Emerging RNA viruses continue to pose serious threats to animal and human health, necessitating the development of broad-spectrum antiviral agents within a One Health framework. In this study, we evaluated the antiviral potential of Desmodesmus multivariabilis extracts against three representative RNA viruses: influenza A virus (IAV), porcine epidemic diarrhea virus (PEDV), and hepatitis A virus (HAV). Extracts derived from four closely related microalgal isolates were tested using pre- and post-treatment protocols in vitro. Among them, strain ME749 demonstrated the most potent and consistent antiviral activity, with low cytotoxicity and high selectivity index (SI), particularly against IAV (SI = 23.8), followed by PEDV (SI = 10.7) and HAV (SI = 6.2). ME752 also showed moderate antiviral efficacy. RT-qPCR confirmed significant reductions in viral RNA levels for all three viruses, with ME749 achieving up to 3-log₁₀ reduction in IAV and 2.4-log₁₀ reduction in HAV. These results suggest that the antiviral effect may be mediated through mechanisms beyond direct virucidal activity, potentially involving host immune modulation. Although the active compounds remain unidentified, carotenoids are hypothesized as key bioactive components. This study highlights Desmodesmus multivariabilis, particularly ME749, as a promising source of novel, natural antiviral agents with applications in both veterinary and zoonotic disease control. Future studies including metabolomic profiling, mechanism-of-action analyses, and in vivo validation are warranted to further explore its potential in combating current and emerging RNA virus threats.

Melia azedarach is commonly used in traditional and folk medicine in Korea and China to treat a variety of diseases including diarrheal, diabetic, rheumatic, and hypertensive disease. The aim of this study was to determine the potential prophylactic and therapeutic effects of Melia azedarach against a broad spectrum of viruses in in vitro cell culture model and the protective effect against different influenza A subtypes in BALB/c mice model. An effective dose of pre-treatment, co-treatment, and post-treatment of Melia azedarach significantly reduced the replication of coxsackievirus, herpes simplex virus, influenza A virus, enterovirus, and bovine rhinovirus in both epithelial and macrophage cell lines. Melia azedarach treatment remarkably promoted the phosphorylation of the key molecules associated with the type-1 interferon and NF-κB signaling pathways. Furthermore, it induced the secretion of type-1 interferon and pro-inflammatory cytokines and the subsequent stimulation of the antiviral state in both epithelial and macrophage cells. Interestingly, oral inoculation of an effective dose of herb extract significantly improved viral clearance in the lungs of BALB/c mice, thus exhibiting protection against several subtypes of influenza A virus. Together with our results indicate that an extracts of Melia azedarach and its components could exhibit a potential natural source of an antiviral drug candidate for a broad spectrum of viruses in animal and humans.