Umifenovir protects Procambarus clarkii against white spot syndrome virus by suppressing viral replication and modulating innate immunity

White spot syndrome virus (WSSV) remains one of the most destructive viral pathogens in crustacean aquaculture, yet effective antiviral agents are still lacking. Umifenovir is a clinically approved broad-spectrum antiviral drug with a favorable safety profile and immunomodulatory properties; however, its antiviral potential in aquatic invertebrates has not been explored. In this study, we systematically evaluated the antiviral efficacy and immunological mechanisms of umifenovir against WSSV infection in the crayfish Procambarus clarkii . Toxicity assessment and ADMET predictive analysis confirmed a wide safety window of umifenovir in crayfish. In vivo infection experiments demonstrated that umifenovir significantly suppressed WSSV replication in a dose-, tissue-, and time-dependent manner, achieving up to 97.26% inhibition at 30 mg/kg after 24 h. The treatment also reduced the transcription of viral genes from immediate-early to late stages and improved the survival rate of infected hosts. Both preventive and therapeutic administration conferred robust antiviral protection. Mechanistically, umifenovir inhibited the transcription of key viral genes required for WSSV replication and disrupted the virus-mediated manipulation of innate immune signaling pathways. Umifenovir further enhanced antioxidant capacity, alleviated oxidative stress and inflammatory responses, and attenuated virus-induced apoptosis. In addition, umifenovir reduced abnormal protein accumulation associated with viral replication while increasing lysosomal enzyme activity, indicating restoration of protein homeostasis and strengthened innate immune function. Collectively, these findings provide the first evidence that umifenovir exerts potent anti-WSSV activity in crustaceans by coordinately suppressing viral replication and modulating host immune responses, offering new mechanistic insights into its immunomodulatory effects. Given its favorable safety profile and feasibility for industrial synthesis, umifenovir represents a promising antiviral candidate for immunological intervention and provides a potential translational basis for future disease control strategies in aquaculture.