Plant extracts as a source of antiviral agents against influenza A virus

The influenza virus, especially influenza A (IAV), has remained a constant global health threat due to its high morbidity rate and ability to undergo antigenic shifts and drifts, causing pandemics and epidemics. Due to the rapid evolution of IAV, novel therapeutics are urgently required to combat these viruses effectively, as they develop resistance against current therapeutics. Natural products have been the subject of debate for alternative IAV therapy, where the abundance of bioactive compounds offers numerous potentials for novel anti-IAV drug discovery. Therefore, this review discusses the antiviral effects of natural plant extracts against IAV. Examples are Silybum marianum, Scutellaria baicalensis, Angelica dahurica, Peganum harmala, Sambucus nigra, Echinacea purpurea, Panax ginseng, and Camellia sinensis. Most studies found that Si. marianum inhibits viral ribonucleic acid (RNA) synthesis. In contrast, Sc. baicalensis, A. dahurica, Sa. nigra, C. sinensis, and E. purpurea were effective in preventing the entry or binding of IAV into host cells. On the other hand, Sc. baicalensis and Pa. ginseng exert their anti-IAV effect via immunomodulation. Peganum harmala, on the contrary, exhibits a direct virucidal effect against IAV. These studies have shown promising results from using natural products against IAV, which may aid in formulating combinatorial compounds as anti-IAV therapy.