animals and iii) animals inoculated with
SJNNV and superinfected with RGNNV. Superinfection was performed 24 h after the
SJNNV inoculation. Control animals were mocked-injected with L-15 medium.
The RGNNV-infected group showed typical symptoms of the disease and displayed
76% cumulative mortality at the end of the experiment, whereas the mortality in the
superinfected group was 4%, and no mortality was recorded in the SJNNV-inoculated
group. The analysis of the Mx transcription by RT-qPCR showed a clearly differential
induction of the sea bass innate immune system by RGNNV and SJNNV, since no
transcription was recorded at any time tested (from 0 h to 48 h p.i.) after the RGNNV
inoculation, whereas the injection of SJNNV resulted in an important increase of the
Mx transcription from 24 h p.i. onwards. In the superinfected group the induction of
the Mx gene transcription follows the same patterns that the ones described for the
groups inoculated with SJNNV and RGNNV separately. These results suggest that the
induction of the IFN mediated system by the previous infection with SJNNV could be
responsible for the decrease in the mortality recorded in the superinfected group,
protecting sea bass of the posterior infection with RGNNV.