Effects of crude extracts of various plants on infectious bovine rhinotracheitis virus-plaque production.

Extracts of 28 plants were tested without demonstable antiviral activity in an agar-overlay plaque-reduction antiviral assay system, using infectious bovine rhinotracheitis virus and bovine endocardial cell cultures. Ethanolic extract of Narcissus tazetta L bulb elicited antiviral activity by inhibition of viral plaque formation. Antiviral activity was demonstrated against infectious bovine rhinotracheitis and equine rhinopneumonitis viruses. Narcissus tazetta L bulb did not directly inactivate the virus extracellularly. The extract exhibited only limited toxicity to rapidly multiplying bovine endocardial cells at plaque-inhibitory levels and was not cytoxic to preformed confluent cell monolayers. Narcissus extract did not induce the formation of drug-resistant viral strains.