Broad-spectrum antiviral activity of molnupiravir against fish rhabdoviruses through entry blockade and induced viral mutagenesis.

Abstract

Viral hemorrhagic septicemia virus (VHSV), spring viremia of carp virus (SVCV), and infectious hematopoietic necrosis virus (IHNV), belonging to the family Rhabdoviridae, are three economically important fish rhabdoviruses for which control options remain limited. In this study, we evaluated the antiviral effect of molnupiravir against these viruses. The half-maximal effective concentration (EC) was determined along with time-of-addition experiments to identify the life cycle stage targeted, and mutation profiling following serial passage under drug pressure to assess the consequences of long-term exposure. The results showed that the ECvalues of molnupiravir against VHSV and SVCV were 5.25 μM and 2.59 μM respectively, with a high selectivity index. This indicates that it has favorable safety and efficacy profiles. The most pronounced inhibitory effect was observed when molnupiravir was added during the early post-infection period. Stage-specific assays showed inhibitory effects during viral attachment and internalization, and molnupiravir treatment also reduced the specific infectivity of progeny virus. Under sustained drug pressure (EC-EC), viruses continued to replicate at low levels and accumulated mutations, predominantly involving base transitions (C→T and A→G) which are a signature characteristic of molnupiravir-induced mutagenesis. This study demonstrates that molnupiravir significantly inhibits three important fish rhabdoviruses in vitro, providing experimental evidence for its application in aquatic virus control, and suggests that attention should be paid to the possible viral adaptive mutations caused by long-term use.