The highly pathogenic Micropterus salmoides rhabdovirus (MSRV) poses a serious threat to largemouth bass aquaculture, leading to considerable economic losses. We assessed the anti‐MSRV activity of milbemycin A4 (A4), a macrolide derived from Streptomyces bingchenggensis in both cell‐based and animal models. Cytotoxicity assays revealed that A4 was not toxic to EPC cells at concentrations up to 6.4 mg/L. It significantly reduce...