In Vitro Screening of Thai Medicinal Plants Identifies Antiviral Candidates Against an Avian Herpesvirus

Viral infections remain a major threat to poultry health and production worldwide, highlighting the need for novel antiviral strategies. This study evaluated the in vitro antiviral and virucidal activities of six Thai medicinal plant extracts against herpesvirus of turkey (HVT), a non-oncogenic avian herpesvirus widely used as a live vaccine vector and employed here as a preliminary in vitro screening model. Cytotoxicity, total phenolic content (TPC), and antioxidant capacity were assessed using the MTT, Folin–Ciocalteu, DPPH, ABTS, and FRAP assays. Among the tested extracts, Caesalpinia sappan (CS) and Persicaria odorata (PO) exhibited the highest TPC and antioxidant activities and were selected for further antiviral evaluation. Both extracts significantly reduced HVT infectivity, infectious viral titers, and intracellular viral DNA levels in DF-1 cells (p < 0.01). CS produced lower infectious viral titers (1.995 × 104 TCID50/mL) than PO (8.24 × 104 TCID50/mL); however, both extracts significantly reduced viral titers compared with the untreated control (1.78 × 106 TCID50/mL). In the virucidal assay, pre-incubation of HVT with CS and PO significantly reduced infectious virus titers to 4.64 × 104 and 2.15 × 104 TCID50/mL, respectively, compared with the untreated control (1.47 × 106 TCID50/mL) (p < 0.01). These findings identify C. sappan and P. odorata as promising sources of plant-derived compounds for future investigation as antiviral agents against avian herpesviruses. However, additional studies using virulent MDV-1, independent biological replicates, and in vivo models are required before their potential application to Marek’s disease control can be established.