CoV is an enveloped, positive-sense single-stranded RNA (ssRNA) virus belonging to the Coronaviridae family. The CoV family consists of several species and causes upper respiratory tract and gastrointestinal infections in mammals and birds. In humans, it mainly causes common cold, but complications including pneumonia and SARS can occur. The known human CoV (HCoV) includes HCoV-229E, -OC43, -NL63, -HKU1, and the more widely known severe acute respiratory syndrome coronavirus (SARS-CoV) which caused a global threat with high mortality in 2003. In 2012, the World Health Organization (WHO) designated a sixth type of HCoV infection identified as the Middle East respiratory syndrome coronavirus (MERS-CoV) which is associated with high fatality.
There are no specific treatments for CoV infection and preventive vaccines are still being explored. Thus, the situation reflects the need to develop effective antivirals for prophylaxis and treatment of CoV infection. We have previously reported that saikosaponins (A, B2, C, and D), which are naturally occurring triterpene glycosides isolated from medicinal plants such as Bupleurum spp. (柴胡 Chái Hú), Heteromorpha spp., and Scrophularia scorodonia (玄參 Xuán Shēn), exert antiviral activity against HCoV-22E9. Upon co-challenge with the virus, these natural compounds effectively prevent the early stage of HCoV-22E9 infection, including viral attachment and penetration. Extracts from Lycoris radiata (石蒜 Shí Suàn), Artemisia annua (黃花蒿 Huáng Huā Hāo), Pyrrosia lingua (石葦 Shí Wěi), and Lindera aggregata (烏藥 Wū Yào) have also been documented to display anti–SARS-CoV effect from a screening analysis using hundreds of Chinese medicinal herbs. Natural inhibitors against the SARS-CoV enzymes, such as the nsP13 helicase and 3CL protease, have been identified as well and include myricetin, scutellarein, and phenolic compounds from Isatis indigotica (板藍根 Bǎn Lán Gēn) and Torreya nucifera (榧 Fěi).[16,17,18] Other anti-CoV natural medicines include the water extract from Houttuynia cordata (魚腥草 Yú Xīng Cǎo), which has been observed to exhibit several antiviral mechanisms against SARS-CoV, such as inhibiting the viral 3CL protease and blocking the viral RNA-dependent RNA polymerase activity.