IPNV carrier Atlantic salmon growers do not express Mx mRNA and poly I:C-induced Mx response does not cure the carrier state

Injection of infectious pancreatic necrosis virus (IPNV) in post-smolt Atlantic salmon induced a rapid and persistent expression of Mx mRNA from day 1 to at least day 11 when Mx:β actin ratios were still at peak values of about 1.0. In contrast, an Atlantic salmon grower population, shown to be carriers of IPNV by culture of the virus from plastic adherent kidney leucocytes, showed no evidence of the expression of Mx transcripts. Nevertheless, IPNV-carrier growers showed a typical Mx response following injection of poly I:C, beginning on day 1, peaking on day 3 (mean Mx:β actin ratio 0.82) and disappearing by day 7. Notwithstanding such treatment, IPNV continued to persist in growers as the virus could still be isolated 14 days after poly I:C injection.