Medicinal plants of Tamil Nadu (Southern India) are a rich source of antiviral activities

In order to evaluate the potential of medicinal plants of Tamil Nadu as sources of antiviral activities, we used seven different viruses to evaluate the methanol extracts of 30 plants, derived from 22 families and recognized for their local medical applications. Antiviral activity was the minimum concentration of extracts required to completely inhibit viral cytopathic effects (CPE), i.e., MIC₁₀₀ values. Many extracts showed strong activities against Herpes simplex virus (HSV) and mouse corona virus (MCV, the surrogate for human SARS virus). Some extracts were also active against influenza virus and Sindbis virus (SINV, surrogate for hepatitis C virus), but fewer were active against the non-membrane viruses feline calicivirus (FCV, the surrogate for Norovirus), rhinovirus (common cold virus), and poliovirus. The most potent extracts (low MIC₁₀₀ and broad spectrum of activity) were obtained from Gymnema sylvestre R. Br. (Asclepiadaceae), Pergularia daemia (Forsskal) Chiov. (Asclepiadaceae), Sphaeranthus indicus L. (Asteraceae), Cassia alata L. (Caesalpiniaceae), Evolvulus alsinoides L. (Convolvulaceae), Clitoria ternatea L. (Fabaceae), Indigofera tinctoria L. (Euphorbiaceae), Abutilon indicum G. Don. (Malvaceae), Vitex trifolia L. (Verbenaceae), Clerodendrum inerme (L.) Gaertn (Verbenaceae), and Leucas aspera Spr. (Lamiaceae), which showed anti-MCV and anti-HSV activities at a concentration as low as 0.4 μg/mL. In some cases the activities were enhanced by light, suggesting the presence of photosensitizers. Some of these antiviral activities could contribute to the medicinal properties of the plants, and also provide more support for the concept of scientific validation of traditional plant medicines in the fight against infectious diseases.